Message-ID: <1457511.dE2lGbyvKX@vostro.rjw.lan>
Date:	Wed, 06 Nov 2013 23:01:55 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Tomi Valkeinen <tomi.valkeinen@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Kevin Hilman <khilman@...aro.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Archit Taneja <archit@...com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: Async runtime put in __device_release_driver()

On Wednesday, November 06, 2013 09:51:42 AM Tomi Valkeinen wrote:
> On 2013-11-05 23:29, Ulf Hansson wrote:
> > On 23 October 2013 12:11, Tomi Valkeinen <tomi.valkeinen@...com> wrote:
> >> Hi,
> >>
> >> I was debugging why clocks were left enabled after removing omapdss
> >> driver, and I found this commit:
> >>
> >> fa180eb448fa263cf18dd930143b515d27d70d7b (PM / Runtime: Idle devices
> >> asynchronously after probe|release)
> >>
> >> I don't understand how that is supposed to work.
> >>
> >> When a driver is removed, instead of using pm_runtime_put_sync() the
> >> commit uses pm_runtime_put(), so the runtime_suspend call is queued. But
> >> who is going to handle the queued suspend call, as the driver is already
> >> removed? At least in my case, obviously nobody, as I only get
> >> runtime_resume call in my driver, never the runtime_suspend.
> >>
> >> Is there something I need to add to my driver to make this work, or
> >> should that part of the patch be reverted?
> > 
> > I believe it is quite common that a device driver calls
> > pm_runtime_get_sync as a part of it's remove callback, then it
> > explicitly returns it's resources that has been fetched during probe.
> > Like a clk_disable_unprepare for example.
> 
> I guess you mean the driver calls pm_runtime_get_sync _and_
> pm_runtime_put_sync as part of its remove callback?
> 
> Probably bus drivers need to do that, but for memory mapped devices in a
> SoC, I don't think there's normally any need to do
> pm_runtime_get/put_sync during the remove callback.
> 
> > The idea behind the change in __device_release_driver, was to try to
> > prevent  devices from going active->idle->active and instead just
> > remain active (if possible).
> >
> > In your case, which seems like a more modern way of implementing
> > "remove", you shall call "pm_runtime_suspend" to make sure the
> > runtime_suspend callbacks gets called.
> 
> And as far as I understand, the change creates an explicit requirement
> to do either pm_runtime_get/put_sync or pm_runtime_suspend inside
> driver's remove callback. If so, that should be mentioned in big red
> letters in the pm-runtime documentation.
> 
> The runtime_pm.txt doc does mention something related to this (and btw,
> the doc says pm_runtime_put_sync is being called, which is no longer
> true), but nothing clear about how the driver remove callback must be
> implemented.

That's correct.

> I tried grepping the kernel sources to find out if pm_runtime_suspend is
> widely used to get SoC platform devices to suspend, but it doesn't seem
> like it is. I didn't see pm_runtime_get/put_sync being used in remove
> callbacks widely either, but that was more difficult one to grep.

I think your observations are valid, which unfortunately means that we'll
need to revert the commit in question, because it has changed the behavior
that drivers are perfectly fine to expect given the existing documentation
etc.  It looks like the change was premature at least.

Greg, I wonder if you can queue up a revert of fa180eb448fa for 3.13, or
do you want me to do that?

Rafael

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