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

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

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.