Re: [PATCH v8 01/34] opp: Add dev_pm_opp_sync() helper

[Dmitry Osipenko <digetx@gmail.com>]

20.08.2021 15:42, Ulf Hansson пишет:
> On Thu, 19 Aug 2021 at 21:35, Dmitry Osipenko <digetx@gmail.com> wrote:
>>
>> 19.08.2021 16:07, Ulf Hansson пишет:
>>> On Wed, 18 Aug 2021 at 17:43, Dmitry Osipenko <digetx@gmail.com> wrote:
>>>>
>>>> 18.08.2021 13:08, Ulf Hansson пишет:
>>>>> On Wed, 18 Aug 2021 at 11:50, Viresh Kumar <viresh.kumar@linaro.org> wrote:
>>>>>>
>>>>>> On 18-08-21, 11:41, Ulf Hansson wrote:
>>>>>>> On Wed, 18 Aug 2021 at 11:14, Viresh Kumar <viresh.kumar@linaro.org> wrote:
>>>>>>>> What we need here is just configure. So something like this then:
>>>>>>>>
>>>>>>>> - genpd->get_performance_state()
>>>>>>>>   -> dev_pm_opp_get_current_opp() //New API
>>>>>>>>   -> dev_pm_genpd_set_performance_state(dev, current_opp->pstate);
>>>>>>>>
>>>>>>>> This can be done just once from probe() then.
>>>>>>>
>>>>>>> How would dev_pm_opp_get_current_opp() work? Do you have a suggestion?
>>>>>>
>>>>>> The opp core already has a way of finding current OPP, that's what
>>>>>> Dmitry is trying to use here. It finds it using clk_get_rate(), if
>>>>>> that is zero, it picks the lowest freq possible.
>>>>>>
>>>>>>> I am sure I understand the problem. When a device is getting probed,
>>>>>>> it needs to consume power, how else can the corresponding driver
>>>>>>> successfully probe it?
>>>>>>
>>>>>> Dmitry can answer that better, but a device doesn't necessarily need
>>>>>> to consume energy in probe. It can consume bus clock, like APB we
>>>>>> have, but the more energy consuming stuff can be left disabled until
>>>>>> the time a user comes up. Probe will just end up registering the
>>>>>> driver and initializing it.
>>>>>
>>>>> That's perfectly fine, as then it's likely that it won't vote for an
>>>>> OPP, but can postpone that as well.
>>>>>
>>>>> Perhaps the problem is rather that the HW may already carry a non-zero
>>>>> vote made from a bootloader. If the consumer driver tries to clear
>>>>> that vote (calling dev_pm_opp_set_rate(dev, 0), for example), it would
>>>>> still not lead to any updates of the performance state in genpd,
>>>>> because genpd internally has initialized the performance-state to
>>>>> zero.
>>>>
>>>> We don't need to discover internal SoC devices because we use
>>>> device-tree on ARM. For most devices power isn't required at a probe
>>>> time because probe function doesn't touch h/w at all, thus devices are
>>>> left in suspended state after probe.
>>>>
>>>> We have three components comprising PM on Tegra:
>>>>
>>>> 1. Power gate
>>>> 2. Clock state
>>>> 3. Voltage state
>>>>
>>>> GENPD on/off represents the 'power gate'.
>>>>
>>>> Clock and reset are controlled by device drivers using clk and rst APIs.
>>>>
>>>> Voltage state is represented by GENPD's performance level.
>>>>
>>>> GENPD core assumes that at a first rpm-resume of a consumer device, its
>>>> genpd_performance=0. Not true for Tegra because h/w of the device is
>>>> preconfigured to a non-zero perf level initially, h/w may not support
>>>> zero level at all.
>>>
>>> I think you may be misunderstanding genpd's behaviour around this, but
>>> let me elaborate.
>>>
>>> In genpd_runtime_resume(), we try to restore the performance state for
>>> the device that genpd_runtime_suspend() *may* have dropped earlier.
>>> That means, if genpd_runtime_resume() is called prior
>>> genpd_runtime_suspend() for the first time, it means that
>>> genpd_runtime_resume() will *not* restore a performance state, but
>>> instead just leave the performance state as is for the device (see
>>> genpd_restore_performance_state()).
>>>
>>> In other words, a consumer driver may use the following sequence to
>>> set an initial performance state for the device during ->probe():
>>>
>>> ...
>>> rate = clk_get_rate()
>>> dev_pm_opp_set_rate(rate)
>>>
>>> pm_runtime_enable()
>>> pm_runtime_resume_and_get()
>>> ...
>>>
>>> Note that, it's the consumer driver's responsibility to manage device
>>> specific resources, in its ->runtime_suspend|resume() callbacks.
>>> Typically that means dealing with clock gating/ungating, for example.
>>>
>>> In the other scenario where a consumer driver prefers to *not* call
>>> pm_runtime_resume_and_get() in its ->probe(), because it doesn't need
>>> to power on the device to complete probing, then we don't want to vote
>>> for an OPP at all - and we also want the performance state for the
>>> device in genpd to be set to zero. Correct?
>>
>> Yes
>>
>>> Is this the main problem you are trying to solve, because I think this
>>> doesn't work out of the box as of today?
>>
>> The main problem is that the restored performance state is zero for the
>> first genpd_runtime_resume(), while it's not zero from the h/w perspective.
> 
> This should not be a problem, but can be handled by the consumer driver.
> 
> genpd_runtime_resume() calls genpd_restore_performance_state() to
> restore a performance state for the device. However, in the scenario
> you describe, "gpd_data->rpm_pstate" is zero, which makes
