[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Daniel Vetter]

On Fri, Jun 20, 2014 at 1:42 AM, Greg KH  wrote:
>> I'm actually concerned about this trend.  Downgrading things to WARN_ON
>> can allow a security bug in the kernel to continue to exist, for
>> example, or make the error message disappear.
>
> A BUG_ON makes any error message disappear pretty quickly :)
>
> I'm talking about foolish "ASSERT-like" BUG_ON that driver authors like
> to add to their code when writing it to catch things they are messing
> up.  After the code is working, they should be removed, like this one.

Well except for cases where it's super performance critical I like to
retain these WARN_ON asserts (not BUG_ON). "Is the logic sufficient
locked down with WARN_ONs?" is actually one of the main review
criteria I have for i915 patches, especially on the modeset side.
They're a bit an annoyance for distro's since they result in a
constant (but ever shifting) stream of backtraces, but for me they
serve as an excellent early warning sign when our driver has yet again
lost its marbles (or at least some) way before something user-visibly
bad happens.

And for those screaming that these checks should be hidden behind a
config option and only enabled for validation: Nope, there's too many
combinations of display hardware out there and I simply need our
entire user base to serve as guinea pigs. There's really no other way
to validate this mess called drm/i915.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Daniel Vetter]

On Fri, Jun 20, 2014 at 12:39 AM, H. Peter Anvin  wrote:
>>> Aside: This is a pet peeve of mine and recently I've switched to
>>> rejecting all patch that have a BUG_ON, period.
>>
>> Please do, I have been for a few years now as well for the same reasons
>> you cite.
>>
>
> I'm actually concerned about this trend.  Downgrading things to WARN_ON
> can allow a security bug in the kernel to continue to exist, for
> example, or make the error message disappear.
>
> I am wondering if the right thing here isn't to have a user (command
> line?) settable policy as to how to proceed on an assert violation,
> instead of hardcoding it at compile time.

I should clarify: If it smells like the issue is a failure of our
ioctl/syscall validation code to catch crap, BUG_ON is the right
choice. And fundamentally I've had this rule since 1-2 years now, the
only recent change I've done is switch my scripts from warning by
default if there's a new BUG_ON to rejecting by default. Mostly
because I'm lazy and let too many BUG_ONs pass through by default.

Also if you add a new interface to i915 I'll make damn sure you supply
a full set of nasty testcases to abuse the ioctl hard. In the end it's
a tradeoff and overall I don't think I'm compromising security with my
current set of rules.

Also, people don't (yet) terribly care about data integrity as soon as
their data has passed once through a gpu.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Dave Airlie]

On 20 June 2014 04:19, Greg KH  wrote:
> On Thu, Jun 19, 2014 at 01:45:30PM -0400, Rob Clark wrote:
>> On Thu, Jun 19, 2014 at 1:00 PM, Greg KH  
>> wrote:
>> > On Thu, Jun 19, 2014 at 10:00:18AM -0400, Rob Clark wrote:
>> >> On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  
>> >> wrote:
>> >> > On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>> >> >> +#define CREATE_TRACE_POINTS
>> >> >> +#include 
>> >> >> +
>> >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
>> >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
>> >> >
>> >> > Are you really willing to live with these as tracepoints for forever?
>> >> > What is the use of them in debugging?  Was it just for debugging the
>> >> > fence code, or for something else?
>> >> >
>> >> >> +/**
>> >> >> + * fence_context_alloc - allocate an array of fence contexts
>> >> >> + * @num: [in]amount of contexts to allocate
>> >> >> + *
>> >> >> + * This function will return the first index of the number of fences 
>> >> >> allocated.
>> >> >> + * The fence context is used for setting fence->context to a unique 
>> >> >> number.
>> >> >> + */
>> >> >> +unsigned fence_context_alloc(unsigned num)
>> >> >> +{
>> >> >> + BUG_ON(!num);
>> >> >> + return atomic_add_return(num, _context_counter) - num;
>> >> >> +}
>> >> >> +EXPORT_SYMBOL(fence_context_alloc);
>> >> >
>> >> > EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
>> >> > Traditionally all of the driver core exports have been with this
>> >> > marking, any objection to making that change here as well?
>> >>
>> >> tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
>> >> wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
>> >> life.  We already went through this debate once with dma-buf.  We
>> >> aren't going to change $evil_vendor's mind about non-gpl modules.  The
>> >> only result will be a more flugly convoluted solution (ie. use syncpt
>> >> EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
>> >> workaround, with the result that no-one benefits.
>> >
>> > It has been proven that using _GPL() exports have caused companies to
>> > release their code "properly" over the years, so as these really are
>> > Linux-only apis, please change them to be marked this way, it helps
>> > everyone out in the end.
>>
>> Well, maybe that is the true in some cases.  But it certainly didn't
>> work out that way for dma-buf.  And I think the end result is worse.
>>
>> I don't really like coming down on the side of EXPORT_SYMBOL() instead
>> of EXPORT_SYMBOL_GPL(), but if we do use EXPORT_SYMBOL_GPL() then the
>> result will only be creative workarounds using the _GPL symbols
>> indirectly by whatever is available via EXPORT_SYMBOL().  I don't
>> really see how that will be better.
>
> You are saying that you _know_ companies will violate our license, so
> you should just "give up"?  And how do you know people aren't working on
> preventing those "indirect" usages as well?  :)
>
> Sorry, I'm not going to give up here, again, it has proven to work in
> the past in changing the ways of _very_ large companies, why stop now?

I've found large companies shipping lots of hw putting pressure on
other large/small companies seems to be only way this has ever
happened, we'd like to cover that up and say its some great GPL
enforcement thing.

To be honest, author's choice is how I'd treat this.

Personally I think _GPL is broken by design, and that Linus's initial
point for them has been so diluted by random lobby groups asking for
every symbol to be _GPL that they are becoming effectively pointless
now. I also dislike the fact that the lobby groups don't just bring
violators to court. I'm also sure someone like the LF could have a
nice income stream if Linus gave them permission to enforce his
copyrights.

But anyways, has someone checked that iOS or Windows don't have a
fence interface? so we know that this is a Linux only interface and
any works using it are derived? Say the nvidia driver isn't a derived
work now, will using this interface magically translate it into a
derived work, so we can go sue them? I don't think so.

But its up to Maarten and Rob, and if they say no _GPL then I don't
think we should be overriding authors intents.

Dave.

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Daniel Vetter]

On Thu, Jun 19, 2014 at 8:19 PM, Greg KH  wrote:
>> >> > EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
>> >> > Traditionally all of the driver core exports have been with this
>> >> > marking, any objection to making that change here as well?
>> >>
>> >> tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
>> >> wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
>> >> life.  We already went through this debate once with dma-buf.  We
>> >> aren't going to change $evil_vendor's mind about non-gpl modules.  The
>> >> only result will be a more flugly convoluted solution (ie. use syncpt
>> >> EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
>> >> workaround, with the result that no-one benefits.
>> >
>> > It has been proven that using _GPL() exports have caused companies to
>> > release their code "properly" over the years, so as these really are
>> > Linux-only apis, please change them to be marked this way, it helps
>> > everyone out in the end.
>>
>> Well, maybe that is the true in some cases.  But it certainly didn't
>> work out that way for dma-buf.  And I think the end result is worse.
>>
>> I don't really like coming down on the side of EXPORT_SYMBOL() instead
>> of EXPORT_SYMBOL_GPL(), but if we do use EXPORT_SYMBOL_GPL() then the
>> result will only be creative workarounds using the _GPL symbols
>> indirectly by whatever is available via EXPORT_SYMBOL().  I don't
>> really see how that will be better.
>
> You are saying that you _know_ companies will violate our license, so
> you should just "give up"?  And how do you know people aren't working on
> preventing those "indirect" usages as well?  :)
>
> Sorry, I'm not going to give up here, again, it has proven to work in
> the past in changing the ways of _very_ large companies, why stop now?

Dave should chime in here since currently dma-buf is _GPL and the
drm_prime.c wrapper for it is not (and he merged that one, contributed
from said $vendor). And since we're gfx people everything we do is MIT
licensed (that's where X is from after all), so _GPL for for drm stuff
really doesn't make a lot of sense for us. ianal and all that applies.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Daniel Vetter]

On Thu, Jun 19, 2014 at 7:00 PM, Greg KH  wrote:
>> >> + BUG_ON(f1->context != f2->context);
>> >
>> > Nice, you just crashed the kernel, making it impossible to debug or
>> > recover :(
>>
>> agreed, that should probably be 'if (WARN_ON(...)) return NULL;'
>>
>> (but at least I wouldn't expect to hit that under console_lock so you
>> should at least see the last N lines of the backtrace on the screen
>> ;-))
>
> Lots of devices don't have console screens :)

Aside: This is a pet peeve of mine and recently I've switched to
rejecting all patch that have a BUG_ON, period. Except when you can
prove that the kernel will die in the next few lines and there's
nothing you can do about it a WARN_ON is always better - I've wasted
_way_ too much time debugging hard hangs because such a "benign"
BUG_ON ended up eating my irq handler or a spinlock required by such.
Or some other nonsense that makes debugging a royal pita, especially
if your remote debugger consists of a frustrated users staring at a
hung machine.



Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Rob Clark]

On Thu, Jun 19, 2014 at 5:50 PM, Dave Airlie  wrote:
> On 20 June 2014 04:19, Greg KH  wrote:
>> On Thu, Jun 19, 2014 at 01:45:30PM -0400, Rob Clark wrote:
>>> On Thu, Jun 19, 2014 at 1:00 PM, Greg KH  
>>> wrote:
>>> > On Thu, Jun 19, 2014 at 10:00:18AM -0400, Rob Clark wrote:
>>> >> On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  
>>> >> wrote:
>>> >> > On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>>> >> >> +#define CREATE_TRACE_POINTS
>>> >> >> +#include 
>>> >> >> +
>>> >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
>>> >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
>>> >> >
>>> >> > Are you really willing to live with these as tracepoints for forever?
>>> >> > What is the use of them in debugging?  Was it just for debugging the
>>> >> > fence code, or for something else?
>>> >> >
>>> >> >> +/**
>>> >> >> + * fence_context_alloc - allocate an array of fence contexts
>>> >> >> + * @num: [in]amount of contexts to allocate
>>> >> >> + *
>>> >> >> + * This function will return the first index of the number of fences 
>>> >> >> allocated.
>>> >> >> + * The fence context is used for setting fence->context to a unique 
>>> >> >> number.
>>> >> >> + */
>>> >> >> +unsigned fence_context_alloc(unsigned num)
>>> >> >> +{
>>> >> >> + BUG_ON(!num);
>>> >> >> + return atomic_add_return(num, _context_counter) - num;
>>> >> >> +}
>>> >> >> +EXPORT_SYMBOL(fence_context_alloc);
>>> >> >
>>> >> > EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
>>> >> > Traditionally all of the driver core exports have been with this
>>> >> > marking, any objection to making that change here as well?
>>> >>
>>> >> tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
>>> >> wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
>>> >> life.  We already went through this debate once with dma-buf.  We
>>> >> aren't going to change $evil_vendor's mind about non-gpl modules.  The
>>> >> only result will be a more flugly convoluted solution (ie. use syncpt
>>> >> EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
>>> >> workaround, with the result that no-one benefits.
>>> >
>>> > It has been proven that using _GPL() exports have caused companies to
>>> > release their code "properly" over the years, so as these really are
>>> > Linux-only apis, please change them to be marked this way, it helps
>>> > everyone out in the end.
>>>
>>> Well, maybe that is the true in some cases.  But it certainly didn't
>>> work out that way for dma-buf.  And I think the end result is worse.
>>>
>>> I don't really like coming down on the side of EXPORT_SYMBOL() instead
>>> of EXPORT_SYMBOL_GPL(), but if we do use EXPORT_SYMBOL_GPL() then the
>>> result will only be creative workarounds using the _GPL symbols
>>> indirectly by whatever is available via EXPORT_SYMBOL().  I don't
>>> really see how that will be better.
>>
>> You are saying that you _know_ companies will violate our license, so
>> you should just "give up"?  And how do you know people aren't working on
>> preventing those "indirect" usages as well?  :)
>>
>> Sorry, I'm not going to give up here, again, it has proven to work in
>> the past in changing the ways of _very_ large companies, why stop now?
>
> I've found large companies shipping lots of hw putting pressure on
> other large/small companies seems to be only way this has ever
> happened, we'd like to cover that up and say its some great GPL
> enforcement thing.
>
> To be honest, author's choice is how I'd treat this.
>
> Personally I think _GPL is broken by design, and that Linus's initial
> point for them has been so diluted by random lobby groups asking for
> every symbol to be _GPL that they are becoming effectively pointless
> now. I also dislike the fact that the lobby groups don't just bring
> violators to court. I'm also sure someone like the LF could have a
> nice income stream if Linus gave them permission to enforce his
> copyrights.
>
> But anyways, has someone checked that iOS or Windows don't have a
> fence interface? so we know that this is a Linux only interface and
> any works using it are derived? Say the nvidia driver isn't a derived
> work now, will using this interface magically translate it into a
> derived work, so we can go sue them? I don't think so.

I've no ideas about what the APIs are in windows, but windows has had
multi-gpu support for a *long* time, which implies some mechanism like
dmabuf and fence.. this isn't exactly an area where we are
trailblazing here.

BR,
-R


> But its up to Maarten and Rob, and if they say no _GPL then I don't
> think we should be overriding authors intents.
>
> Dave.

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Thu, Jun 19, 2014 at 03:39:47PM -0700, H. Peter Anvin wrote:
> On 06/19/2014 01:01 PM, Greg KH wrote:
> > On Thu, Jun 19, 2014 at 09:15:36PM +0200, Daniel Vetter wrote:
> >> On Thu, Jun 19, 2014 at 7:00 PM, Greg KH  
> >> wrote:
> >> + BUG_ON(f1->context != f2->context);
> >
> > Nice, you just crashed the kernel, making it impossible to debug or
> > recover :(
> 
>  agreed, that should probably be 'if (WARN_ON(...)) return NULL;'
> 
>  (but at least I wouldn't expect to hit that under console_lock so you
>  should at least see the last N lines of the backtrace on the screen
>  ;-))
> >>>
> >>> Lots of devices don't have console screens :)
> >>
> >> Aside: This is a pet peeve of mine and recently I've switched to
> >> rejecting all patch that have a BUG_ON, period.
> > 
> > Please do, I have been for a few years now as well for the same reasons
> > you cite.
> > 
> 
> I'm actually concerned about this trend.  Downgrading things to WARN_ON
> can allow a security bug in the kernel to continue to exist, for
> example, or make the error message disappear.

A BUG_ON makes any error message disappear pretty quickly :)

I'm talking about foolish "ASSERT-like" BUG_ON that driver authors like
to add to their code when writing it to catch things they are messing
up.  After the code is working, they should be removed, like this one.

Don't enforce an api requirement with a kernel crash, warn and return an
error which the caller should always be checking anyway.

thanks,

greg k-h

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[James Bottomley]

On Thu, 2014-06-19 at 15:39 -0700, H. Peter Anvin wrote:
> On 06/19/2014 01:01 PM, Greg KH wrote:
> > On Thu, Jun 19, 2014 at 09:15:36PM +0200, Daniel Vetter wrote:
> >> On Thu, Jun 19, 2014 at 7:00 PM, Greg KH  
> >> wrote:
> >> + BUG_ON(f1->context != f2->context);
> >
> > Nice, you just crashed the kernel, making it impossible to debug or
> > recover :(
> 
>  agreed, that should probably be 'if (WARN_ON(...)) return NULL;'
> 
>  (but at least I wouldn't expect to hit that under console_lock so you
>  should at least see the last N lines of the backtrace on the screen
>  ;-))
> >>>
> >>> Lots of devices don't have console screens :)
> >>
> >> Aside: This is a pet peeve of mine and recently I've switched to
> >> rejecting all patch that have a BUG_ON, period.
> > 
> > Please do, I have been for a few years now as well for the same reasons
> > you cite.
> > 
> 
> I'm actually concerned about this trend.  Downgrading things to WARN_ON
> can allow a security bug in the kernel to continue to exist, for
> example, or make the error message disappear.

Me too.  We use BUG_ON in the I/O subsystem where we're forced to
violate a guarantee.  When the choice is corrupt something or panic the
system, I prefer the latter every time.

> I am wondering if the right thing here isn't to have a user (command
> line?) settable policy as to how to proceed on an assert violation,
> instead of hardcoding it at compile time.

I'd say it depends on the consequence of the assertion violation.  We
have assertions that are largely theoretical, ones that govern process
internal state (so killing the process mostly sanitizes the system) and
a few that imply data loss or data corruption.

James

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[H. Peter Anvin]

On 06/19/2014 01:01 PM, Greg KH wrote:
> On Thu, Jun 19, 2014 at 09:15:36PM +0200, Daniel Vetter wrote:
>> On Thu, Jun 19, 2014 at 7:00 PM, Greg KH  
>> wrote:
>> + BUG_ON(f1->context != f2->context);
>
> Nice, you just crashed the kernel, making it impossible to debug or
> recover :(

 agreed, that should probably be 'if (WARN_ON(...)) return NULL;'

 (but at least I wouldn't expect to hit that under console_lock so you
 should at least see the last N lines of the backtrace on the screen
 ;-))
>>>
>>> Lots of devices don't have console screens :)
>>
>> Aside: This is a pet peeve of mine and recently I've switched to
>> rejecting all patch that have a BUG_ON, period.
> 
> Please do, I have been for a few years now as well for the same reasons
> you cite.
> 

I'm actually concerned about this trend.  Downgrading things to WARN_ON
can allow a security bug in the kernel to continue to exist, for
example, or make the error message disappear.

I am wondering if the right thing here isn't to have a user (command
line?) settable policy as to how to proceed on an assert violation,
instead of hardcoding it at compile time.

-hpa

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Rob Clark]

On Thu, Jun 19, 2014 at 2:19 PM, Greg KH  wrote:
> On Thu, Jun 19, 2014 at 01:45:30PM -0400, Rob Clark wrote:
>> On Thu, Jun 19, 2014 at 1:00 PM, Greg KH  
>> wrote:
>> > On Thu, Jun 19, 2014 at 10:00:18AM -0400, Rob Clark wrote:
>> >> On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  
>> >> wrote:
>> >> > On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>> >> >> +#define CREATE_TRACE_POINTS
>> >> >> +#include 
>> >> >> +
>> >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
>> >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
>> >> >
>> >> > Are you really willing to live with these as tracepoints for forever?
>> >> > What is the use of them in debugging?  Was it just for debugging the
>> >> > fence code, or for something else?
>> >> >
>> >> >> +/**
>> >> >> + * fence_context_alloc - allocate an array of fence contexts
>> >> >> + * @num: [in]amount of contexts to allocate
>> >> >> + *
>> >> >> + * This function will return the first index of the number of fences 
>> >> >> allocated.
>> >> >> + * The fence context is used for setting fence->context to a unique 
>> >> >> number.
>> >> >> + */
>> >> >> +unsigned fence_context_alloc(unsigned num)
>> >> >> +{
>> >> >> + BUG_ON(!num);
>> >> >> + return atomic_add_return(num, _context_counter) - num;
>> >> >> +}
>> >> >> +EXPORT_SYMBOL(fence_context_alloc);
>> >> >
>> >> > EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
>> >> > Traditionally all of the driver core exports have been with this
>> >> > marking, any objection to making that change here as well?
>> >>
>> >> tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
>> >> wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
>> >> life.  We already went through this debate once with dma-buf.  We
>> >> aren't going to change $evil_vendor's mind about non-gpl modules.  The
>> >> only result will be a more flugly convoluted solution (ie. use syncpt
>> >> EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
>> >> workaround, with the result that no-one benefits.
>> >
>> > It has been proven that using _GPL() exports have caused companies to
>> > release their code "properly" over the years, so as these really are
>> > Linux-only apis, please change them to be marked this way, it helps
>> > everyone out in the end.
>>
>> Well, maybe that is the true in some cases.  But it certainly didn't
>> work out that way for dma-buf.  And I think the end result is worse.
>>
>> I don't really like coming down on the side of EXPORT_SYMBOL() instead
>> of EXPORT_SYMBOL_GPL(), but if we do use EXPORT_SYMBOL_GPL() then the
>> result will only be creative workarounds using the _GPL symbols
>> indirectly by whatever is available via EXPORT_SYMBOL().  I don't
>> really see how that will be better.
>
> You are saying that you _know_ companies will violate our license, so
> you should just "give up"?  And how do you know people aren't working on
> preventing those "indirect" usages as well?  :)

Well, all I know is what happened with dmabuf.  This seems like the
exact same scenario (same vendor, same driver, same use-case).

