linux-kernel - Re: [PATCH] PM: runtime: add RPM_IDLE_SUSPEND / RPM_IDLE_NO

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Message-ID: <CAJZ5v0i-6PBa=UuOkwZhmGvEpuUYkMeu9eubbvUNcGvc=DmDuA@mail.gmail.com>
Date:   Fri, 5 Jun 2020 14:10:30 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM: runtime: add RPM_IDLE_SUSPEND / RPM_IDLE_NO_SUSPEND constants

On Sat, May 30, 2020 at 6:33 PM Heiner Kallweit <hkallweit1@...il.com> wrote:
>
> runtime_idle() callback implementations have to return a non-zero value
> if they don't intend to suspend now. Several drivers use an errno like
> -EBUSY for this purpose. This can be problematic because the return
> value is propagated up the call chain, from rpm_idle() to
> __pm_runtime_idle(), and from there to callers like
> pm_runtime_put_sync(). A driver author checking the return value of
> e.g. pm_runtime_put_sync() may as usual check for retval < 0 and
> bail out.

Which would be a bug anyway, because rpm_idle() may return -EAGAIN or
-EINPROGRESS due to concurrency and -EBUSY should be treated
similarly.

> Therefore a positive value should be returned.

While it is recommended to return a positive value then, returning
-EBUSY or -EAGAIN should still work if the callers are careful enough
(and they should be).

Thanks!