linux-kernel - Re: RFC: a failing pm_runtime

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Date:   Sun, 14 Jun 2020 12:00:21 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     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?

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?

> > > 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?

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

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds