linux-kernel - Re: [PATCH] driver core: Call pm_runtime_put_sync() only after device

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Date:   Fri, 12 May 2023 16:04:59 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH] driver core: Call pm_runtime_put_sync() only after device_remove()

On Fri, May 12, 2023 at 9:39 AM Johan Hovold <johan@...nel.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@...nel.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.

> 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.

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.