linux-kernel - Re: [PATCH] PM / runtime: Drop children check from __pm_runtime_set

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Date:   Mon, 13 Nov 2017 22:58:59 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH] PM / runtime: Drop children check from __pm_runtime_set_status()

On Mon, Nov 13, 2017 at 10:50 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> On Monday, November 13, 2017 2:26:28 PM CET Ulf Hansson wrote:
>> On 12 November 2017 at 01:27, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> >
>> > The check for "active" children in __pm_runtime_set_status(), when
>> > trying to set the parent device status to "suspended", doesn't
>> > really make sense, because in fact it is not invalid to set the
>> > status of a device with runtime PM disabled to "suspended" in any
>> > case.  It is invalid to enable runtime PM for a device with its
>> > status set to "suspended" while its child_count reference counter
>> > is nonzero, but the check in __pm_runtime_set_status() doesn't
>> > really cover that situation.
>>
>> The reason to why I changed this in commit a8636c89648a ("PM /
>> Runtime: Don't allow to suspend a device with an active child") was
>> because to get a consistent behavior.
>
> Well, it causes the function to return an error in a non-error situation,
> which IMnsHO is a bug.
>
>> Because, setting the device's status to active (RPM_ACTIVE) via
>> __pm_runtime_set_status(), requires its parent to also be active (in
>> case the parent has runtime PM enabled).
>
> No!!!
>
> The check is in there, because the parent's usage_count is affected by that

Actually, the parent's child_count is affected, but that doesn't matter here.

> code and incrementing it in case the parent has runtime PM enabled and is
> RPM_SUSPENDED leads to an inconsistent runtime PM state of the parent.  IOW,
> it would confuse the framework.
>
> There's no such issue if the runtime PM status of a child is set to RPM_SUSPENDED.
>
> It is all consistent without the check I'm removing and is made inconsistent
> by that very check.
>
>> I would prefer to try maintain this consistency, but I also I realize
>> that commit a8636c89648a, should also have been checking if the parent
>> has runtime PM enabled (again for consistency), which it doesn't.
>>
>> What about fixing that instead?
>
> Runtime PM is *disabled* for the parent at this point, guaranteed, so there's
> nothing to check, I'm afraid ...
>
> OTOH, the runtime PM status of the children doesn't matter here, because their
> reference counters are not updated.

Thanks,
Rafael