Re: [PATCH 2/6] cpuidle: Introduce CPUIDLE_FLAG_MWAIT

[Sudeep Holla <sudeep.holla@arm.com>]

On Tue, Jan 14, 2025 at 03:01:26PM +0100, Rafael J. Wysocki wrote:
> On Thu, Jan 2, 2025 at 4:02 PM Frederic Weisbecker <frederic@kernel.org> wrote:
> >
> > From: Peter Zijlstra <peterz@infradead.org>
> >
> > Provide a way to tell the cpuidle core about states monitoring
> > TIF_NEED_RESCHED on the hardware level, monitor/mwait users being the
> > only examples in use.
> >
> > This will allow cpuidle core to manage TIF_NR_POLLING on behalf of all
> > kinds of TIF_NEED_RESCHED watching states while keeping a necessary
> > distinction for the governors between software loops polling on
> > TIF_NEED_RESCHED and hardware monitored writes to thread flags.
> >
> > [fweisbec: _ Initialize flag from acpi_processor_setup_cstates() instead
> >              of acpi_processor_setup_lpi_states(), as the latter seem to
> >              be about arm64...
> >            _ Rename CPUIDLE_FLAG_NO_IPI to CPUIDLE_FLAG_MWAIT]
> >
> > Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > ---
> >  drivers/acpi/processor_idle.c | 3 +++
> >  drivers/idle/intel_idle.c     | 5 ++++-
> >  include/linux/cpuidle.h       | 1 +
> >  3 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> > index 698897b29de2..66cb5536d91e 100644
> > --- a/drivers/acpi/processor_idle.c
> > +++ b/drivers/acpi/processor_idle.c
> > @@ -806,6 +806,9 @@ static int acpi_processor_setup_cstates(struct acpi_processor *pr)
> >                 if (cx->type == ACPI_STATE_C1 || cx->type == ACPI_STATE_C2)
> >                         drv->safe_state_index = count;
> >
> > +               if (cx->entry_method == ACPI_CSTATE_FFH)
> > +                       state->flags |= CPUIDLE_FLAG_MWAIT;
> 
> FFH need not mean MWAIT in principle.
> 
> FFH in _CST means MWAIT in practice because _CST is used on x86 which
> implements FFH through MWAIT, but it would be good at least to put a
> comment here to explain that this code is only expected to run on x86.
> 
> Or better still, add something like acpi_arch_idle_state_flags(u8
> entry_method) that will return CPUIDLE_FLAG_MWAIT for ACPI_CSTATE_FFH
> and 0 otherwise and then do
> 
>     state->flags |= acpi_arch_idle_state_flags(cx->entry_method);
>

+1, was about to suggest the same. Though I am not aware of any Arm platforms
using C-States(LPI was added to suit Arm requirements), it is better to keep
the FFH definition arch specific.


-- 
Regards,
Sudeep