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

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Date:   Tue, 19 Jan 2021 21:38:22 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Kevin Hilman <khilman@...libre.com>,
        Nicolas Pitre <npitre@...libre.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Linux PM list <linux-pm@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM / clk: make PM clock layer compatible with clocks that
 must sleep

Hi Kevin, Nicolas,

On Tue, Jan 19, 2021 at 7:45 PM Kevin Hilman <khilman@...libre.com> wrote:
> [ + Geert.. renesas SoCs are the primary user of PM clk ]

Thanks!

> Nicolas Pitre <npitre@...libre.com> writes:
> > 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.
> >
> > Signed-off-by: Nicolas Pitre <npitre@...libre.com>

Thanks for your patch!

> > --- a/drivers/base/power/clock_ops.c
> > +++ b/drivers/base/power/clock_ops.c

> > +/**
> > + * pm_clk_op_lock - ensure exclusive access for performing clock operations.
> > + * @psd: pm_subsys_data instance corresponding to the PM clock entry list
> > + *    and clk_op_might_sleep count being used.
> > + * @flags: stored irq flags.
> > + * @fn: string for the caller function's name.
> > + *
> > + * This is used by pm_clk_suspend() and pm_clk_resume() to guard
> > + * against concurrent modifications to the clock entry list and the
> > + * clock_op_might_sleep count. If clock_op_might_sleep is != 0 then
> > + * only the mutex can be locked and those functions can only be used in
> > + * non atomic context. If clock_op_might_sleep == 0 then these functions
> > + * may be used in any context and only the spinlock can be locked.
> > + * Returns -EINVAL if called in atomic context when clock ops might sleep.
> > + */
> > +static int pm_clk_op_lock(struct pm_subsys_data *psd, unsigned long *flags,
> > +                       const char *fn)
> > +{
> > +     bool atomic_context = in_atomic() || irqs_disabled();

Is this safe? Cfr.
https://lore.kernel.org/dri-devel/20200914204209.256266093@linutronix.de/

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