linux-kernel - Re: RFC: a failing pm_runtime

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Message-ID: <CAJZ5v0i87NGcy9+kxubScdPDyByr8ypQWcGgBFn+V-wDd69BHQ@mail.gmail.com>
Date:   Sun, 14 Jun 2020 15:59:15 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Wolfram Sang <wsa@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-i2c <linux-i2c@...r.kernel.org>
Subject: Re: RFC: a failing pm_runtime_get increases the refcnt?

On Sun, Jun 14, 2020 at 12:00 PM Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
>
> Hi Andy,
>
> On Sun, Jun 14, 2020 at 11:43 AM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> > On Sun, Jun 14, 2020 at 12:34 PM Andy Shevchenko
> > <andy.shevchenko@...il.com> wrote:
> > >
> > > On Sun, Jun 14, 2020 at 12:10 PM Wolfram Sang <wsa@...nel.org> wrote:
> > > > both in the I2C subsystem and also for Renesas drivers I maintain, I am
> > > > starting to get boilerplate patches doing some pm_runtime_put_* variant
> > > > because a failing pm_runtime_get is supposed to increase the ref
> > > > counters? Really? This feels wrong and unintuitive to me.
> > >
> > > Yeah, that is a well known issue with PM (I even have for a long time
> > > a coccinelle script, when I realized myself that there are a lot of
> > > cases like this, but someone else discovered this recently, like
> > > opening a can of worms).
> > >
> > > > I expect there
> > > > has been a discussion around it but I couldn't find it.
> > >
> > > Rafael explained (again) recently this. I can't find it quickly, unfortunately.
> >
> > I _think_ this discussion, but may be it's simple another tentacle of
> > the same octopus.
> > https://patchwork.ozlabs.org/project/linux-tegra/patch/20200520095148.10995-1-dinghao.liu@zju.edu.cn/
>
> Thanks, hadn't read that one! (so I was still at -1 from
> http://sweng.the-davies.net/Home/rustys-api-design-manifesto ;-)
>
> So "pm_runtime_put_noidle()" is the (definitive?) one to pair with a
> pm_runtime_get_sync() failure?

If you bail out immediately on errors, then yes, it is.

If you'd rather to something like

        ret = pm_runtime_get_sync(dev);
        if (ret < 0)
               goto fail;

        ... code depending on PM ...

fail:
       pm_runtime_put_autosuspend(dev);

then it will still work correctly.

It actually doesn't matter which pm_runtime_put*() variant you call
after a pm_runtime_get_sync() failure, but the _noidle() is the
simplest one and it is sufficient.

> > > > I wonder why we
> > > > don't fix the code where the incremented refcount is expected for some
> > > > reason.
> > >
> > > The main idea behind API that a lot of drivers do *not* check error
> > > codes from runtime PM, so, we need to keep balance in case of
> > >
> > > pm_runtime_get(...);
> > > ...
> > > pm_runtime_put(...);
>
> I've always[*] considered a pm_runtime_get_sync() failure to be fatal
> (or: cannot happen), and that there's nothing that can be done to
> recover.  Hence I never checked the function's return value.
> Was that wrong?

No, it wasn't.  It is the right thing to do in the majority of cases.

> [*] at least on Renesas SoCs with Clock and/or Power Domains.

Cheers!