Not really sure how we could completely prevent indirect usage, given
that drm core and many of the drivers are dual MIT/GPL.   (But ofc,
IANAL.)

> Sorry, I'm not going to give up here, again, it has proven to work in
> the past in changing the ways of _very_ large companies, why stop now?

In the general case, I would agree.  But in this specific case, I am
not very optimistic.

That said, it isn't really my loss if it is _GPL()..  I don't have to
use or support that particular driver.  But given that we have some
history from the same debate with dma-buf, I think it is pretty easy
to infer the result from making fence EXPORT_SYMBOL_GPL().

BR,
-R

> thanks,
>
> greg k-h

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Rob Clark]

On Thu, Jun 19, 2014 at 1:00 PM, Greg KH  wrote:
> On Thu, Jun 19, 2014 at 10:00:18AM -0400, Rob Clark wrote:
>> On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  
>> wrote:
>> > On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>> >> +#define CREATE_TRACE_POINTS
>> >> +#include 
>> >> +
>> >> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
>> >> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
>> >
>> > Are you really willing to live with these as tracepoints for forever?
>> > What is the use of them in debugging?  Was it just for debugging the
>> > fence code, or for something else?
>> >
>> >> +/**
>> >> + * fence_context_alloc - allocate an array of fence contexts
>> >> + * @num: [in]amount of contexts to allocate
>> >> + *
>> >> + * This function will return the first index of the number of fences 
>> >> allocated.
>> >> + * The fence context is used for setting fence->context to a unique 
>> >> number.
>> >> + */
>> >> +unsigned fence_context_alloc(unsigned num)
>> >> +{
>> >> + BUG_ON(!num);
>> >> + return atomic_add_return(num, _context_counter) - num;
>> >> +}
>> >> +EXPORT_SYMBOL(fence_context_alloc);
>> >
>> > EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
>> > Traditionally all of the driver core exports have been with this
>> > marking, any objection to making that change here as well?
>>
>> tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
>> wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
>> life.  We already went through this debate once with dma-buf.  We
>> aren't going to change $evil_vendor's mind about non-gpl modules.  The
>> only result will be a more flugly convoluted solution (ie. use syncpt
>> EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
>> workaround, with the result that no-one benefits.
>
> It has been proven that using _GPL() exports have caused companies to
> release their code "properly" over the years, so as these really are
> Linux-only apis, please change them to be marked this way, it helps
> everyone out in the end.

Well, maybe that is the true in some cases.  But it certainly didn't
work out that way for dma-buf.  And I think the end result is worse.

I don't really like coming down on the side of EXPORT_SYMBOL() instead
of EXPORT_SYMBOL_GPL(), but if we do use EXPORT_SYMBOL_GPL() then the
result will only be creative workarounds using the _GPL symbols
indirectly by whatever is available via EXPORT_SYMBOL().  I don't
really see how that will be better.

>> >> +int __fence_signal(struct fence *fence)
>> >> +{
>> >> + struct fence_cb *cur, *tmp;
>> >> + int ret = 0;
>> >> +
>> >> + if (WARN_ON(!fence))
>> >> + return -EINVAL;
>> >> +
>> >> + if (!ktime_to_ns(fence->timestamp)) {
>> >> + fence->timestamp = ktime_get();
>> >> + smp_mb__before_atomic();
>> >> + }
>> >> +
>> >> + if (test_and_set_bit(FENCE_FLAG_SIGNALED_BIT, >flags)) {
>> >> + ret = -EINVAL;
>> >> +
>> >> + /*
>> >> +  * we might have raced with the unlocked fence_signal,
>> >> +  * still run through all callbacks
>> >> +  */
>> >> + } else
>> >> + trace_fence_signaled(fence);
>> >> +
>> >> + list_for_each_entry_safe(cur, tmp, >cb_list, node) {
>> >> + list_del_init(>node);
>> >> + cur->func(fence, cur);
>> >> + }
>> >> + return ret;
>> >> +}
>> >> +EXPORT_SYMBOL(__fence_signal);
>> >
>> > Don't export a function with __ in front of it, you are saying that an
>> > internal function is somehow "valid" for everyone else to call?  Why?
>> > You aren't even documenting the function, so who knows how to use it?
>>
>> fwiw, the __ versions appear to mainly be concessions for android
>> syncpt.  That is the only user outside of fence.c, and it should stay
>> that way.
>
> How are you going to "ensure" this?  And where did you document it?
> Please fix this up, it's a horrid way to create a new api.
>
> If the android code needs to be fixed to fit into this model, then fix
> it.

heh, and in fact I was wrong about this.. the __ versions are actually
for when the lock is already held.  Maarten needs to rename (ie
_locked suffix) and add some API docs for this.

>> >> +void
>> >> +__fence_init(struct fence *fence, const struct fence_ops *ops,
>> >> +  spinlock_t *lock, unsigned context, unsigned seqno)
>> >> +{
>> >> + BUG_ON(!lock);
>> >> + BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
>> >> +!ops->get_driver_name || !ops->get_timeline_name);
>> >> +
>> >> + kref_init(>refcount);
>> >> + fence->ops = ops;
>> >> + INIT_LIST_HEAD(>cb_list);
>> >> + fence->lock = lock;
>> >> + fence->context = context;
>> >> + fence->seqno = seqno;
>> >> + fence->flags = 0UL;
>> >> +
>> >> + trace_fence_init(fence);
>> >> +}
>> >> +EXPORT_SYMBOL(__fence_init);
>> >
>> > Again with the __ exported function...

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Thu, Jun 19, 2014 at 09:15:36PM +0200, Daniel Vetter wrote:
> On Thu, Jun 19, 2014 at 7:00 PM, Greg KH  
> wrote:
> >> >> + BUG_ON(f1->context != f2->context);
> >> >
> >> > Nice, you just crashed the kernel, making it impossible to debug or
> >> > recover :(
> >>
> >> agreed, that should probably be 'if (WARN_ON(...)) return NULL;'
> >>
> >> (but at least I wouldn't expect to hit that under console_lock so you
> >> should at least see the last N lines of the backtrace on the screen
> >> ;-))
> >
> > Lots of devices don't have console screens :)
> 
> Aside: This is a pet peeve of mine and recently I've switched to
> rejecting all patch that have a BUG_ON, period.

Please do, I have been for a few years now as well for the same reasons
you cite.

greg k-h

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Eric Boxer]

Eric Boxer liked your message with Boxer. On June 19, 2014 at 12:45:30 PM CDT, 
Rob Clark  wrote: 
-- next part --
An HTML attachment was scrubbed...
URL: 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[James Bottomley]

On Thu, 2014-06-19 at 11:19 -0700, Greg KH wrote:
> On Thu, Jun 19, 2014 at 01:45:30PM -0400, Rob Clark wrote:
> > On Thu, Jun 19, 2014 at 1:00 PM, Greg KH  
> > wrote:
> > > On Thu, Jun 19, 2014 at 10:00:18AM -0400, Rob Clark wrote:
> > >> On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  
> > >> wrote:
> > >> > On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
> > >> >> +#define CREATE_TRACE_POINTS
> > >> >> +#include 
> > >> >> +
> > >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
> > >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
> > >> >
> > >> > Are you really willing to live with these as tracepoints for forever?
> > >> > What is the use of them in debugging?  Was it just for debugging the
> > >> > fence code, or for something else?
> > >> >
> > >> >> +/**
> > >> >> + * fence_context_alloc - allocate an array of fence contexts
> > >> >> + * @num: [in]amount of contexts to allocate
> > >> >> + *
> > >> >> + * This function will return the first index of the number of fences 
> > >> >> allocated.
> > >> >> + * The fence context is used for setting fence->context to a unique 
> > >> >> number.
> > >> >> + */
> > >> >> +unsigned fence_context_alloc(unsigned num)
> > >> >> +{
> > >> >> + BUG_ON(!num);
> > >> >> + return atomic_add_return(num, _context_counter) - num;
> > >> >> +}
> > >> >> +EXPORT_SYMBOL(fence_context_alloc);
> > >> >
> > >> > EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
> > >> > Traditionally all of the driver core exports have been with this
> > >> > marking, any objection to making that change here as well?
> > >>
> > >> tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
> > >> wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
> > >> life.  We already went through this debate once with dma-buf.  We
> > >> aren't going to change $evil_vendor's mind about non-gpl modules.  The
> > >> only result will be a more flugly convoluted solution (ie. use syncpt
> > >> EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
> > >> workaround, with the result that no-one benefits.
> > >
> > > It has been proven that using _GPL() exports have caused companies to
> > > release their code "properly" over the years, so as these really are
> > > Linux-only apis, please change them to be marked this way, it helps
> > > everyone out in the end.
> > 
> > Well, maybe that is the true in some cases.  But it certainly didn't
> > work out that way for dma-buf.  And I think the end result is worse.
> > 
> > I don't really like coming down on the side of EXPORT_SYMBOL() instead
> > of EXPORT_SYMBOL_GPL(), but if we do use EXPORT_SYMBOL_GPL() then the
> > result will only be creative workarounds using the _GPL symbols
> > indirectly by whatever is available via EXPORT_SYMBOL().  I don't
> > really see how that will be better.
> 
> You are saying that you _know_ companies will violate our license, so
> you should just "give up"?  And how do you know people aren't working on
> preventing those "indirect" usages as well?  :)
> 
> Sorry, I'm not going to give up here, again, it has proven to work in
> the past in changing the ways of _very_ large companies, why stop now?

