linux-kernel - Re: Async runtime put in __device_release

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Date:	Wed, 6 Nov 2013 17:02:12 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
cc:	Tomi Valkeinen <tomi.valkeinen@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	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 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).

Alan Stern

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