[I'm going to reply to several points in this thread in one reply. I
have restored context that was trimmed in later replys when I wanted to
speak to.]
David Gibson wrote at 2008-11-18 00:28:28:
On Mon, Nov 17, 2008 at 01:41:24PM -0600, Timur Tabi wrote:
On Thu, Oct 23, 2008 at 9:54 PM, David Gibson
<[EMAIL PROTECTED]> wrote:
This patch adds a new backend for the hvc console based on the
low-level udbg callbacks. This effectively implements a working
runtime console in terms of the simple udbg primitives. This is kind
of a hack - since udbg isn't something you really want to be using
routinely - but it's really useful during bringup.
Why is udbg hvc something you want to not use routinely?
Stated differently, if your routine (1) fundamently works one character
at a time and (2) is not interrupt driven, and (3) only supports one
channel, what avantage is there to an explicit hvc driver?
This can be used to quickly implement a userspace usable console
while
you're working on a proper driver for whatever console I/O device the
hardware has. Or, it can be used to avoid writing a full blown
tty/console driver entirely for quick-and-dirty I/O hardware that
will
later be replaced by something else.
Actually it looks remarkably similar to a cleaned up version of a patch
i've been using since hvc_console was split to be a hookable shell. Or
was it the motivation for adding the hooks? The code is different in
how it structures the error checks and that it is implemented in
drivers/char where it belongs (hence the cleaned up comment) but the
fact that David recreated what I did does speak to its general utility.
David, I have a stupid question. I already have an HVC console driver
that works. Do I need to do anything, other than enable HVC_UDBG, to
get my hvc console drive to use udbg?
As was pointed out elsewhere, this is hvc calling udbg not udbg hooks
calling hvc_cosole methods, which might need a different context. But
I wanted to comment on David's reply:
Um.. well.. if you have both activated, I think you can select which
HVC console backend will be used by using console=/dev/hvcNN on the
commandline, where values of NN correspond to different backends, in
order depending on link order in some complex fashion.
It doesn't just work that way because of the way the hvc_console shell
implements the console hooks.
If the drivers request different hvc channel numbers, then yes, you can
select which hvc backend is used by choosing console=hvcNN. If all
request 0, then which ever one registers with hvc_console first wins
the slot, and the others get a busy error return. They can still
register their tty later, at which point they get a dynamically
determined /dev/hvcN slot, above all registered console slots.
There are multiple reasons for this design. The original user, pseries
vterm, wants to assign the channel number based on what the hypervisor
tells it the channel should be. There are two loops though the device
tree, searching for different protocols (and consequently different
backends), so they need to be able to specify the number and not just
use the order they are found. The platform describes not only the
protocol, but which instance that is, and the backend currently
registers with that number. Another reason is its a bad idea to have
console=/dev/hvc0 depend on link order instead of knowing which driver
is selected. If a user is specifing what the console is, it should not
depend on other linked drivers.
Until this point, all mainline drivers have been exclusive in that only
one will actually register with hvc_console midlayer on any given
platform. This is the first backend that is not exclusive, and
therefore its coexistence needs special attention.
In my internal tree, I register (my version of) this not as hvc0 but as
hvc4. hvc0 is rtas, hvc1-3 are an internal backend, and this one is
hvc4. The order I chose is arbitrary, but the main point is it does't
compete with hvc_rtas for slot 0. (All of these drivers coexist, and I
can choose which one I want to use based on my needs for that boot.)
We could make it configurable via Kconfig, or just choose a slot. (I
think vterm can have 2 channels on some boxes where they drive real
serial ports on the box.)
But if you already have a working HVC console driver, I don't see why
you'd want to use HVC_UDBG - it's essentially a bringup hack.
As I said, why should we need a fancier hvc_console backend if you are
polling and gain no efficency processing multiple characters at once.
The udbg drivers know if they are sharing the interface with a
hvc_console some other driver. If we trigger the registration of this
backend on the udbg hook saying its useful (eg by setting a flag word
in the function where we assign the udbg putc and getc pointers) then
this driver will not need to be compiled out for "production" kernels.
David Gibson wrote at 2008-11-19 00:42:20:
On Tue, Nov 18, 2008 at 09:06:17AM -0600, Timur Tabi wrote:
David Gibson wrote: [at 2008-11-18 05:14:08]
Given the variety of strange I/O configurations in prototype and
embedded platforms, I can't imagine this was a unique situation. So
I've pushed my patch out, so anyone else in a similar situation can
immediately turn their little udbg methods for whatever strange I/O
they have into a fully-functional console. Maybe it's not something
you'd want to go to release with, but it certainly simplifies life
during bringup.
Ok, I understand now. However, I would like to see two changes:
1) Re-arrange the Makefile as I pointed out in another post.
[the other post]
Timur Tabi - 2008-11-17 20:18:31
One other thing ...
HVC console drivers must be compiled before hvc_console.o
see
http://git.kernel.org/?p=linux/kernel/git/paulus/powerpc.git;
a=commit;h=938473b24636d77dc5e9c3f41090d071b6cf4389
[end other post]
Um.. yeah.. I'm a bit baffled by this.. all the existing backends
are listed after hvc_console, I just added hvc_udbg to the end. I
didn't really understand the rationale in that commit, but then I
haven't had time to look at it very much yet.
I too challanged the necessity of that change. I carefully designed
the hvc_console layer to find the selected console weither it was
registered before or after the console_initcall of hvc_console.
I am very disapointed that the changelog is so sparse.
However, I think the bottom line was that add_preferred_console is
suppsoed to be called by architecture setup code. Doing it the
console_initcall is almost abuse. But if the console_initcall is going
to call add_preferred_console, then it must link before the hvc_console
driver. I would have to go back and find the discussion to remember
the exact details. (maybe it only needed to call
add_preferreed_console before registering itself if its the only hvc
backend).
Which does bring up the point of avoinding calling
add_preferred_console to udbg just because it is linked in. The udbg
getc routines might work for xmon, but one would probably not like the
result of the real driver and udbg trying to read the same device at
the same time. Perhaps the same flag that says "consider me or
udbg_hvc" should apply, or we do add_preferred_console in arch code
before console_initcall. (its ok to preferr a console that never
registers as long as another preferred console does register).
2) Update the Kconfig help file to be very clear that this feature
is only meaningful if the platform has a udbg back-end but no other
console or TTY driver.
Alrighty...
That does bring up the point, I don't see any kconfig help for this
user-selectable option.
And if we don't have the udbg hooks say to enable it, then we really
need to have text discouraging its use as it might break their console.
milton
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@ozlabs.org
https://ozlabs.org/mailman/listinfo/linuxppc-dev
