Re: [PATCH V2 4/8] PM / OPP: Pass struct dev_pm_opp_supply to _set_opp_voltage()

[Viresh Kumar]

On 25-10-16, 13:26, Stephen Boyd wrote:
> For things like AVS we'll probably want to do that, although it's
> sort of funny because replacing RCU with rw-locks is the opposite
> direction most people go.

Yes, that would be very funny :)

> With AVS we would be updating the
> voltage(s) in use for the current OPP, and we would want that
> update to block any OPP transition until the voltage is adjusted.
> I don't know how we would do that with RCU very well. Plus, RCU
> is for reader heavy things, but we mostly have one or two
> readers.

Not just that, think of opp_disable() function. What guarantees currently that
an OPP being disabled isn't already used right now? Or is on the way of getting
used?

I strongly feel RCU is not the best fit for OPP core at least.

> I guess it's ok for now to do all this copying, but it feels like
> we'll need to undo a large portion of it later with things like
> AVS.

Yes.

> Or at least we'll be doing copies for almost no reason
> because we'll want to hold the read lock across the whole OPP
> transition. I was going to suggest we pass around information
> about what we want to grab from the RCU protected data
> structures, think index of regulator, etc. and then have small
> RCU read-side critical sections to grab that info during the OPP
> transition but I'm not sure that's any better. It might be worse
> because the OPP could change during the OPP transition and we
> could be using half of the old and half of the new data.

The problem is that this code is getting harder to read for everybody. If we are
finding it difficult to understand, what about newbies..

-- 
viresh

Re: [PATCH V2 4/8] PM / OPP: Pass struct dev_pm_opp_supply to _set_opp_voltage()

[Viresh Kumar]

On 25-10-16, 13:26, Stephen Boyd wrote:
> For things like AVS we'll probably want to do that, although it's
> sort of funny because replacing RCU with rw-locks is the opposite
> direction most people go.

Yes, that would be very funny :)

> With AVS we would be updating the
> voltage(s) in use for the current OPP, and we would want that
> update to block any OPP transition until the voltage is adjusted.
> I don't know how we would do that with RCU very well. Plus, RCU
> is for reader heavy things, but we mostly have one or two
> readers.

Not just that, think of opp_disable() function. What guarantees currently that
an OPP being disabled isn't already used right now? Or is on the way of getting
used?

I strongly feel RCU is not the best fit for OPP core at least.

> I guess it's ok for now to do all this copying, but it feels like
> we'll need to undo a large portion of it later with things like
> AVS.

Yes.

> Or at least we'll be doing copies for almost no reason
> because we'll want to hold the read lock across the whole OPP
> transition. I was going to suggest we pass around information
> about what we want to grab from the RCU protected data
> structures, think index of regulator, etc. and then have small
> RCU read-side critical sections to grab that info during the OPP
> transition but I'm not sure that's any better. It might be worse
> because the OPP could change during the OPP transition and we
> could be using half of the old and half of the new data.

The problem is that this code is getting harder to read for everybody. If we are
finding it difficult to understand, what about newbies..

-- 
viresh

Re: [PATCH V2 4/8] PM / OPP: Pass struct dev_pm_opp_supply to _set_opp_voltage()

[Stephen Boyd]

On 10/25, Viresh Kumar wrote:
> On 24-10-16, 16:14, Stephen Boyd wrote:
> > On 10/20, Viresh Kumar wrote:
> > > Pass the entire supply structure instead of all of its fields.
> > > 
> > > Signed-off-by: Viresh Kumar 
> > > ---
> > 
> > This patch should be combined with the previous one.
> 
> I think it is a fair to do this separately as this is a completely different
> logical change.

Let's agree to disagree.

> 
> > I'm still
> > not sure if it even makes sense to do this though.
> 
> :)
> 
> > Do we really
> > have to make duplicate "OPP snapshot" structures just because of
> > how OPPs use RCU?
> 
> I agree. With RCU, yes this change is probably required. But I am not sure if
> RCU fits that well to OPP core anymore. A rw-lock may be much easier to help.
> 

For things like AVS we'll probably want to do that, although it's
sort of funny because replacing RCU with rw-locks is the opposite
direction most people go. With AVS we would be updating the
voltage(s) in use for the current OPP, and we would want that
update to block any OPP transition until the voltage is adjusted.
I don't know how we would do that with RCU very well. Plus, RCU
is for reader heavy things, but we mostly have one or two
readers.

