netdev - Re: [PATCH v3 1/2] PM: runtime: Add pm_runtime_resume_and

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Message-ID: <CAJZ5v0iALE+oSXmJ7mWGCEG7MwFptfMwa-_SS8BusMUx7C7urA@mail.gmail.com>
Date:   Mon, 30 Nov 2020 20:12:12 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Zhang Qilong <zhangqilong3@...wei.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Fugang Duan <fugang.duan@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH v3 1/2] PM: runtime: Add pm_runtime_resume_and_get to deal
 with usage counter

On Mon, Nov 30, 2020 at 7:50 PM Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Rafael,
>
> On Mon, Nov 30, 2020 at 06:55:57PM +0100, Rafael J. Wysocki wrote:
> > On Mon, Nov 30, 2020 at 6:35 PM Laurent Pinchart wrote:
> > > On Mon, Nov 30, 2020 at 05:37:52PM +0100, Rafael J. Wysocki wrote:
> > > > On Fri, Nov 27, 2020 at 11:16 AM Geert Uytterhoeven wrote:
> > > > > On Tue, Nov 10, 2020 at 10:29 AM Zhang Qilong <zhangqilong3@...wei.com> wrote:
> > > > > > In many case, we need to check return value of pm_runtime_get_sync, but
> > > > > > it brings a trouble to the usage counter processing. Many callers forget
> > > > > > to decrease the usage counter when it failed, which could resulted in
> > > > > > reference leak. It has been discussed a lot[0][1]. So we add a function
> > > > > > to deal with the usage counter for better coding.
> > > > > >
> > > > > > [0]https://lkml.org/lkml/2020/6/14/88
> > > > > > [1]https://patchwork.ozlabs.org/project/linux-tegra/list/?series=178139
> > > > > > Signed-off-by: Zhang Qilong <zhangqilong3@...wei.com>
> > > > >
> > > > > Thanks for your patch, which is now commit dd8088d5a8969dc2 ("PM:
> > > > > runtime: Add pm_runtime_resume_and_get to deal with usage counter") in
> > > > > v5.10-rc5.
> > > > >
> > > > > > --- a/include/linux/pm_runtime.h
> > > > > > +++ b/include/linux/pm_runtime.h
> > > > > > @@ -386,6 +386,27 @@ static inline int pm_runtime_get_sync(struct device *dev)
> > > > > >         return __pm_runtime_resume(dev, RPM_GET_PUT);
> > > > > >  }
> > > > > >
> > > > > > +/**
> > > > > > + * pm_runtime_resume_and_get - Bump up usage counter of a device and resume it.
> > > > > > + * @dev: Target device.
> > > > > > + *
> > > > > > + * Resume @dev synchronously and if that is successful, increment its runtime
> > > > > > + * PM usage counter. Return 0 if the runtime PM usage counter of @dev has been
> > > > > > + * incremented or a negative error code otherwise.
> > > > > > + */
> > > > > > +static inline int pm_runtime_resume_and_get(struct device *dev)
> > > > >
> > > > > Perhaps this function should be called pm_runtime_resume_and_get_sync(),
> > > >
> > > > No, really.
> > > >
> > > > I might consider calling it pm_runtime_acquire(), and adding a
> > > > matching _release() as a pm_runtime_get() synonym for that matter, but
> > > > not the above.
> > >
> > > pm_runtime_acquire() seems better to me too. Would pm_runtime_release()
> > > would be an alias for pm_runtime_put() ?
> >
> > Yes.  This covers all of the use cases relevant for drivers AFAICS.
> >
> > > We would also likely need a pm_runtime_release_autosuspend() too then.
> >
> > Why would we?
> >
> > > But on that topic, I was wondering, is there a reason we can't select
> > > autosuspend behaviour automatically when autosuspend is enabled ?
> >
> > That is the case already.
> >
> > pm_runtime_put() will autosuspend if enabled and the usage counter is
> > 0, as long as ->runtime_idle() returns 0 (or is absent).
> >
> > pm_runtime_put_autosuspend() is an optimization allowing
> > ->runtime_idle() to be skipped entirely, but I'm wondering how many
> > users really need that.
>
> Ah, I didn't know that, that's good to know. We then don't need
> pm_runtime_release_autosuspend() (unless the optimization really makes a
> big difference).
>
> Should I write new drievr code with pm_runtime_put() instead of
> pm_runtime_put_autosuspend() ?

If you don't have ->runtime_idle() in the driver (and in the bus type
generally speaking, but none of them provide it IIRC),
pm_runtime_put() is basically equivalent to
pm_runtime_put_autosuspend() AFAICS, except for some extra checks done
by the former.

Otherwise it all depends on what the ->runtime_idle() callback does,
but it is hard to imagine a practical use case when the difference
would be really meaningful.

> I haven't found clear guidelines on this in the documentation.

Yes, that's one of the items I need to take care of.