Re: [PATCH 1/3] PM: domains: Drop the performance state vote for a device at detach

[Dmitry Osipenko <digetx@gmail.com>]

06.09.2021 13:24, Ulf Hansson пишет:
> On Sun, 5 Sept 2021 at 10:26, Dmitry Osipenko <digetx@gmail.com> wrote:
>>
>> 03.09.2021 17:03, Ulf Hansson пишет:
>>> On Fri, 3 Sept 2021 at 11:58, Dmitry Osipenko <digetx@gmail.com> wrote:
>>>>
>>>> 03.09.2021 11:22, Ulf Hansson пишет:
>>>>> On Fri, 3 Sept 2021 at 08:01, Dmitry Osipenko <digetx@gmail.com> wrote:
>>>>>>
>>>>>> 02.09.2021 13:16, Ulf Hansson пишет:
>>>>>>> When a device is detached from its genpd, genpd loses track of the device,
>>>>>>> including its performance state vote that may have been requested for it.
>>>>>>>
>>>>>>> Rather than relying on the consumer driver to drop the performance state
>>>>>>> vote for its device, let's do it internally in genpd when the device is
>>>>>>> getting detached. In this way, we makes sure that the aggregation of the
>>>>>>> votes in genpd becomes correct.
>>>>>>
>>>>>> This is a dangerous behaviour in a case where performance state
>>>>>> represents voltage. If hardware is kept active on detachment, say it's
>>>>>> always-on, then it may be a disaster to drop the voltage for the active
>>>>>> hardware.
>>>>>>
>>>>>> It's safe to drop performance state only if you assume that there is a
>>>>>> firmware behind kernel which has its own layer of performance management
>>>>>> and it will prevent the disaster by saying 'nope, I'm not doing this'.
>>>>>>
>>>>>> The performance state should be persistent for a device and it should be
>>>>>> controlled in a conjunction with runtime PM. If platform wants to drop
>>>>>> performance state to zero on detachment, then this behaviour should be
>>>>>> specific to that platform.
>>>>>
>>>>> I understand your concern, but at this point, genpd can't help to fix this.
>>>>>
>>>>> Genpd has no information about the device, unless it's attached to it.
>>>>> For now and for these always on HWs, we simply need to make sure the
>>>>> device stays attached, in one way or the other.
>>>>
>>>> This indeed requires to redesign GENPD to make it more coupled with a
>>>> device, but this is not a real problem for any of the current API users
>>>> AFAIK. Ideally the state should be persistent to make API more universal.
>>>
>>> Right. In fact this has been discussed in the past. In principle, the
>>> idea was to attach to genpd at device registration, rather than at
>>> driver probe.
>>>
>>> Although, this is not very easy to implement - and it seems like the
>>> churns to do, have not been really worth it. At least so far.
>>>
>>>>
>>>> Since for today we assume that device should be suspended at the time of
>>>> the detachment (if the default OPP state isn't used), it may be better
>>>> to add a noisy warning message if pstate!=0, keeping the state untouched
>>>> if it's not zero.
>>>
>>> That would just be very silly in my opinion.
>>>
>>> When the device is detached (suspended or not), it may cause it's PM
>>> domain to be powered off - and there is really nothing we can do about
>>> that from the genpd point of view.
>>>
>>> As stated, the only current short term solution is to avoid detaching
>>> the device. Anything else, would just be papering of the issue.
>>
>> What about to re-evaluate the performance state of the domain after
>> detachment instead of setting the state to zero?
> 
> I am not suggesting to set the performance state of the genpd to zero,
> but to drop a potential vote for a performance state for the *device*
> that is about to be detached.

By removing the vote of the *device*, you will drop the performance
state of the genpd. If device is active and it's wrong to drop its
state, then you may cause the damage.

> Calling genpd_set_performance_state(dev, 0), during detach will have
> the same effect as triggering a re-evaluation of the performance state
> for the genpd, but after the detach.

Yes

>> This way PD driver may
>> take an action on detachment if performance isn't zero, before hardware
>> is crashed, for example it may emit a warning.
> 
> Not sure I got that. Exactly when do you want to emit a warning and
> for what reason?
> 
> Do you want to add a check somewhere to see if
> 'gpd_data->performance_state' is non zero - and then print a warning?

I want to check the 'gpd_data->performance_state' from the detachment
callback and emit the warning + lock further performance changes in the
PD driver since it's a error condition.