Re: Async runtime put in __device_release_driver()

[Kevin Hilman]

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.

Kevin

> ---
>  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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