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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 12 May 2023 17:04:07 +0200
Message-ID: <CAJZ5v0j14X2SweUds+dT3pGsWC79xGR6s8jguPseFVLmftK9wg@mail.gmail.com>
Subject: Re: [PATCH] driver core: Call pm_runtime_put_sync() only after device_remove()
To:     Johan Hovold <johan@kernel.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, kernel@pengutronix.de
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On Fri, May 12, 2023 at 5:00 PM Johan Hovold <johan@kernel.org> wrote:
>
> On Fri, May 12, 2023 at 04:04:59PM +0200, Rafael J. Wysocki wrote:
> > On Fri, May 12, 2023 at 9:39 AM Johan Hovold <johan@kernel.org> wrote:
> > >
> > > On Thu, May 11, 2023 at 04:44:25PM +0200, Rafael J. Wysocki wrote:
> > > > On Thu, May 11, 2023 at 1:48 PM Johan Hovold <johan@kernel.org> wrote:
> > >
> > > > > No, this seems like very bad idea and even violates the documentation
> > > > > which clearly states that the usage counter is balanced before calling
> > > > > remove() so that drivers can use pm_runtime_suspend() to put devices
> > > > > into suspended state.
> > > >
> > > > I missed that, sorry.
> > > >
> > > > > There's is really no good reason to even try to change as this is in no
> > > > > way a fast path.
> > > >
> > > > Still, I think that while the "put" part needs to be done before
> > > > device_remove(), the actual state change can be carried out later.
> > > >
> > > > So something like
> > > >
> > > >     pm_runtime_put_noidle(dev);
> > > >
> > > >     device_remove(dev);
> > > >
> > > >     pm_runtime_suspend(dev);
> > > >
> > > > would generally work, wouldn't it?
> > >
> > > No, as drivers typically disable runtime pm in their remove callbacks,
> >
> > What exactly do you mean by "typically"?  None of the PCI drivers
> > should do that, for instance.
>
> I had platform drivers in mind, but so do i2c drivers for example.
>
> > > that pm_runtime_suspend() would amount to a no-op (and calling the
> > > driver pm ops post unbind and the driver having freed its data would
> > > not work either).
> >
> > Well, not really.
> >
> > There are drivers and there are bus types/PM domains.  Drivers need
> > not disable PM-runtime in their "remove" callbacks if they know that
> > the bus type/PM domain will take care of handling PM-runtime properly
> > after the driver's remove callback has run and the bus type/PM domain
> > may very well want its PM-runtime suspend callback to run then (for
> > example, to remove power from the unused device).  Arguably it can
> > invoke runtime_suspend() from its "remove" callback, so it's not like
> > this is a big deal, but IMO it helps if the most general case is
> > considered.
>
> My point was that hundreds of drivers do and for these this call becomes
> a no-op. Same for buses that disable runtime pm at remove.
>
> > Anyway, the question here really is: Does it make sense to carry out a
> > runtime suspend immediately before device_remove()?  Honestly, I'm not
> > sure about that.
>
> I'd say it doesn't really matter as driver unbind is not a common
> operation and drivers using autosuspend would generally not be affected
> either.
>
> You can try to rework this, but clearly it needs more thought than
> simply moving the put sync and some drivers may also be relying on the
> current behaviour.
>
> A quick grep reveals a few which would be left active if you change
> pm_runtime_put_sync() to pm_runtime_put_noidle(), even if that could be
> fixed driver by driver of course.

OK, fair enough.