I guess it's ok for now to do all this copying, but it feels like
we'll need to undo a large portion of it later with things like
AVS. Or at least we'll be doing copies for almost no reason
because we'll want to hold the read lock across the whole OPP
transition. I was going to suggest we pass around information
about what we want to grab from the RCU protected data
structures, think index of regulator, etc. and then have small
RCU read-side critical sections to grab that info during the OPP
transition but I'm not sure that's any better. It might be worse
because the OPP could change during the OPP transition and we
could be using half of the old and half of the new data.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

Re: [PATCH V2 4/8] PM / OPP: Pass struct dev_pm_opp_supply to _set_opp_voltage()

[Stephen Boyd]

On 10/25, Viresh Kumar wrote:
> On 24-10-16, 16:14, Stephen Boyd wrote:
> > On 10/20, Viresh Kumar wrote:
> > > Pass the entire supply structure instead of all of its fields.
> > > 
> > > Signed-off-by: Viresh Kumar 
> > > ---
> > 
> > This patch should be combined with the previous one.
> 
> I think it is a fair to do this separately as this is a completely different
> logical change.

Let's agree to disagree.

> 
> > I'm still
> > not sure if it even makes sense to do this though.
> 
> :)
> 
> > Do we really
> > have to make duplicate "OPP snapshot" structures just because of
> > how OPPs use RCU?
> 
> I agree. With RCU, yes this change is probably required. But I am not sure if
> RCU fits that well to OPP core anymore. A rw-lock may be much easier to help.
> 

For things like AVS we'll probably want to do that, although it's
sort of funny because replacing RCU with rw-locks is the opposite
direction most people go. With AVS we would be updating the
voltage(s) in use for the current OPP, and we would want that
update to block any OPP transition until the voltage is adjusted.
I don't know how we would do that with RCU very well. Plus, RCU
is for reader heavy things, but we mostly have one or two
readers.

I guess it's ok for now to do all this copying, but it feels like
we'll need to undo a large portion of it later with things like
AVS. Or at least we'll be doing copies for almost no reason
because we'll want to hold the read lock across the whole OPP
transition. I was going to suggest we pass around information
about what we want to grab from the RCU protected data
structures, think index of regulator, etc. and then have small
RCU read-side critical sections to grab that info during the OPP
transition but I'm not sure that's any better. It might be worse
because the OPP could change during the OPP transition and we
could be using half of the old and half of the new data.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

Re: [PATCH V2 4/8] PM / OPP: Pass struct dev_pm_opp_supply to _set_opp_voltage()

[Viresh Kumar]

On 24-10-16, 16:14, Stephen Boyd wrote:
> On 10/20, Viresh Kumar wrote:
> > Pass the entire supply structure instead of all of its fields.
> > 
> > Signed-off-by: Viresh Kumar 
> > ---
> 
> This patch should be combined with the previous one.

I think it is a fair to do this separately as this is a completely different
logical change.

> I'm still
> not sure if it even makes sense to do this though.

:)

> Do we really
> have to make duplicate "OPP snapshot" structures just because of
> how OPPs use RCU?

I agree. With RCU, yes this change is probably required. But I am not sure if
RCU fits that well to OPP core anymore. A rw-lock may be much easier to help.

-- 
viresh

Re: [PATCH V2 4/8] PM / OPP: Pass struct dev_pm_opp_supply to _set_opp_voltage()

[Viresh Kumar]

On 24-10-16, 16:14, Stephen Boyd wrote:
> On 10/20, Viresh Kumar wrote:
> > Pass the entire supply structure instead of all of its fields.
> > 
> > Signed-off-by: Viresh Kumar 
> > ---
> 
> This patch should be combined with the previous one.

I think it is a fair to do this separately as this is a completely different
logical change.

> I'm still
> not sure if it even makes sense to do this though.

:)

> Do we really
> have to make duplicate "OPP snapshot" structures just because of
> how OPPs use RCU?

I agree. With RCU, yes this change is probably required. But I am not sure if
RCU fits that well to OPP core anymore. A rw-lock may be much easier to help.

-- 
viresh

Re: [PATCH V2 4/8] PM / OPP: Pass struct dev_pm_opp_supply to _set_opp_voltage()

[Stephen Boyd]

On 10/20, Viresh Kumar wrote:
> Pass the entire supply structure instead of all of its fields.
> 
> Signed-off-by: Viresh Kumar 
> ---

This patch should be combined with the previous one. I'm still
not sure if it even makes sense to do this though. Do we really
have to make duplicate "OPP snapshot" structures just because of
how OPPs use RCU?

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

Re: [PATCH V2 4/8] PM / OPP: Pass struct dev_pm_opp_supply to _set_opp_voltage()

[Stephen Boyd]

On 10/20, Viresh Kumar wrote:
> Pass the entire supply structure instead of all of its fields.
> 
> Signed-off-by: Viresh Kumar 
> ---

This patch should be combined with the previous one. I'm still
not sure if it even makes sense to do this though. Do we really
have to make duplicate "OPP snapshot" structures just because of
how OPPs use RCU?

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project