> genpd_restore_performance_state() to just leave the device's
> performance state as is - it will *not* restore the performance state
> to zero.
> 
> To make the consumer driver deal with this, it would need to call
> dev_pm_opp_set_rate() from within its ->runtime_resume() callback.
> 
>>
>>> There is another concern though, but perhaps it's not a problem after
>>> all. Viresh told us that dev_pm_opp_set_rate() may turn on resources
>>> like clock/regulators. That could certainly be problematic, in
>>> particular if the device and its genpd have OPP tables associated with
>>> it and the consumer driver wants to follow the above sequence in
>>> probe.
>>
>> dev_pm_opp_set_rate() won't enable clocks and regulators, but it may
>> change the clock rate and voltage. This is also platform/driver specific
>> because it's up to OPP user how to configure OPP table. On Tegra we only
>> assign clock to OPP table, regulators are unused.
>>
>>> Viresh, can you please chime in here and elaborate on some of the
>>> magic happening behind dev_pm_opp_set_rate() API - is there a problem
>>> here or not?
>>>
>>>>
>>>> GENPD core assumes that consumer devices can work at any performance
>>>> level. Not true for Tegra because voltage needs to be set in accordance
>>>> to the clock rate before clock is enabled, otherwise h/w won't work
>>>> properly, perhaps clock may be unstable or h/w won't be latching.
>>>
>>> Correct. Genpd relies on the callers to use the OPP framework if there
>>> are constraints like you describe above.
>>>
>>> That said, it's not forbidden for a consumer driver to call
>>> dev_pm_genpd_set_performance_state() directly, but then it better
>>> knows exactly what it's doing.
>>>
>>>>
>>>> Performance level should be set to 0 while device is suspended.
>>>
>>> Do you mean system suspend or runtime suspend? Or both?
>>
>> Runtime suspend.
> 
> Alright. So that's already taken care of for us in genpd_runtime_suspend().
> 
> Or perhaps you have discovered some problem with this?
> 
>>
>>>> Performance level needs to be bumped on rpm-resume of a device in
>>>> accordance to h/w state before hardware is enabled.
>>>
>>> Assuming there was a performance state set for the device when
>>> genpd_runtime_suspend() was called, genpd_runtime_resume() will
>>> restore that state according to the sequence you described.
>>
>> What do you think about adding API that will allow drivers to explicitly
>> set the restored performance state of a power domain?
>>
>> Another option could be to change the GENPD core, making it to set the
>> rpm_pstate when dev_pm_genpd_set_performance_state(dev) is invoked and
>> device is rpm-suspended, instead of calling the
>> genpd->set_performance_state callback.
>>
>> Then drivers will be able to sync the perf state at a probe time.
>>
>> What do you think?
> 
> I don't think it's needed, see my reply earlier above. However your
> change touches another problem though, see below.
> 
>>
>> diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
>> index a934c679e6ce..cc15ab9eacc9 100644
>> --- a/drivers/base/power/domain.c
>> +++ b/drivers/base/power/domain.c
>> @@ -435,7 +435,7 @@ static void genpd_restore_performance_state(struct
>> device *dev,
>>  int dev_pm_genpd_set_performance_state(struct device *dev, unsigned int
>> state)
>>  {
>>         struct generic_pm_domain *genpd;
>> -       int ret;
>> +       int ret = 0;
>>
>>         genpd = dev_to_genpd_safe(dev);
>>         if (!genpd)
>> @@ -446,7 +446,10 @@ int dev_pm_genpd_set_performance_state(struct
>> device *dev, unsigned int state)
>>                 return -EINVAL;
>>
>>         genpd_lock(genpd);
>> -       ret = genpd_set_performance_state(dev, state);
>> +       if (pm_runtime_suspended(dev))
>> +               dev_gpd_data(dev)->rpm_pstate = state;
>> +       else
>> +               ret = genpd_set_performance_state(dev, state);
>>         genpd_unlock(genpd);
> 
> This doesn't work for all cases. For example, when a consumer driver
> deploys runtime PM support in its ->probe() according to the below
> sequence:
> 
> ...
> dev_pm_opp_set_rate(rate)
> pm_runtime_get_noresume()
> pm_runtime_set_active()
> pm_runtime_enable()
> ...
> pm_runtime_put()
> ...
> 
> We need to call genpd_set_performance_state() independently of whether
> the device is runtime suspended or not.

I don't see where is the problem in yours example.

pm_runtime_suspended() = false while RPM is disabled. When device is
resumed, the rpm_pstate=0, so it won't change the pstate on resume.

> Although, it actually seems like good idea to update
> dev_gpd_data(dev)->rpm_pstate = state here, as to make sure
> genpd_runtime_resume() doesn't restore an old/invalid value that was
> saved while dropping the performance state vote for the device in
> genpd_runtime_suspend() earlier.
> 
> Let me send a patch for this shortly, to close this window of a possible error.

It will also remove the need to resume device just to change the clock
rate, like I needed to do it in the PWM patch of this series.