Re: [PATCH 3/4] ACPI: processor idle: Only flush cache on entering C3

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Dec 9, 2021 at 2:33 PM Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> On Wed, Dec 08, 2021 at 05:26:12PM +0100, Rafael J. Wysocki wrote:
> > On Mon, Dec 6, 2021 at 4:03 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Mon, Dec 06, 2021 at 03:29:51PM +0300, Kirill A. Shutemov wrote:
> > > > According to the ACPI spec v6.4, section 8.2, cache flushing required
> > > > on entering C3 power state.
> > > >
> > > > Avoid flushing cache on entering other power states.
> > > >
> > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > > ---
> > > >  drivers/acpi/processor_idle.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> > > > index 76ef1bcc8848..01495aca850e 100644
> > > > --- a/drivers/acpi/processor_idle.c
> > > > +++ b/drivers/acpi/processor_idle.c
> > > > @@ -567,7 +567,8 @@ static int acpi_idle_play_dead(struct cpuidle_device *dev, int index)
> > > >  {
> > > >       struct acpi_processor_cx *cx = per_cpu(acpi_cstate[index], dev->cpu);
> > > >
> > > > -     ACPI_FLUSH_CPU_CACHE();
> > > > +     if (cx->type == ACPI_STATE_C3)
> > > > +             ACPI_FLUSH_CPU_CACHE();
> > > >
> > >
> > > acpi_idle_enter() already does this, acpi_idle_enter_s2idle() has it
> > > confused again,
> >
> > No, they do the same thing: acpi_idle_enter_bm() if flags.bm_check is set.
> >
> > > Also, I think acpi_idle_enter() does it too late; consider
> > > acpi_idle_enter_mb(). Either that or the BM crud needs more comments.
> >
> > I think the latter.
> >
> > Evidently, acpi_idle_play_dead(() doesn't support FFH and the BM
> > thing, so it is only necessary to flush the cache when using
> > ACPI_CSTATE_SYSTEMIO and when cx->type is C3.
>
> I'm new to this and not completely follow what I need to change.
>
> Does it look correct?

It does, but I liked the original one more and so that one has been
applied as 5.17 material (with some edits in the changelog).

Thanks!

> From 3c544bc95a16d6a23dcb0aa50ee905d5e97c9ce5 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Date: Thu, 9 Dec 2021 16:24:44 +0300
> Subject: [PATCH] ACPI: processor idle: Only flush cache on entering C3
>
> According to the ACPI spec v6.4, section 8.2, cache flushing required
> on entering C3 power state.
>
> Avoid flushing cache on entering other power states.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  drivers/acpi/processor_idle.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index 76ef1bcc8848..d2a4d4446eff 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -567,7 +567,9 @@ static int acpi_idle_play_dead(struct cpuidle_device *dev, int index)
>  {
>         struct acpi_processor_cx *cx = per_cpu(acpi_cstate[index], dev->cpu);
>
> -       ACPI_FLUSH_CPU_CACHE();
> +       if (cx->entry_method == ACPI_CSTATE_SYSTEMIO &&
> +           cx->type == ACPI_STATE_C3)
> +               ACPI_FLUSH_CPU_CACHE();
>
>         while (1) {
>
> --
>  Kirill A. Shutemov