Re: Async runtime put in __device_release_driver()

[Ulf Hansson]

On 7 November 2013 02:05, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> wrote:
>> > On Wednesday, November 06, 2013 11:48:24 PM Ulf Hansson wrote:
>> >>
>> >> "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx> 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@xxxxxx> 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@xxxxxxxxx>
>> > 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@xxxxxx>
>> > Fixes: fa180eb448fa (PM / Runtime: Idle devices asynchronously after probe|release)
>> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>> > Cc: 3.10+ <stable@xxxxxxxxxxxxxxx> # 3.10+
>>
>> Acked-by: Kevin Hilman <khilman@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>

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

Kind regards
Ulf Hansson

>
> Thanks!
>
>
>> > ---
>> >  drivers/base/dd.c |    2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > Index: linux-pm/drivers/base/dd.c
>> > ===================================================================
>> > --- linux-pm.orig/drivers/base/dd.c
>> > +++ linux-pm/drivers/base/dd.c
>> > @@ -499,7 +499,7 @@ static void __device_release_driver(stru
>> >                                                      BUS_NOTIFY_UNBIND_DRIVER,
>> >                                                      dev);
>> >
>> > -               pm_runtime_put(dev);
>> > +               pm_runtime_put_sync(dev);
>> >
>> >                 if (dev->bus && dev->bus->remove)
>> >                         dev->bus->remove(dev);
>> >
>> --
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> --
> I speak only for myself.
> Rafael J. Wysocki, Intel Open Source Technology Center.
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