When you try to train a dog, you have to be consistent about it.  We're
fantastically inconsistent in symbol exports.

For instance, the mutex primitives are all EXPORT_SYMBOL(), so we're
telling proprietary modules they can use them.  However, when the kernel
is built with CONFIG_DEBUG_MUTEX, they all become
EXPORT_SYMBOL_GPL() ... what type of crazy message is that supposed to
send?  It's OK to use mutexes but it's potentially a GPL violation to
debug them?

We either need to decide that we have a defined and consistent part of
our API that's GPL only or make the bold statement that we don't have
any part of our API that's usable by non-GPL modules.  Right at the
minute we do neither and it confuses people no end about what is and
isn't allowed.

James

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Thu, Jun 19, 2014 at 01:45:30PM -0400, Rob Clark wrote:
> On Thu, Jun 19, 2014 at 1:00 PM, Greg KH  
> wrote:
> > On Thu, Jun 19, 2014 at 10:00:18AM -0400, Rob Clark wrote:
> >> On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  
> >> wrote:
> >> > On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
> >> >> +#define CREATE_TRACE_POINTS
> >> >> +#include 
> >> >> +
> >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
> >> >> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
> >> >
> >> > Are you really willing to live with these as tracepoints for forever?
> >> > What is the use of them in debugging?  Was it just for debugging the
> >> > fence code, or for something else?
> >> >
> >> >> +/**
> >> >> + * fence_context_alloc - allocate an array of fence contexts
> >> >> + * @num: [in]amount of contexts to allocate
> >> >> + *
> >> >> + * This function will return the first index of the number of fences 
> >> >> allocated.
> >> >> + * The fence context is used for setting fence->context to a unique 
> >> >> number.
> >> >> + */
> >> >> +unsigned fence_context_alloc(unsigned num)
> >> >> +{
> >> >> + BUG_ON(!num);
> >> >> + return atomic_add_return(num, _context_counter) - num;
> >> >> +}
> >> >> +EXPORT_SYMBOL(fence_context_alloc);
> >> >
> >> > EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
> >> > Traditionally all of the driver core exports have been with this
> >> > marking, any objection to making that change here as well?
> >>
> >> tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
> >> wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
> >> life.  We already went through this debate once with dma-buf.  We
> >> aren't going to change $evil_vendor's mind about non-gpl modules.  The
> >> only result will be a more flugly convoluted solution (ie. use syncpt
> >> EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
> >> workaround, with the result that no-one benefits.
> >
> > It has been proven that using _GPL() exports have caused companies to
> > release their code "properly" over the years, so as these really are
> > Linux-only apis, please change them to be marked this way, it helps
> > everyone out in the end.
> 
> Well, maybe that is the true in some cases.  But it certainly didn't
> work out that way for dma-buf.  And I think the end result is worse.
> 
> I don't really like coming down on the side of EXPORT_SYMBOL() instead
> of EXPORT_SYMBOL_GPL(), but if we do use EXPORT_SYMBOL_GPL() then the
> result will only be creative workarounds using the _GPL symbols
> indirectly by whatever is available via EXPORT_SYMBOL().  I don't
> really see how that will be better.

You are saying that you _know_ companies will violate our license, so
you should just "give up"?  And how do you know people aren't working on
preventing those "indirect" usages as well?  :)

Sorry, I'm not going to give up here, again, it has proven to work in
the past in changing the ways of _very_ large companies, why stop now?

thanks,

greg k-h

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Sumit Semwal]

On 19 June 2014 10:24, Greg KH  wrote:
> On Thu, Jun 19, 2014 at 09:57:35AM +0530, Sumit Semwal wrote:
>> Hi Greg,
>>

>> >>
>> >> Who is going to sign up to maintain this code?  (hint, it's not me...)
>> >
>> > that would be Sumit (dma-buf tree)..
>> >
>> > probably we should move fence/reservation/dma-buf into drivers/dma-buf
>> > (or something approximately like that)
>> Yes, that would be me - it might be better to create a new directory
>> as suggested above (drivers/dma-buf).
>
> That's fine with me, there is going to be more than just one file in
> there, right?  :)
>
> greg k-h
Certainly atleast 3 :)

~sumit

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Thu, Jun 19, 2014 at 10:00:18AM -0400, Rob Clark wrote:
> On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  
> wrote:
> > On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
> >> +#define CREATE_TRACE_POINTS
> >> +#include 
> >> +
> >> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
> >> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
> >
> > Are you really willing to live with these as tracepoints for forever?
> > What is the use of them in debugging?  Was it just for debugging the
> > fence code, or for something else?
> >
> >> +/**
> >> + * fence_context_alloc - allocate an array of fence contexts
> >> + * @num: [in]amount of contexts to allocate
> >> + *
> >> + * This function will return the first index of the number of fences 
> >> allocated.
> >> + * The fence context is used for setting fence->context to a unique 
> >> number.
> >> + */
> >> +unsigned fence_context_alloc(unsigned num)
> >> +{
> >> + BUG_ON(!num);
> >> + return atomic_add_return(num, _context_counter) - num;
> >> +}
> >> +EXPORT_SYMBOL(fence_context_alloc);
> >
> > EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
> > Traditionally all of the driver core exports have been with this
> > marking, any objection to making that change here as well?
> 
> tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
> wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
> life.  We already went through this debate once with dma-buf.  We
> aren't going to change $evil_vendor's mind about non-gpl modules.  The
> only result will be a more flugly convoluted solution (ie. use syncpt
> EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
> workaround, with the result that no-one benefits.

It has been proven that using _GPL() exports have caused companies to
release their code "properly" over the years, so as these really are
Linux-only apis, please change them to be marked this way, it helps
everyone out in the end.

> >> +int __fence_signal(struct fence *fence)
> >> +{
> >> + struct fence_cb *cur, *tmp;
> >> + int ret = 0;
> >> +
> >> + if (WARN_ON(!fence))
> >> + return -EINVAL;
> >> +
> >> + if (!ktime_to_ns(fence->timestamp)) {
> >> + fence->timestamp = ktime_get();
> >> + smp_mb__before_atomic();
> >> + }
> >> +
> >> + if (test_and_set_bit(FENCE_FLAG_SIGNALED_BIT, >flags)) {
> >> + ret = -EINVAL;
> >> +
> >> + /*
> >> +  * we might have raced with the unlocked fence_signal,
> >> +  * still run through all callbacks
> >> +  */
> >> + } else
> >> + trace_fence_signaled(fence);
> >> +
> >> + list_for_each_entry_safe(cur, tmp, >cb_list, node) {
> >> + list_del_init(>node);
> >> + cur->func(fence, cur);
> >> + }
> >> + return ret;
> >> +}
> >> +EXPORT_SYMBOL(__fence_signal);
> >
> > Don't export a function with __ in front of it, you are saying that an
> > internal function is somehow "valid" for everyone else to call?  Why?
> > You aren't even documenting the function, so who knows how to use it?
> 
> fwiw, the __ versions appear to mainly be concessions for android
> syncpt.  That is the only user outside of fence.c, and it should stay
> that way.

How are you going to "ensure" this?  And where did you document it?
Please fix this up, it's a horrid way to create a new api.

If the android code needs to be fixed to fit into this model, then fix
it.

> >> +void
> >> +__fence_init(struct fence *fence, const struct fence_ops *ops,
> >> +  spinlock_t *lock, unsigned context, unsigned seqno)
> >> +{
> >> + BUG_ON(!lock);
> >> + BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
> >> +!ops->get_driver_name || !ops->get_timeline_name);
> >> +
> >> + kref_init(>refcount);
> >> + fence->ops = ops;
> >> + INIT_LIST_HEAD(>cb_list);
> >> + fence->lock = lock;
> >> + fence->context = context;
> >> + fence->seqno = seqno;
> >> + fence->flags = 0UL;
> >> +
> >> + trace_fence_init(fence);
> >> +}
> >> +EXPORT_SYMBOL(__fence_init);
> >
> > Again with the __ exported function...
> >
> > I don't even see a fence_init() function anywhere, why the __ ?
> >
> 
> think of it as a 'protected' constructor.. only the derived fence
> subclass should call.

Where do you say this?  Again, not a good reason, fix up the api please.

> >> + kref_get(>refcount);
> >> +}
> >
> > Why is this inline?
> 
> performance can be critical.. especially if the driver is using this
> fence mechanism for internal buffers as well as shared buffers (which
> is what I'd like to do to avoid having to deal with two different
> fencing mechanisms for shared vs non-shared buffers), since you could
> easily have 100's or perhaps 1000's of buffers involved in a submit.

"can be".  Did you actually measure it?  Please do so.

