linux-kernel - Re: [PATCH v4] PM / clk: make PM clock layer compatible with clocks that must sleep

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Date:   Thu, 11 Feb 2021 08:55:07 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Nicolas Pitre <npitre@...libre.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Turquette <mturquette@...libre.com>,
        Russell King <linux@...linux.org.uk>,
        Linux PM <linux-pm@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v4] PM / clk: make PM clock layer compatible with clocks
 that must sleep

Hi Stephen,

On Thu, Feb 11, 2021 at 4:35 AM Stephen Boyd <sboyd@...nel.org> wrote:
> Quoting Nicolas Pitre (2021-01-25 11:29:18)
> > The clock API splits its interface into sleepable ant atomic contexts:
> >
> > - clk_prepare/clk_unprepare for stuff that might sleep
> >
> > - clk_enable_clk_disable for anything that may be done in atomic context
> >
> > The code handling runtime PM for clocks only calls clk_disable() on
> > suspend requests, and clk_enable on resume requests. This means that
> > runtime PM with clock providers that only have the prepare/unprepare
> > methods implemented is basically useless.
> >
> > Many clock implementations can't accommodate atomic contexts. This is
> > often the case when communication with the clock happens through another
> > subsystem like I2C or SCMI.
> >
> > Let's make the clock PM code useful with such clocks by safely invoking
> > clk_prepare/clk_unprepare upon resume/suspend requests. Of course, when
> > such clocks are registered with the PM layer then pm_runtime_irq_safe()
> > can't be used, and neither pm_runtime_suspend() nor pm_runtime_resume()
> > may be invoked in atomic context.
> >
> > For clocks that do implement the enable and disable methods then
> > everything just works as before.
> >
> > A note on sparse:
> > According to https://lwn.net/Articles/109066/ there are things
> > that sparse can't cope with. In particular, pm_clk_op_lock() and
> > pm_clk_op_unlock() may or may not lock/unlock psd->lock depending on
> > some runtime condition. To work around that we tell it the lock is
> > always untaken for the purpose of static analisys.
> >
> > Thanks to Naresh Kamboju for reporting issues with the initial patch.
> >
> > Signed-off-by: Nicolas Pitre <npitre@...libre.com>
> > Tested-by: Naresh Kamboju <naresh.kamboju@...aro.org>
> >
> > ---
>
> Thanks for doing this. I think it's the main reason why nobody uses the
> PM clock code so far.

"git grep pm_clk_add" tells you otherwise?

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