Message-ID: <2120140.KNmcSko2m0@vostro.rjw.lan>
Date:	Thu, 07 Nov 2013 19:55:55 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	Kevin Hilman <khilman@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	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 Thursday, November 07, 2013 09:18:52 AM Ulf Hansson wrote:
> On 7 November 2013 02:05, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > On Wednesday, November 06, 2013 04:21:48 PM Kevin Hilman wrote:
> >> On Wed, Nov 6, 2013 at 4:16 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >> > On Wednesday, November 06, 2013 11:48:24 PM Ulf Hansson wrote:
> >> >>
> >> >> "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.
> >> >
> >> > There you go.
> >> >
> >> > ---
> >> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >> > Subject: PM / runtime: Use pm_runtime_put_sync() in __device_release_driver()
> >> >
> >> > Commit fa180eb448fa (PM / Runtime: Idle devices asynchronously after
> >> > probe|release) modified __device_release_driver() to call
> >> > pm_runtime_put(dev) instead of pm_runtime_put_sync(dev) before
> >> > detaching the driver from the device.  However, that was a mistake,
> >> > because pm_runtime_put(dev) causes rpm_idle() to be queued up and
> >> > the driver may be gone already when that function is executed.
> >> > That breaks the assumptions the drivers have the right to make
> >> > about the core's behavior on the basis of the existing documentation
> >> > and actually causes problems to happen, so revert that part of
> >> > commit fa180eb448fa and restore the previous behavior of
> >> > __device_release_driver().
> >> >
> >> > Reported-by: Tomi Valkeinen <tomi.valkeinen@...com>
> >> > Fixes: fa180eb448fa (PM / Runtime: Idle devices asynchronously after probe|release)
> >> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >> > Cc: 3.10+ <stable@...r.kernel.org> # 3.10+
> >>
> >> Acked-by: Kevin Hilman <khilman@...aro.org>
> >>
> >> I like this fix since I don't want to add any more requirements to drivers.
> 
> Agree!
> 
> >
> > OK, I've queued this up for 3.13.
> 
> If not to late:
> 
> Acked-by: Ulf Hansson <ulf.hansson@...aro.org>

No, it isn't, thanks!

> BTW, I start creating a patch on the doc to align it to the changes
> that the "async" patches made.

I've seen it, please address the Alan's comments in that patch.

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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