Re: [PATCH v4 3/3] cpufreq: Return failure if fast_switch is not set and fast_switch_possible is set

[Wyes Karny]

Hi Rafael,

On 24 May 19:44, Rafael J. Wysocki wrote:
> On Wed, May 17, 2023 at 6:30 PM Wyes Karny <wyes.karny@xxxxxxx> wrote:
> >
> > If fast_switch_possible flag is set by the scaling driver, the governor
> > is free to select fast_switch function even if adjust_perf is set.  Some
> > scaling drivers which use adjust_perf don't set fast_switch thinking
> > that the governor would never fall back to fast_switch. But the governor
> > can fall back to fast_switch even in runtime if frequency invariance is
> > disabled due to some reason. This could crash the kernel if the driver
> > didn't set the fast_switch function pointer.
> >
> > Therefore, return failure in cpufreq_online function if fast_switch is
> > not set and fast_switch_possible is set.
> >
> > Signed-off-by: Wyes Karny <wyes.karny@xxxxxxx>
> > ---
> >  drivers/cpufreq/cpufreq.c | 5 +++++
> >  include/linux/cpufreq.h   | 4 +++-
> >  2 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 6b52ebe5a890..7835ba4fa34c 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -1376,6 +1376,11 @@ static int cpufreq_online(unsigned int cpu)
> >                         goto out_free_policy;
> >                 }
> >
> > +               if (policy->fast_switch_possible && !cpufreq_driver->fast_switch) {
> > +                       pr_err("fast_switch_possible is enabled but fast_switch callback is not set\n");
> > +                       ret = -EINVAL;
> > +                       goto out_destroy_policy;
> > +               }
> 
> The driver registration can fail if the driver has ->adjust_perf
> without ->fast_switch.  Then the check above would not be necessary
> any more.

Sure. Will do that.

> 
> >                 /*
> >                  * The initialization has succeeded and the policy is online.
> >                  * If there is a problem with its frequency table, take it
> > diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> > index 26e2eb399484..8cdf77bb3bc1 100644
> > --- a/include/linux/cpufreq.h
> > +++ b/include/linux/cpufreq.h
> > @@ -340,7 +340,9 @@ struct cpufreq_driver {
> >         /*
> >          * ->fast_switch() replacement for drivers that use an internal
> >          * representation of performance levels and can pass hints other than
> > -        * the target performance level to the hardware.
> > +        * the target performance level to the hardware. If driver is setting this,
> > +        * then it needs to set fast_switch also. Because in certain scenario scale
> > +        * invariance could be disabled and governor can switch back to fast_switch.
> 
> I would say something like "This can only be set if ->fast_switch is
> set too, because in those cases (under specific conditions) scale
> invariance can be disabled, which causes the schedutil governor to
> fall back to the latter."

Sure. Will update and send the updated patch in-reply-to this.

Thanks & Regards,
Wyes

> 
> >          */
> >         void            (*adjust_perf)(unsigned int cpu,
> >                                        unsigned long min_perf,
> > --
> > 2.34.1
> >