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    SubjectRe: 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@ti.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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