Re: [PATCH v5 4/4] platform/x86/intel/pmc: core: Report duration of time in HW sleep state

[Limonciello, Mario]

On 4/3/2023 13:00, Box, David E wrote:
> On Fri, 2023-03-31 at 20:05 +0200, Rafael J. Wysocki wrote:
> > On Thu, Mar 30, 2023 at 9:45 PM Mario Limonciello
> > <mario.limonciello@xxxxxxx> wrote:
> > > intel_pmc_core displays a warning when the module parameter
> > > `warn_on_s0ix_failures` is set and a suspend didn't get to a HW sleep
> > > state.
> > >
> > > Report this to the standard kernel reporting infrastructure so that
> > > userspace software can query after the suspend cycle is done.
> > >
> > > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > > ---
> > > v4->v5:
> > >   * Reword commit message
> > > ---
> > >   drivers/platform/x86/intel/pmc/core.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > >
> > > diff --git a/drivers/platform/x86/intel/pmc/core.c
> > > b/drivers/platform/x86/intel/pmc/core.c
> > > index e2f171fac094..980af32dd48a 100644
> > > --- a/drivers/platform/x86/intel/pmc/core.c
> > > +++ b/drivers/platform/x86/intel/pmc/core.c
> > > @@ -1203,6 +1203,8 @@ static inline bool pmc_core_is_s0ix_failed(struct
> > > pmc_dev *pmcdev)
> > >          if (pmc_core_dev_state_get(pmcdev, &s0ix_counter))
> > >                  return false;
> > >
> > > +       pm_set_hw_sleep_time(s0ix_counter - pmcdev->s0ix_counter);
> > > +
> >
> > Maybe check if this is really accumulating?  In case of a counter
> > overflow, for instance?
>
> Overflow is likely on some systems. The counter is only 32-bit and at our
> smallest granularity of 30.5us per tick it could overflow after a day and a half
> of s0ix time, though most of our systems have a higher granularity that puts
> them around 6 days.
>
> This brings up an issue that the attribute cannot be trusted if the system is
> suspended for longer than the maximum hardware counter time. Should be noted in
> the Documentation.

I think it would be rather confusing for userspace having to account for 
this and it's better to abstract it in the kernel.

How can you discover the granularity a system can support?
How would you know overflow actually happened?  Is there a bit somewhere 
else that could tell you?

In terms of ABI how about when we know overflow occurred and userspace 
reads the sysfs file we return -EOVERFLOW instead of a potentially bad 
value?