linux-kernel - Re: Async runtime put in __device_release

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Message-ID: <d067c3bd-40e6-4691-ae4a-a853566f9c13@email.android.com>
Date:	Wed, 06 Nov 2013 23:48:24 +0100
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Alan Stern <stern@...land.harvard.edu>
CC:	Tomi Valkeinen <tomi.valkeinen@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.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()



"Rafael J. Wysocki" <rjw@...ysocki.net> skrev:
>On Wednesday, November 06, 2013 05:02:12 PM Alan Stern wrote:
>> On Wed, 6 Nov 2013, Rafael J. Wysocki wrote:
>> 
>> > 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?
>> 
>> Would it be better to leave the runtime-idle callbacks (invoked
>during
>> probe) async and revert only the change to __device_release_driver()?
>> 
>> Having an async callback after probe shouldn't cause problems,
>because 
>> the driver will then be bound (assuming the probe succeeded).
>
>Right.  OK, I'll prepare a patch.

That seems like a good way forward. Also I appoligize for not updating the doc as part of the original patch.

Kind regards
Ulf Hansson

>
>Thanks,
>Rafael

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