Re: [PATCH] cpufreq: intel_pstate: Override parameters if HWP forced by BIOS

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

On Thu, Sep 9, 2021 at 3:20 PM Doug Smythies <dsmythies@telus.net> wrote:
>
> On Thu, Sep 9, 2021 at 4:18 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
> >
> > On Thu, Sep 9, 2021 at 8:52 AM Srinivas Pandruvada
> > <srinivas.pandruvada@linux.intel.com> wrote:
> > >
> > > On Wed, 2021-09-08 at 20:48 -0700, Doug Smythies wrote:
> > > > If HWP has been already been enabled by BIOS, it may be
> > > > necessary to override some kernel command line parameters.
> > > > Once it has been enabled it requires a reset to be disabled.
> > > >
> > > > Signed-off-by: Doug Smythies <dsmythies@telus.net>
> > > > ---
> > > >  drivers/cpufreq/intel_pstate.c | 22 ++++++++++++++++------
> > > >  1 file changed, 16 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/drivers/cpufreq/intel_pstate.c
> > > > b/drivers/cpufreq/intel_pstate.c
> > > > index bb4549959b11..073bae5d4498 100644
> > > > --- a/drivers/cpufreq/intel_pstate.c
> > > > +++ b/drivers/cpufreq/intel_pstate.c
> > > > @@ -3267,7 +3267,7 @@ static int __init intel_pstate_init(void)
> > > >                  */
> > > >                 if ((!no_hwp && boot_cpu_has(X86_FEATURE_HWP_EPP)) ||
> > > >                     intel_pstate_hwp_is_enabled()) {
> > > > -                       hwp_active++;
> > > > +                       hwp_active = 1;
> > > Why this change?
> >
> > I think hwp_active can be changed to bool and then it would make sense
> > to update this line.
> >
> > > >                         hwp_mode_bdw = id->driver_data;
> > > >                         intel_pstate.attr = hwp_cpufreq_attrs;
> > > >                         intel_cpufreq.attr = hwp_cpufreq_attrs;
> > > > @@ -3347,17 +3347,27 @@ device_initcall(intel_pstate_init);
> > > >
> > > >  static int __init intel_pstate_setup(char *str)
> > > >  {
> > > > +       /*
> > > > +        * If BIOS is forcing HWP, then parameter
> > > > +        * overrides might be needed. Only print
> > > > +        * the message once, and regardless of
> > > > +        * any overrides.
> > > > +        */
> > > > +       if(!hwp_active
> > > This part of code is from early_param, Is it possible that
> > > hwp_active is not 0?
> >
> > Well, it wouldn't matter even if it were nonzero.  This check is just
> > pointless anyway.
> >
> > > > && boot_cpu_has(X86_FEATURE_HWP))
> > > > +               if(intel_pstate_hwp_is_enabled()){
> >
> > This should be
> >
> > if (boot_cpu_has(X86_FEATURE_HWP) && intel_pstate_hwp_is_enabled()) {
>
> Disagree.
> This routine gets executed once per intel_pstate related grub command
> line entry. The purpose of the "if(!hwp_active" part is to prevent the
> printing of the message to the logs multiple times.

Ah OK.  Fair enough.

You can do all of the checks in one conditional, though.  They will be
processed left-to-right anyway.

But then it would be good to avoid calling
intel_pstate_hwp_is_enabled() multiple times if it returns false.

And having said all that I'm not sure why you are trying to make
no_hwp depend on !hwp_active?  I will not be taken into account anyway
if intel_pstate_hwp_is_enabled() returns 'true'?

So if no_hwp is covered regardless, you may move the
intel_pstate_hwp_is_enabled() inside the no_load conditional.

Alternatively, and I would do that, intel_pstate_hwp_is_enabled()
could be evaluated earlier in intel_pstate_init() and if it returned
'true', both no_load and no_hwp would be disregarded.