> The fence stuff does try to inline as much stuff as possible,
> 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Rob Clark]

On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  wrote:
> On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>> +#define CREATE_TRACE_POINTS
>> +#include 
>> +
>> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
>> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
>
> Are you really willing to live with these as tracepoints for forever?
> What is the use of them in debugging?  Was it just for debugging the
> fence code, or for something else?
>
>> +/**
>> + * fence_context_alloc - allocate an array of fence contexts
>> + * @num: [in]amount of contexts to allocate
>> + *
>> + * This function will return the first index of the number of fences 
>> allocated.
>> + * The fence context is used for setting fence->context to a unique number.
>> + */
>> +unsigned fence_context_alloc(unsigned num)
>> +{
>> + BUG_ON(!num);
>> + return atomic_add_return(num, _context_counter) - num;
>> +}
>> +EXPORT_SYMBOL(fence_context_alloc);
>
> EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
> Traditionally all of the driver core exports have been with this
> marking, any objection to making that change here as well?

tbh, I prefer EXPORT_SYMBOL()..  well, I'd prefer even more if there
wasn't even a need for EXPORT_SYMBOL_GPL(), but sadly it is a fact of
life.  We already went through this debate once with dma-buf.  We
aren't going to change $evil_vendor's mind about non-gpl modules.  The
only result will be a more flugly convoluted solution (ie. use syncpt
EXPORT_SYMBOL() on top of fence EXPORT_SYMBOL_GPL()) just as a
workaround, with the result that no-one benefits.

>> +int __fence_signal(struct fence *fence)
>> +{
>> + struct fence_cb *cur, *tmp;
>> + int ret = 0;
>> +
>> + if (WARN_ON(!fence))
>> + return -EINVAL;
>> +
>> + if (!ktime_to_ns(fence->timestamp)) {
>> + fence->timestamp = ktime_get();
>> + smp_mb__before_atomic();
>> + }
>> +
>> + if (test_and_set_bit(FENCE_FLAG_SIGNALED_BIT, >flags)) {
>> + ret = -EINVAL;
>> +
>> + /*
>> +  * we might have raced with the unlocked fence_signal,
>> +  * still run through all callbacks
>> +  */
>> + } else
>> + trace_fence_signaled(fence);
>> +
>> + list_for_each_entry_safe(cur, tmp, >cb_list, node) {
>> + list_del_init(>node);
>> + cur->func(fence, cur);
>> + }
>> + return ret;
>> +}
>> +EXPORT_SYMBOL(__fence_signal);
>
> Don't export a function with __ in front of it, you are saying that an
> internal function is somehow "valid" for everyone else to call?  Why?
> You aren't even documenting the function, so who knows how to use it?

fwiw, the __ versions appear to mainly be concessions for android
syncpt.  That is the only user outside of fence.c, and it should stay
that way.

>> +/**
>> + * fence_signal - signal completion of a fence
>> + * @fence: the fence to signal
>> + *
>> + * Signal completion for software callbacks on a fence, this will unblock
>> + * fence_wait() calls and run all the callbacks added with
>> + * fence_add_callback(). Can be called multiple times, but since a fence
>> + * can only go from unsignaled to signaled state, it will only be effective
>> + * the first time.
>> + */
>> +int fence_signal(struct fence *fence)
>> +{
>> + unsigned long flags;
>> +
>> + if (!fence)
>> + return -EINVAL;
>> +
>> + if (!ktime_to_ns(fence->timestamp)) {
>> + fence->timestamp = ktime_get();
>> + smp_mb__before_atomic();
>> + }
>> +
>> + if (test_and_set_bit(FENCE_FLAG_SIGNALED_BIT, >flags))
>> + return -EINVAL;
>> +
>> + trace_fence_signaled(fence);
>> +
>> + if (test_bit(FENCE_FLAG_ENABLE_SIGNAL_BIT, >flags)) {
>> + struct fence_cb *cur, *tmp;
>> +
>> + spin_lock_irqsave(fence->lock, flags);
>> + list_for_each_entry_safe(cur, tmp, >cb_list, node) {
>> + list_del_init(>node);
>> + cur->func(fence, cur);
>> + }
>> + spin_unlock_irqrestore(fence->lock, flags);
>> + }
>> + return 0;
>> +}
>> +EXPORT_SYMBOL(fence_signal);
>
> So, why should I use this over __fence_signal?  What is the difference?
> Am I missing some documentation somewhere?
>
>> +void release_fence(struct kref *kref)
>> +{
>> + struct fence *fence =
>> + container_of(kref, struct fence, refcount);
>> +
>> + trace_fence_destroy(fence);
>> +
>> + BUG_ON(!list_empty(>cb_list));
>> +
>> + if (fence->ops->release)
>> + fence->ops->release(fence);
>> + else
>> + kfree(fence);
>> +}
>> +EXPORT_SYMBOL(release_fence);
>
> fence_release() makes it more unified with the other functions in this
> file, right?
>
>> +/**
>> + * fence_default_wait - default sleep until the fence gets signaled
>> + * or until timeout elapses
>> + * @fence:   [in]the fence to wait on
>> + * 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Sumit Semwal]

Hi Greg,

On 19 June 2014 06:55, Rob Clark  wrote:
> On Wed, Jun 18, 2014 at 9:16 PM, Greg KH  
> wrote:
>> On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>>> A fence can be attached to a buffer which is being filled or consumed
>>> by hw, to allow userspace to pass the buffer without waiting to another
>>> device.  For example, userspace can call page_flip ioctl to display the
>>> next frame of graphics after kicking the GPU but while the GPU is still
>>> rendering.  The display device sharing the buffer with the GPU would
>>> attach a callback to get notified when the GPU's rendering-complete IRQ
>>> fires, to update the scan-out address of the display, without having to
>>> wake up userspace.
>>>
>>> A driver must allocate a fence context for each execution ring that can
>>> run in parallel. The function for this takes an argument with how many
>>> contexts to allocate:
>>>   + fence_context_alloc()
>>>
>>> A fence is transient, one-shot deal.  It is allocated and attached
>>> to one or more dma-buf's.  When the one that attached it is done, with
>>> the pending operation, it can signal the fence:
>>>   + fence_signal()
>>>
>>> To have a rough approximation whether a fence is fired, call:
>>>   + fence_is_signaled()
>>>
>>> The dma-buf-mgr handles tracking, and waiting on, the fences associated
>>> with a dma-buf.
>>>
>>> The one pending on the fence can add an async callback:
>>>   + fence_add_callback()
>>>
>>> The callback can optionally be cancelled with:
>>>   + fence_remove_callback()
>>>
>>> To wait synchronously, optionally with a timeout:
>>>   + fence_wait()
>>>   + fence_wait_timeout()
>>>
>>> When emitting a fence, call:
>>>   + trace_fence_emit()
>>>
>>> To annotate that a fence is blocking on another fence, call:
>>>   + trace_fence_annotate_wait_on(fence, on_fence)
>>>
>>> A default software-only implementation is provided, which can be used
>>> by drivers attaching a fence to a buffer when they have no other means
>>> for hw sync.  But a memory backed fence is also envisioned, because it
>>> is common that GPU's can write to, or poll on some memory location for
>>> synchronization.  For example:
>>>
>>>   fence = custom_get_fence(...);
>>>   if ((seqno_fence = to_seqno_fence(fence)) != NULL) {
>>> dma_buf *fence_buf = seqno_fence->sync_buf;
>>> get_dma_buf(fence_buf);
>>>
>>> ... tell the hw the memory location to wait ...
>>> custom_wait_on(fence_buf, seqno_fence->seqno_ofs, fence->seqno);
>>>   } else {
>>> /* fall-back to sw sync * /
>>> fence_add_callback(fence, my_cb);
>>>   }
>>>
>>> On SoC platforms, if some other hw mechanism is provided for synchronizing
>>> between IP blocks, it could be supported as an alternate implementation
>>> with it's own fence ops in a similar way.
>>>
>>> enable_signaling callback is used to provide sw signaling in case a cpu
>>> waiter is requested or no compatible hardware signaling could be used.
>>>
>>> The intention is to provide a userspace interface (presumably via eventfd)
>>> later, to be used in conjunction with dma-buf's mmap support for sw access
>>> to buffers (or for userspace apps that would prefer to do their own
>>> synchronization).
>>>
>>> v1: Original
>>> v2: After discussion w/ danvet and mlankhorst on #dri-devel, we decided
>>> that dma-fence didn't need to care about the sw->hw signaling path
>>> (it can be handled same as sw->sw case), and therefore the fence->ops
>>> can be simplified and more handled in the core.  So remove the signal,
>>> add_callback, cancel_callback, and wait ops, and replace with a simple
>>> enable_signaling() op which can be used to inform a fence supporting
>>> hw->hw signaling that one or more devices which do not support hw
>>> signaling are waiting (and therefore it should enable an irq or do
>>> whatever is necessary in order that the CPU is notified when the
>>> fence is passed).
>>> v3: Fix locking fail in attach_fence() and get_fence()
>>> v4: Remove tie-in w/ dma-buf..  after discussion w/ danvet and mlankorst
>>> we decided that we need to be able to attach one fence to N dma-buf's,
>>> so using the list_head in dma-fence struct would be problematic.
>>> v5: [ Maarten Lankhorst ] Updated for dma-bikeshed-fence and 
>>> dma-buf-manager.
>>> v6: [ Maarten Lankhorst ] I removed dma_fence_cancel_callback and some 
>>> comments
>>> about checking if fence fired or not. This is broken by design.
>>> waitqueue_active during destruction is now fatal, since the signaller
>>> should be holding a reference in enable_signalling until it signalled
>>> the fence. Pass the original dma_fence_cb along, and call __remove_wait
>>> in the dma_fence_callback handler, so that no cleanup needs to be
>>> performed.
>>> v7: [ Maarten Lankhorst ] Set cb->func and only enable sw signaling if
>>> fence wasn't signaled yet, for example for hardware fences that may
>>> choose to signal blindly.
>>> v8: [ 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Thu, Jun 19, 2014 at 09:57:35AM +0530, Sumit Semwal wrote:
> Hi Greg,
> 
> On 19 June 2014 06:55, Rob Clark  wrote:
> > On Wed, Jun 18, 2014 at 9:16 PM, Greg KH  
> > wrote:
> >> On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
> >>> A fence can be attached to a buffer which is being filled or consumed
> >>> by hw, to allow userspace to pass the buffer without waiting to another
> >>> device.  For example, userspace can call page_flip ioctl to display the
> >>> next frame of graphics after kicking the GPU but while the GPU is still
> >>> rendering.  The display device sharing the buffer with the GPU would
> >>> attach a callback to get notified when the GPU's rendering-complete IRQ
> >>> fires, to update the scan-out address of the display, without having to
> >>> wake up userspace.
> >>>
> >>> A driver must allocate a fence context for each execution ring that can
> >>> run in parallel. The function for this takes an argument with how many
> >>> contexts to allocate:
> >>>   + fence_context_alloc()
> >>>
> >>> A fence is transient, one-shot deal.  It is allocated and attached
> >>> to one or more dma-buf's.  When the one that attached it is done, with
> >>> the pending operation, it can signal the fence:
> >>>   + fence_signal()
> >>>
> >>> To have a rough approximation whether a fence is fired, call:
> >>>   + fence_is_signaled()
> >>>
> >>> The dma-buf-mgr handles tracking, and waiting on, the fences associated
> >>> with a dma-buf.
> >>>
> >>> The one pending on the fence can add an async callback:
> >>>   + fence_add_callback()
> >>>
> >>> The callback can optionally be cancelled with:
> >>>   + fence_remove_callback()
> >>>
> >>> To wait synchronously, optionally with a timeout:
> >>>   + fence_wait()
> >>>   + fence_wait_timeout()
> >>>
> >>> When emitting a fence, call:
> >>>   + trace_fence_emit()
> >>>
> >>> To annotate that a fence is blocking on another fence, call:
> >>>   + trace_fence_annotate_wait_on(fence, on_fence)
> >>>
> >>> A default software-only implementation is provided, which can be used
> >>> by drivers attaching a fence to a buffer when they have no other means
> >>> for hw sync.  But a memory backed fence is also envisioned, because it
> >>> is common that GPU's can write to, or poll on some memory location for
> >>> synchronization.  For example:
> >>>
> >>>   fence = custom_get_fence(...);
> >>>   if ((seqno_fence = to_seqno_fence(fence)) != NULL) {
> >>> dma_buf *fence_buf = seqno_fence->sync_buf;
> >>> get_dma_buf(fence_buf);
> >>>
> >>> ... tell the hw the memory location to wait ...
> >>> custom_wait_on(fence_buf, seqno_fence->seqno_ofs, fence->seqno);
> >>>   } else {
> >>> /* fall-back to sw sync * /
> >>> fence_add_callback(fence, my_cb);
> >>>   }
> >>>
> >>> On SoC platforms, if some other hw mechanism is provided for synchronizing
> >>> between IP blocks, it could be supported as an alternate implementation
> >>> with it's own fence ops in a similar way.
> >>>
> >>> enable_signaling callback is used to provide sw signaling in case a cpu
> >>> waiter is requested or no compatible hardware signaling could be used.
> >>>
> >>> The intention is to provide a userspace interface (presumably via eventfd)
> >>> later, to be used in conjunction with dma-buf's mmap support for sw access
> >>> to buffers (or for userspace apps that would prefer to do their own
> >>> synchronization).
> >>>
> >>> v1: Original
> >>> v2: After discussion w/ danvet and mlankhorst on #dri-devel, we decided
> >>> that dma-fence didn't need to care about the sw->hw signaling path
> >>> (it can be handled same as sw->sw case), and therefore the fence->ops
> >>> can be simplified and more handled in the core.  So remove the signal,
> >>> add_callback, cancel_callback, and wait ops, and replace with a simple
> >>> enable_signaling() op which can be used to inform a fence supporting
> >>> hw->hw signaling that one or more devices which do not support hw
> >>> signaling are waiting (and therefore it should enable an irq or do
> >>> whatever is necessary in order that the CPU is notified when the
> >>> fence is passed).
> >>> v3: Fix locking fail in attach_fence() and get_fence()
> >>> v4: Remove tie-in w/ dma-buf..  after discussion w/ danvet and mlankorst
> >>> we decided that we need to be able to attach one fence to N dma-buf's,
> >>> so using the list_head in dma-fence struct would be problematic.
> >>> v5: [ Maarten Lankhorst ] Updated for dma-bikeshed-fence and 
> >>> dma-buf-manager.
> >>> v6: [ Maarten Lankhorst ] I removed dma_fence_cancel_callback and some 
> >>> comments
> >>> about checking if fence fired or not. This is broken by design.
> >>> waitqueue_active during destruction is now fatal, since the signaller
> >>> should be holding a reference in enable_signalling until it signalled
> >>> the fence. Pass the original dma_fence_cb along, and call 
> >>> __remove_wait
> >>> in the 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Rob Clark]

