Re: [PATCH v2 1/3] phy: core: Move runtime PM reference counting to the parent device

[Ulf Hansson <ulf.hansson@linaro.org>]

On 23 December 2017 at 02:35, Rafael J. Wysocki <rafael@kernel.org> wrote:
> On Thu, Dec 21, 2017 at 11:50 AM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
>> On 21 December 2017 at 02:39, Rafael J. Wysocki <rafael@kernel.org> wrote:
>>> On Wed, Dec 20, 2017 at 3:09 PM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
>>>> The runtime PM deployment in the phy core is deployed using the phy core
>>>> device, which is created by the phy core and assigned as a child device of
>>>> the phy provider device.
>>>>
>>>> The behaviour around the runtime PM deployment cause some issues during
>>>> system suspend, in cases when the phy provider device is put into a low
>>>> power state via a call to the pm_runtime_force_suspend() helper, as is the
>>>> case for a Renesas SoC, which has its phy provider device attached to the
>>>> generic PM domain.
>>>>
>>>> In more detail, the problem in this case is that pm_runtime_force_suspend()
>>>> expects the child device of the provider device, which is the phy core
>>>> device, to be runtime suspended, else a WARN splat will be printed
>>>> (correctly) when runtime PM gets re-enabled at system resume.
>>>
>>> So we are now trying to work around issues with
>>> pm_runtime_force_suspend().  Lovely. :-/
>>
>> Yes, we have to, as pm_runtime_force_suspend() is widely deployed. Or
>> are you saying we should just ignore all issues related to it?
>
> Or get rid of it?
>
> Seriously, is the resulting pain worth it?

I am not sure what pain you refer to? :-) So far I haven't got much
errors being reported because of its use, have you?

Moreover, to me, this small series fixes the problems in a rather trivial way.

>
>> Of course, if we had something that could replace
>> pm_runtime_force_suspend(), that would be great, but there isn't.
>
> I beg to differ.
>
> At least some of it could be replaced with the driver flags.

Yes, true.

On the other hand that is exactly the problem, only *some*. And more
importantly, genpd can't use them, because it can't cope with have
system wide PM callbacks to be skipped.

In other words, so far, the driver PM flags can't solve this issue.

>
>>>> In the current scenario, even if a call to phy_power_off() triggers it to
>>>> invoke pm_runtime_put() during system suspend, the phy core device doesn't
>>>> get runtime suspended, because this is prevented in the system suspend
>>>> phases by the PM core.
>>>>
>>>> To solve this problem, let's move the runtime PM deployment from the phy
>>>> core device to the phy provider device, as this provides the similar
>>>> behaviour. Changing this makes it redundant to enable runtime PM for the
>>>> phy core device, so let's avoid doing that.
>>>
>>> I'm not really convinced that this approach is the best one to be honest.
>>>
>>> I'll have a deeper look at this in the next few days, stay tuned.
>>
>> There is different ways to solve this, for sure. I picked this one,
>> because I think it's the most trivial thing to do, and it shouldn't
>> cause any other problems.
>>
>> I think any other option would involve assigning ->suspend|resume()
>> callbacks to the phy core device, but that's fine too, if you prefer
>> that.
>>
>> Also, I have considered how to deal with wakeup paths for phys,
>> although I didn't want to post changes as a part of this series, but
>> maybe I should to give a more complete picture?
>
> Yes, you should.

Okay!

>
> The point is that without genpd using pm_runtime_force_suspend() the
> phy code could very well stay the way it is.  And it is logical,
> because having a parent with enabled runtime PM without enabling
> runtime PM for its children is at least conceptually questionable.

I agree that the phy core is today logical okay. But I think that's
also the case, if we pick up this small series.

There are many reasons and cases where a children is runtime PM
disabled, while the parent is runtime PM enabled. Moreover the runtime
PM core copes fine with that.

>
> So the conclusion may be that the phy code is OK, but calling
> pm_runtime_force_suspend() from genpd isn't.

So I am worried that changing genpd in this regards, may introduce
other regressions. @subject series seems far less risky - and again,
to me, the changes are trivial.

Anyway, I will keep your suggestion in mind and think about, how/if
genpd can be changed.

Kind regards
Uffe