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

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Date:   Thu, 11 May 2023 16:46:41 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH] driver core: Call pm_runtime_put_sync() only after
 device_remove()

Hello Johan,

On Thu, May 11, 2023 at 01:48:25PM +0200, Johan Hovold 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 grepped around a bit and found:

	To allow bus types and drivers to put devices into the suspended state by
	calling pm_runtime_suspend() from their ->remove() routines, the driver core
	executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
	notifications in __device_release_driver().  This requires bus types and
	drivers to make their ->remove() callbacks avoid races with runtime PM directly,
	but it also allows more flexibility in the handling of devices during the
	removal of their drivers.

Hmm, while I see your point, it's still ugly. I'll think about it.

Best regards
Uwe


-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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