linux-kernel - Re: [PATCH] driver core: Call pm_runtime_put_sync() only after device

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Message-ID: <CAJZ5v0jvJT4JkHtO3RCUEzkfawxLCwR=QO2Y2CsL=cYN9s4hXw@mail.gmail.com>
Date:   Thu, 11 May 2023 16:44:25 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH] driver core: Call pm_runtime_put_sync() only after device_remove()

On Thu, May 11, 2023 at 1:48 PM Johan Hovold <johan@...nel.org> wrote:
>
> On Thu, May 11, 2023 at 12:39:23PM +0200, Uwe Kleine-König wrote:
> > On Thu, May 11, 2023 at 12:18:09PM +0200, Rafael J. Wysocki wrote:
> > > On Thu, May 11, 2023 at 9:34 AM Uwe Kleine-König
> > > <u.kleine-koenig@...gutronix.de> wrote:
> > > >
> > > > Many drivers that use runtime PM call pm_runtime_get_sync() or one of
> > > > its variants in their remove callback. So calling pm_runtime_put_sync()
> > > > directly before calling the remove callback results (under some
> > > > conditions) in the driver's suspend routine being called just to resume
> > > > it again afterwards.
> > > >
> > > > So delay the pm_runtime_put_sync() call until after device_remove().
> > > >
> > > > Confirmed on a stm32mp157a that doing
> > > >
> > > >         echo 4400e000.can > /sys/bus/platform/drivers/m_can_platform/unbind
> > > >
> > > > (starting with a runtime-pm suspended 4400e000.can) results in one call
> > > > less of m_can_runtime_resume() and m_can_runtime_suspend() each after
> > > > this change was applied.
> > > >
> > > > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> > >
> > > I'm not against this change, although I kind of expect it to trigger
> > > some fallout that will need to be addressed.  So caveat emtor.
> > >
> > > Anyway
> > >
> > > Reviewed-by: Rafael J. Wysocki <rafael@...nel.org>
> >
> > Thanks for your review tag. I wondered if there will be some fallout,
> > and don't know what to expect yet. Sounds like getting it into next soon
> > is a good idea?!
>
> No, this seems like very bad idea and even violates the documentation
> which clearly states that the usage counter is balanced before calling
> remove() so that drivers can use pm_runtime_suspend() to put devices
> into suspended state.

I missed that, sorry.

> There's is really no good reason to even try to change as this is in no
> way a fast path.

Still, I think that while the "put" part needs to be done before
device_remove(), the actual state change can be carried out later.

So something like

    pm_runtime_put_noidle(dev);

    device_remove(dev);

    pm_runtime_suspend(dev);

would generally work, wouldn't it?