On Wed, Jun 18, 2014 at 9:16 PM, Greg KH  wrote:
> On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>> A fence can be attached to a buffer which is being filled or consumed
>> by hw, to allow userspace to pass the buffer without waiting to another
>> device.  For example, userspace can call page_flip ioctl to display the
>> next frame of graphics after kicking the GPU but while the GPU is still
>> rendering.  The display device sharing the buffer with the GPU would
>> attach a callback to get notified when the GPU's rendering-complete IRQ
>> fires, to update the scan-out address of the display, without having to
>> wake up userspace.
>>
>> A driver must allocate a fence context for each execution ring that can
>> run in parallel. The function for this takes an argument with how many
>> contexts to allocate:
>>   + fence_context_alloc()
>>
>> A fence is transient, one-shot deal.  It is allocated and attached
>> to one or more dma-buf's.  When the one that attached it is done, with
>> the pending operation, it can signal the fence:
>>   + fence_signal()
>>
>> To have a rough approximation whether a fence is fired, call:
>>   + fence_is_signaled()
>>
>> The dma-buf-mgr handles tracking, and waiting on, the fences associated
>> with a dma-buf.
>>
>> The one pending on the fence can add an async callback:
>>   + fence_add_callback()
>>
>> The callback can optionally be cancelled with:
>>   + fence_remove_callback()
>>
>> To wait synchronously, optionally with a timeout:
>>   + fence_wait()
>>   + fence_wait_timeout()
>>
>> When emitting a fence, call:
>>   + trace_fence_emit()
>>
>> To annotate that a fence is blocking on another fence, call:
>>   + trace_fence_annotate_wait_on(fence, on_fence)
>>
>> A default software-only implementation is provided, which can be used
>> by drivers attaching a fence to a buffer when they have no other means
>> for hw sync.  But a memory backed fence is also envisioned, because it
>> is common that GPU's can write to, or poll on some memory location for
>> synchronization.  For example:
>>
>>   fence = custom_get_fence(...);
>>   if ((seqno_fence = to_seqno_fence(fence)) != NULL) {
>> dma_buf *fence_buf = seqno_fence->sync_buf;
>> get_dma_buf(fence_buf);
>>
>> ... tell the hw the memory location to wait ...
>> custom_wait_on(fence_buf, seqno_fence->seqno_ofs, fence->seqno);
>>   } else {
>> /* fall-back to sw sync * /
>> fence_add_callback(fence, my_cb);
>>   }
>>
>> On SoC platforms, if some other hw mechanism is provided for synchronizing
>> between IP blocks, it could be supported as an alternate implementation
>> with it's own fence ops in a similar way.
>>
>> enable_signaling callback is used to provide sw signaling in case a cpu
>> waiter is requested or no compatible hardware signaling could be used.
>>
>> The intention is to provide a userspace interface (presumably via eventfd)
>> later, to be used in conjunction with dma-buf's mmap support for sw access
>> to buffers (or for userspace apps that would prefer to do their own
>> synchronization).
>>
>> v1: Original
>> v2: After discussion w/ danvet and mlankhorst on #dri-devel, we decided
>> that dma-fence didn't need to care about the sw->hw signaling path
>> (it can be handled same as sw->sw case), and therefore the fence->ops
>> can be simplified and more handled in the core.  So remove the signal,
>> add_callback, cancel_callback, and wait ops, and replace with a simple
>> enable_signaling() op which can be used to inform a fence supporting
>> hw->hw signaling that one or more devices which do not support hw
>> signaling are waiting (and therefore it should enable an irq or do
>> whatever is necessary in order that the CPU is notified when the
>> fence is passed).
>> v3: Fix locking fail in attach_fence() and get_fence()
>> v4: Remove tie-in w/ dma-buf..  after discussion w/ danvet and mlankorst
>> we decided that we need to be able to attach one fence to N dma-buf's,
>> so using the list_head in dma-fence struct would be problematic.
>> v5: [ Maarten Lankhorst ] Updated for dma-bikeshed-fence and dma-buf-manager.
>> v6: [ Maarten Lankhorst ] I removed dma_fence_cancel_callback and some 
>> comments
>> about checking if fence fired or not. This is broken by design.
>> waitqueue_active during destruction is now fatal, since the signaller
>> should be holding a reference in enable_signalling until it signalled
>> the fence. Pass the original dma_fence_cb along, and call __remove_wait
>> in the dma_fence_callback handler, so that no cleanup needs to be
>> performed.
>> v7: [ Maarten Lankhorst ] Set cb->func and only enable sw signaling if
>> fence wasn't signaled yet, for example for hardware fences that may
>> choose to signal blindly.
>> v8: [ Maarten Lankhorst ] Tons of tiny fixes, moved __dma_fence_init to
>> header and fixed include mess. dma-fence.h now includes dma-buf.h
>> All members 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Rob Clark]

On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  wrote:
> On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>> +#define CREATE_TRACE_POINTS
>> +#include 
>> +
>> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
>> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
>
> Are you really willing to live with these as tracepoints for forever?
> What is the use of them in debugging?  Was it just for debugging the
> fence code, or for something else?

fwiw, the goal is something like this:

http://people.freedesktop.org/~robclark/perf-supertuxkart.svg

but without needing to make perf understand each driver's custom trace events

(from: 
http://bloggingthemonkey.blogspot.com/2013/09/freedreno-update-moar-fps.html
)

