linux-kernel - Re: [PATCH v2] PM: runtime: Return properly from rpm_resume() if dev->power.needs_force

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Date:   Mon, 26 Sep 2022 11:47:42 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Liu Ying <victor.liu@....com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-imx@....com,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] PM: runtime: Return properly from rpm_resume() if
 dev->power.needs_force_resume flag is set

On Fri, 23 Sept 2022 at 17:23, Liu Ying <victor.liu@....com> wrote:
>
> On Fri, 2022-09-23 at 15:48 +0200, Ulf Hansson wrote:
> > On Fri, 23 Sept 2022 at 14:47, Liu Ying <victor.liu@....com> wrote:
> > >
> > > After a device transitions to sleep state through it's system
> > > suspend
> > > callback pm_runtime_force_suspend(), the device's driver may still
> > > try
> > > to do runtime PM for the device(runtime suspend first and then
> > > runtime
> > > resume) although runtime PM is disabled by that callback.  The
> > > runtime
> > > PM operations would not touch the device effectively and the device
> > > is
> > > assumed to be resumed through it's system resume callback
> > > pm_runtime_force_resume().
> >
> > This sounds like a fragile use case to me. In principle you want to
> > allow the device to be runtime resumed/suspended, after the device
> > has
> > already been put into a low power state through the regular system
> > suspend callback. Normally it seems better to prevent this from
> > happening, completely.
>
> Not sure if we really may prevent this from happening completely.
>
> >
> > That said, in this case, I wonder if a better option would be to
> > point
> > ->suspend_late() to pm_runtime_force_suspend() and ->resume_early()
> > to
> > pm_runtime_force_resume(), rather than using the regular
> > ->suspend|resume() callbacks. This should avoid the problem, I think,
> > no?
>
> I thought about this and it actually works for my particular
> panel-simple case.  What worries me is that the device(DRM device in my
> case) which triggers the runtime PM operations may also use
> ->suspend_late/resume_early() callbacks for whatever reasons, hence no
> fixed order to suspend/resume the two devices(like panel device and DRM
> device).
>
> Also, not sure if there is any sequence issue by using the
> ->suspend_late/resume_early() callbacks in the panel-simple driver,
> since it's written for quite a few display panels which may work with
> various DRM devices - don't want to break any of them.

What you are describing here, is the classical problem we have with
suspend/resume ordering of devices.

There are in principle two ways to solve this.
1. If it makes sense, the devices might be assigned as parent/child.
2. If it's more a consumer/supplier thing, we can add a device-link
between them.

In this way, the PM core can guarantee that the order becomes correct.

Kind regards
Uffe