Re: [PATCH] PM / Runtime: Only force-resume device if it has been force-suspended

[Ulf Hansson]

On 21 April 2016 at 14:41, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
> Hi Ulf,
>
> On Thursday 21 Apr 2016 11:10:19 Ulf Hansson wrote:
>> On 21 April 2016 at 01:30, Laurent Pinchart wrote:
>> > On Monday 07 Mar 2016 11:10:08 Ulf Hansson wrote:
>> >> [...]
>> >>
>> >>>> I agree, that's a better idea. Drivers shouldn't call
>> >>>> pm_runtime_force_resume() if they haven't called
>> >>>> pm_runtime_force_suspend(), so checking the PM use count should be
>> >>>> fine. I'll modify the patch, test it and resubmit.
>> >>>
>> >>> I gave it an unfortunately unsuccessful try. The problem I ran into is
>> >>> that device_prepare() calls pm_runtime_get_noresume() calls
>> >>> pm_runtime_get_noresume(), with the corresponding pm_runtime_put() call
>> >>> being performed in device_complete(). The device power usage_count is
>> >>> thus always non-zero in the system resume handler, so I can't base the
>> >>> decision on that.
>> >>
>> >> As Alan said, let's just check against 1 instead.
>> >
>> > I gave this a try, and unfortunately it won't work.
>> >
>> > pm_genpd_prepare() resumes devices without increasing the usage count,
>> > which
>>
>> It doesn't resume them, it only increases the usage count.
>
> Maybe there's something I don't get, but pm_genpd_prepare() ends with

I realize that I did read *pm_genpd_prepare()* as *device_prepare()*,
which represents the behaviour of the PM core during the prepare phase
(drivers/base/power/main.c). In such cases my earlier reply would make
more sense, I believe.

Apologize for giving you the wrong input by not reading your response
in more detail!

>
>         /*
>          * The PM domain must be in the GPD_STATE_ACTIVE state at this point,
>          * so genpd_poweron() will return immediately, but if the device
>          * is suspended (e.g. it's been stopped by genpd_stop_dev()), we need
>          * to make it operational.
>          */
>         pm_runtime_resume(dev);
>         __pm_runtime_disable(dev, false);
>
>         ret = pm_generic_prepare(dev);
>         if (ret) {
>                 ... /* irrelevant error handling */
>         }
>
>         pm_runtime_put(dev);
>         return ret;
>
> The device is thus resumed. Note that the pm_runtime_put() call will decrease
> the usage count that was increased at the beginning of the function (and thus
> makes pm_genpd_prepare() neutral from a usage count point of view) but won't
> resume the device as the disable depth is increased by __pm_runtime_disable().
>
> As far as I understand, the device is thus effectively active when entering
> the driver's system suspend handler, with a usage count equal to 1 or more.
> pm_runtime_force_suspend(), which decides whether to suspend the device based
> on the device pm state and not the usage count, will thus always pm suspend
> the device.
>
> Is the above analysis correct ?

Yes.

Although, genpd is not behaving as it should here. It must *not*
resume all devices, just because the domain is powered.

>
> Now I'm a bit lost as to what happens (and what should happen) at resume time.
> Does genpd and the PM core try to suspend the device after calling the
> driver's resume handler (pm_runtime_force_resume() in this case) under some
> conditions, or does the resume handler have the last word in deciding the PM
> runtime status of the device after system resume ?

The resume callback of genpd decides what will happen, as its a PM
domain and it sits on top of the hierarchy of a devices PM callbacks.

As you have realized, there are issues in genpd regarding the system
PM code, there have even been reports about it.

For example, genpd prevents subsystem/drivers from manage their
devices during system PM - when it start the system PM phase with the
domain in powered off state.

That's not an okay behaviour, as it can't know whether a device needs
to be used to serve a request after that point, causing the device to
be runtime resumed. As genpd prevents it to be suspended via system
PM, it will stay resumed during the system PM suspend phase, at least
that is my understanding and it seems like bad idea.

In general, the system PM code in genpd needs to be modernized. It
somewhat duplicating some code from the PM core and I think the code
can be greatly simplified.

I have started to work on this quite recently, once the first steps
are done I will consider optimizations, such as not runtime resuming
devices unnecessarily.

>
>> > leads to the device always being active in pm_runtime_force_suspend(). The
>> > usage count will be 1 if the device was suspended prior to entering system
>> > suspend (due to the pm_runtime_get_noresume() call in device_prepare()) or
>> > higher than 1 if the device was active.
>>
>> It's only greater than 1 - if someone else than the PM core has
>> increased the usage count.
>
> That I agree with.

Okay, great! :-)

[...]

Again, sorry for the giving the wrong input earlier!

Future wise, I will keep you on cc on any related genpd patches.

Kind regards
Uffe