BR,
-R


>> +/**
>> + * fence_context_alloc - allocate an array of fence contexts
>> + * @num: [in]amount of contexts to allocate
>> + *
>> + * This function will return the first index of the number of fences 
>> allocated.
>> + * The fence context is used for setting fence->context to a unique number.
>> + */
>> +unsigned fence_context_alloc(unsigned num)
>> +{
>> + BUG_ON(!num);
>> + return atomic_add_return(num, _context_counter) - num;
>> +}
>> +EXPORT_SYMBOL(fence_context_alloc);
>
> EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
> Traditionally all of the driver core exports have been with this
> marking, any objection to making that change here as well?
>
>> +int __fence_signal(struct fence *fence)
>> +{
>> + struct fence_cb *cur, *tmp;
>> + int ret = 0;
>> +
>> + if (WARN_ON(!fence))
>> + return -EINVAL;
>> +
>> + if (!ktime_to_ns(fence->timestamp)) {
>> + fence->timestamp = ktime_get();
>> + smp_mb__before_atomic();
>> + }
>> +
>> + if (test_and_set_bit(FENCE_FLAG_SIGNALED_BIT, >flags)) {
>> + ret = -EINVAL;
>> +
>> + /*
>> +  * we might have raced with the unlocked fence_signal,
>> +  * still run through all callbacks
>> +  */
>> + } else
>> + trace_fence_signaled(fence);
>> +
>> + list_for_each_entry_safe(cur, tmp, >cb_list, node) {
>> + list_del_init(>node);
>> + cur->func(fence, cur);
>> + }
>> + return ret;
>> +}
>> +EXPORT_SYMBOL(__fence_signal);
>
> Don't export a function with __ in front of it, you are saying that an
> internal function is somehow "valid" for everyone else to call?  Why?
> You aren't even documenting the function, so who knows how to use it?
>
>> +/**
>> + * fence_signal - signal completion of a fence
>> + * @fence: the fence to signal
>> + *
>> + * Signal completion for software callbacks on a fence, this will unblock
>> + * fence_wait() calls and run all the callbacks added with
>> + * fence_add_callback(). Can be called multiple times, but since a fence
>> + * can only go from unsignaled to signaled state, it will only be effective
>> + * the first time.
>> + */
>> +int fence_signal(struct fence *fence)
>> +{
>> + unsigned long flags;
>> +
>> + if (!fence)
>> + return -EINVAL;
>> +
>> + if (!ktime_to_ns(fence->timestamp)) {
>> + fence->timestamp = ktime_get();
>> + smp_mb__before_atomic();
>> + }
>> +
>> + if (test_and_set_bit(FENCE_FLAG_SIGNALED_BIT, >flags))
>> + return -EINVAL;
>> +
>> + trace_fence_signaled(fence);
>> +
>> + if (test_bit(FENCE_FLAG_ENABLE_SIGNAL_BIT, >flags)) {
>> + struct fence_cb *cur, *tmp;
>> +
>> + spin_lock_irqsave(fence->lock, flags);
>> + list_for_each_entry_safe(cur, tmp, >cb_list, node) {
>> + list_del_init(>node);
>> + cur->func(fence, cur);
>> + }
>> + spin_unlock_irqrestore(fence->lock, flags);
>> + }
>> + return 0;
>> +}
>> +EXPORT_SYMBOL(fence_signal);
>
> So, why should I use this over __fence_signal?  What is the difference?
> Am I missing some documentation somewhere?
>
>> +void release_fence(struct kref *kref)
>> +{
>> + struct fence *fence =
>> + container_of(kref, struct fence, refcount);
>> +
>> + trace_fence_destroy(fence);
>> +
>> + BUG_ON(!list_empty(>cb_list));
>> +
>> + if (fence->ops->release)
>> + fence->ops->release(fence);
>> + else
>> + kfree(fence);
>> +}
>> +EXPORT_SYMBOL(release_fence);
>
> fence_release() makes it more unified with the other functions in this
> file, right?
>
>> +/**
>> + * fence_default_wait - default sleep until the fence gets signaled
>> + * or until timeout elapses
>> + * @fence:   [in]the fence to wait on
>> + * @intr:[in]if true, do an interruptible wait
>> + * @timeout: [in]timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
>> + *
>> + * Returns -ERESTARTSYS if interrupted, 0 if the wait timed out, or the
>> + * remaining timeout in jiffies on success.
>> + */
>> +long
>
> Shouldn't this be signed to be explicit?
>
>> 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Wed, Jun 18, 2014 at 09:23:06PM -0400, Rob Clark wrote:
> On Wed, Jun 18, 2014 at 9:13 PM, Greg KH  
> wrote:
> > On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
> >> +#define CREATE_TRACE_POINTS
> >> +#include 
> >> +
> >> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
> >> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);
> >
> > Are you really willing to live with these as tracepoints for forever?
> > What is the use of them in debugging?  Was it just for debugging the
> > fence code, or for something else?
> 
> fwiw, the goal is something like this:
> 
> http://people.freedesktop.org/~robclark/perf-supertuxkart.svg
> 
> but without needing to make perf understand each driver's custom trace events
> 
> (from: 
> http://bloggingthemonkey.blogspot.com/2013/09/freedreno-update-moar-fps.html
> )

Will these tracepoints provide something like that?  If so, great, but I
want to make sure as these now become a user/kernel ABI that you can not
break.

thanks,

greg k-h

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
> A fence can be attached to a buffer which is being filled or consumed
> by hw, to allow userspace to pass the buffer without waiting to another
> device.  For example, userspace can call page_flip ioctl to display the
> next frame of graphics after kicking the GPU but while the GPU is still
> rendering.  The display device sharing the buffer with the GPU would
> attach a callback to get notified when the GPU's rendering-complete IRQ
> fires, to update the scan-out address of the display, without having to
> wake up userspace.
> 
> A driver must allocate a fence context for each execution ring that can
> run in parallel. The function for this takes an argument with how many
> contexts to allocate:
>   + fence_context_alloc()
> 
> A fence is transient, one-shot deal.  It is allocated and attached
> to one or more dma-buf's.  When the one that attached it is done, with
> the pending operation, it can signal the fence:
>   + fence_signal()
> 
> To have a rough approximation whether a fence is fired, call:
>   + fence_is_signaled()
> 
> The dma-buf-mgr handles tracking, and waiting on, the fences associated
> with a dma-buf.
> 
> The one pending on the fence can add an async callback:
>   + fence_add_callback()
> 
> The callback can optionally be cancelled with:
>   + fence_remove_callback()
> 
> To wait synchronously, optionally with a timeout:
>   + fence_wait()
>   + fence_wait_timeout()
> 
> When emitting a fence, call:
>   + trace_fence_emit()
> 
> To annotate that a fence is blocking on another fence, call:
>   + trace_fence_annotate_wait_on(fence, on_fence)
> 
> A default software-only implementation is provided, which can be used
> by drivers attaching a fence to a buffer when they have no other means
> for hw sync.  But a memory backed fence is also envisioned, because it
> is common that GPU's can write to, or poll on some memory location for
> synchronization.  For example:
> 
>   fence = custom_get_fence(...);
>   if ((seqno_fence = to_seqno_fence(fence)) != NULL) {
> dma_buf *fence_buf = seqno_fence->sync_buf;
> get_dma_buf(fence_buf);
> 
> ... tell the hw the memory location to wait ...
> custom_wait_on(fence_buf, seqno_fence->seqno_ofs, fence->seqno);
>   } else {
> /* fall-back to sw sync * /
> fence_add_callback(fence, my_cb);
>   }
> 
> On SoC platforms, if some other hw mechanism is provided for synchronizing
> between IP blocks, it could be supported as an alternate implementation
> with it's own fence ops in a similar way.
> 
> enable_signaling callback is used to provide sw signaling in case a cpu
> waiter is requested or no compatible hardware signaling could be used.
> 
> The intention is to provide a userspace interface (presumably via eventfd)
> later, to be used in conjunction with dma-buf's mmap support for sw access
> to buffers (or for userspace apps that would prefer to do their own
> synchronization).
> 
> v1: Original
> v2: After discussion w/ danvet and mlankhorst on #dri-devel, we decided
> that dma-fence didn't need to care about the sw->hw signaling path
> (it can be handled same as sw->sw case), and therefore the fence->ops
> can be simplified and more handled in the core.  So remove the signal,
> add_callback, cancel_callback, and wait ops, and replace with a simple
> enable_signaling() op which can be used to inform a fence supporting
> hw->hw signaling that one or more devices which do not support hw
> signaling are waiting (and therefore it should enable an irq or do
> whatever is necessary in order that the CPU is notified when the
> fence is passed).
> v3: Fix locking fail in attach_fence() and get_fence()
> v4: Remove tie-in w/ dma-buf..  after discussion w/ danvet and mlankorst
> we decided that we need to be able to attach one fence to N dma-buf's,
> so using the list_head in dma-fence struct would be problematic.
> v5: [ Maarten Lankhorst ] Updated for dma-bikeshed-fence and dma-buf-manager.
> v6: [ Maarten Lankhorst ] I removed dma_fence_cancel_callback and some 
> comments
> about checking if fence fired or not. This is broken by design.
> waitqueue_active during destruction is now fatal, since the signaller
> should be holding a reference in enable_signalling until it signalled
> the fence. Pass the original dma_fence_cb along, and call __remove_wait
> in the dma_fence_callback handler, so that no cleanup needs to be
> performed.
> v7: [ Maarten Lankhorst ] Set cb->func and only enable sw signaling if
> fence wasn't signaled yet, for example for hardware fences that may
> choose to signal blindly.
> v8: [ Maarten Lankhorst ] Tons of tiny fixes, moved __dma_fence_init to
> header and fixed include mess. dma-fence.h now includes dma-buf.h
> All members are now initialized, so kmalloc can be used for
> allocating a dma-fence. More documentation added.
> v9: Change compiler bitfields to 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
> + * This program is distributed in the hope that it will be useful, but 
> WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.

