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

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Message-ID: <CAJZ5v0gKnHMgN0xokp1twLUCc5MFiXo_4tOORk_-Vxv0iFLLag@mail.gmail.com>
Date:   Sat, 23 Dec 2017 02:35:31 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Kishon Vijay Abraham I <kishon@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] phy: core: Move runtime PM reference counting to
 the parent device

On Thu, Dec 21, 2017 at 11:50 AM, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> On 21 December 2017 at 02:39, Rafael J. Wysocki <rafael@...nel.org> wrote:
>> On Wed, Dec 20, 2017 at 3:09 PM, Ulf Hansson <ulf.hansson@...aro.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?

> 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.

>>> 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.

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.

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

Thanks,
Rafael