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

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Date:   Fri, 23 Sep 2022 15:48:13 +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 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.

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?

Note that, the PM core also disables runtime PM for the device in
__device_suspend_late(). For good reasons.

[...]

Kind regards
Uffe