I don't like this paragraph in all of the files, but if you insist that
some lawyer wants it there, I'll live with it...

> + * You should have received a copy of the GNU General Public License along 
> with
> + * this program.  If not, see .

That's just not needed at all and is fluff.  Please remove.

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Greg KH]

On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
> +#define CREATE_TRACE_POINTS
> +#include 
> +
> +EXPORT_TRACEPOINT_SYMBOL(fence_annotate_wait_on);
> +EXPORT_TRACEPOINT_SYMBOL(fence_emit);

Are you really willing to live with these as tracepoints for forever?
What is the use of them in debugging?  Was it just for debugging the
fence code, or for something else?

> +/**
> + * fence_context_alloc - allocate an array of fence contexts
> + * @num: [in]amount of contexts to allocate
> + *
> + * This function will return the first index of the number of fences 
> allocated.
> + * The fence context is used for setting fence->context to a unique number.
> + */
> +unsigned fence_context_alloc(unsigned num)
> +{
> + BUG_ON(!num);
> + return atomic_add_return(num, _context_counter) - num;
> +}
> +EXPORT_SYMBOL(fence_context_alloc);

EXPORT_SYMBOL_GPL()?  Same goes for all of the exports in here.
Traditionally all of the driver core exports have been with this
marking, any objection to making that change here as well?

> +int __fence_signal(struct fence *fence)
> +{
> + struct fence_cb *cur, *tmp;
> + int ret = 0;
> +
> + if (WARN_ON(!fence))
> + return -EINVAL;
> +
> + if (!ktime_to_ns(fence->timestamp)) {
> + fence->timestamp = ktime_get();
> + smp_mb__before_atomic();
> + }
> +
> + if (test_and_set_bit(FENCE_FLAG_SIGNALED_BIT, >flags)) {
> + ret = -EINVAL;
> +
> + /*
> +  * we might have raced with the unlocked fence_signal,
> +  * still run through all callbacks
> +  */
> + } else
> + trace_fence_signaled(fence);
> +
> + list_for_each_entry_safe(cur, tmp, >cb_list, node) {
> + list_del_init(>node);
> + cur->func(fence, cur);
> + }
> + return ret;
> +}
> +EXPORT_SYMBOL(__fence_signal);

Don't export a function with __ in front of it, you are saying that an
internal function is somehow "valid" for everyone else to call?  Why?
You aren't even documenting the function, so who knows how to use it?

> +/**
> + * fence_signal - signal completion of a fence
> + * @fence: the fence to signal
> + *
> + * Signal completion for software callbacks on a fence, this will unblock
> + * fence_wait() calls and run all the callbacks added with
> + * fence_add_callback(). Can be called multiple times, but since a fence
> + * can only go from unsignaled to signaled state, it will only be effective
> + * the first time.
> + */
> +int fence_signal(struct fence *fence)
> +{
> + unsigned long flags;
> +
> + if (!fence)
> + return -EINVAL;
> +
> + if (!ktime_to_ns(fence->timestamp)) {
> + fence->timestamp = ktime_get();
> + smp_mb__before_atomic();
> + }
> +
> + if (test_and_set_bit(FENCE_FLAG_SIGNALED_BIT, >flags))
> + return -EINVAL;
> +
> + trace_fence_signaled(fence);
> +
> + if (test_bit(FENCE_FLAG_ENABLE_SIGNAL_BIT, >flags)) {
> + struct fence_cb *cur, *tmp;
> +
> + spin_lock_irqsave(fence->lock, flags);
> + list_for_each_entry_safe(cur, tmp, >cb_list, node) {
> + list_del_init(>node);
> + cur->func(fence, cur);
> + }
> + spin_unlock_irqrestore(fence->lock, flags);
> + }
> + return 0;
> +}
> +EXPORT_SYMBOL(fence_signal);

So, why should I use this over __fence_signal?  What is the difference?
Am I missing some documentation somewhere?

> +void release_fence(struct kref *kref)
> +{
> + struct fence *fence =
> + container_of(kref, struct fence, refcount);
> +
> + trace_fence_destroy(fence);
> +
> + BUG_ON(!list_empty(>cb_list));
> +
> + if (fence->ops->release)
> + fence->ops->release(fence);
> + else
> + kfree(fence);
> +}
> +EXPORT_SYMBOL(release_fence);

fence_release() makes it more unified with the other functions in this
file, right?

> +/**
> + * fence_default_wait - default sleep until the fence gets signaled
> + * or until timeout elapses
> + * @fence:   [in]the fence to wait on
> + * @intr:[in]if true, do an interruptible wait
> + * @timeout: [in]timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
> + *
> + * Returns -ERESTARTSYS if interrupted, 0 if the wait timed out, or the
> + * remaining timeout in jiffies on success.
> + */
> +long

Shouldn't this be signed to be explicit?

> +fence_default_wait(struct fence *fence, bool intr, signed long timeout)
> +{
> + struct default_wait_cb cb;
> + unsigned long flags;
> + long ret = timeout;
> + bool was_set;
> +
> + if (test_bit(FENCE_FLAG_SIGNALED_BIT, >flags))
> + return timeout;
> +
> + spin_lock_irqsave(fence->lock, flags);
> +
> + if (intr && signal_pending(current)) {
> + ret = -ERESTARTSYS;
> + goto out;
> + }
> +
> + was_set = 

[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

[Maarten Lankhorst]

A fence can be attached to a buffer which is being filled or consumed
by hw, to allow userspace to pass the buffer without waiting to another
device.  For example, userspace can call page_flip ioctl to display the
next frame of graphics after kicking the GPU but while the GPU is still
rendering.  The display device sharing the buffer with the GPU would
attach a callback to get notified when the GPU's rendering-complete IRQ
fires, to update the scan-out address of the display, without having to
wake up userspace.

A driver must allocate a fence context for each execution ring that can
run in parallel. The function for this takes an argument with how many
contexts to allocate:
  + fence_context_alloc()

A fence is transient, one-shot deal.  It is allocated and attached
to one or more dma-buf's.  When the one that attached it is done, with
the pending operation, it can signal the fence:
  + fence_signal()

To have a rough approximation whether a fence is fired, call:
  + fence_is_signaled()

The dma-buf-mgr handles tracking, and waiting on, the fences associated
with a dma-buf.

The one pending on the fence can add an async callback:
  + fence_add_callback()

The callback can optionally be cancelled with:
  + fence_remove_callback()

To wait synchronously, optionally with a timeout:
  + fence_wait()
  + fence_wait_timeout()

When emitting a fence, call:
  + trace_fence_emit()

To annotate that a fence is blocking on another fence, call:
  + trace_fence_annotate_wait_on(fence, on_fence)

A default software-only implementation is provided, which can be used
by drivers attaching a fence to a buffer when they have no other means
for hw sync.  But a memory backed fence is also envisioned, because it
is common that GPU's can write to, or poll on some memory location for
synchronization.  For example:

  fence = custom_get_fence(...);
  if ((seqno_fence = to_seqno_fence(fence)) != NULL) {
dma_buf *fence_buf = seqno_fence->sync_buf;
get_dma_buf(fence_buf);

... tell the hw the memory location to wait ...
custom_wait_on(fence_buf, seqno_fence->seqno_ofs, fence->seqno);
  } else {
/* fall-back to sw sync * /
fence_add_callback(fence, my_cb);
  }

On SoC platforms, if some other hw mechanism is provided for synchronizing
between IP blocks, it could be supported as an alternate implementation
with it's own fence ops in a similar way.

enable_signaling callback is used to provide sw signaling in case a cpu
waiter is requested or no compatible hardware signaling could be used.

The intention is to provide a userspace interface (presumably via eventfd)
later, to be used in conjunction with dma-buf's mmap support for sw access
to buffers (or for userspace apps that would prefer to do their own
synchronization).

v1: Original
v2: After discussion w/ danvet and mlankhorst on #dri-devel, we decided
that dma-fence didn't need to care about the sw->hw signaling path
(it can be handled same as sw->sw case), and therefore the fence->ops
can be simplified and more handled in the core.  So remove the signal,
add_callback, cancel_callback, and wait ops, and replace with a simple
enable_signaling() op which can be used to inform a fence supporting
hw->hw signaling that one or more devices which do not support hw
signaling are waiting (and therefore it should enable an irq or do
whatever is necessary in order that the CPU is notified when the
fence is passed).
v3: Fix locking fail in attach_fence() and get_fence()
v4: Remove tie-in w/ dma-buf..  after discussion w/ danvet and mlankorst
we decided that we need to be able to attach one fence to N dma-buf's,
so using the list_head in dma-fence struct would be problematic.
v5: [ Maarten Lankhorst ] Updated for dma-bikeshed-fence and dma-buf-manager.
v6: [ Maarten Lankhorst ] I removed dma_fence_cancel_callback and some comments
about checking if fence fired or not. This is broken by design.
waitqueue_active during destruction is now fatal, since the signaller
should be holding a reference in enable_signalling until it signalled
the fence. Pass the original dma_fence_cb along, and call __remove_wait
in the dma_fence_callback handler, so that no cleanup needs to be
performed.
v7: [ Maarten Lankhorst ] Set cb->func and only enable sw signaling if
fence wasn't signaled yet, for example for hardware fences that may
choose to signal blindly.
v8: [ Maarten Lankhorst ] Tons of tiny fixes, moved __dma_fence_init to
header and fixed include mess. dma-fence.h now includes dma-buf.h
All members are now initialized, so kmalloc can be used for
allocating a dma-fence. More documentation added.
v9: Change compiler bitfields to flags, change return type of
enable_signaling to bool. Rework dma_fence_wait. Added
dma_fence_is_signaled and dma_fence_wait_timeout.
s/dma// and change exports to non GPL. Added fence_is_signaled and
fence_enable_sw_signaling calls, add ability to override