Re: [PATCH v2 1/7] cpufreq: move invariance setter calls in cpufreq core

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi guys,

On Thursday 30 Jul 2020 at 09:11:28 (+0530), Viresh Kumar wrote:
> On 27-07-20, 15:48, Rafael J. Wysocki wrote:
> > On Wed, Jul 22, 2020 at 11:38 AM Ionela Voinescu
> > <ionela.voinescu@arm.com> wrote:
> > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > > index 036f4cc42ede..bac4101546db 100644
> > > --- a/drivers/cpufreq/cpufreq.c
> > > +++ b/drivers/cpufreq/cpufreq.c
> > > @@ -2058,9 +2058,16 @@ EXPORT_SYMBOL(cpufreq_unregister_notifier);
> > >  unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
> > >                                         unsigned int target_freq)
> > >  {
> > > +       unsigned int freq;
> > > +
> > >         target_freq = clamp_val(target_freq, policy->min, policy->max);
> > > +       freq = cpufreq_driver->fast_switch(policy, target_freq);
> > > +
> > > +       if (freq)
> > > +               arch_set_freq_scale(policy->related_cpus, freq,
> > > +                                   policy->cpuinfo.max_freq);
> > 
> > Why can't arch_set_freq_scale() handle freq == 0?
> 

Sorry, I seem to have missed this question the first time around.

arch_set_freq_scale() could handle freq == 0, but given that freq == 0
is signaling an error here, I do believe this check is well placed, to
prevent a useless call to arch_set_freq_scale(). Also [1]:

"""
 * If 0 is returned by the driver's ->fast_switch() callback to indicate an
 * error condition, the hardware configuration must be preserved.
 */
"""

> Actually there is no need to. AFAIU the freq returned by fast_switch
> can never be 0 (yeah qcom driver does it right now and I am fixing
> it). And so we can drop this check altogether.
> 

It's not only the qcom driver, it's also the scmi driver that could
return 0 [2]. But I don't think "fixing" these drivers is the solution,
given that 0 is indicated as a valid return value of .fast_switch() to
signal an error condition [1], while schedutil (the caller), also does
validation that the returned frequency is !0 before setting it as
current frequency [3].

Therefore, it is know and (somewhat) documented that 0 indicates an
error condition and it should be allowed as a return value for
.fast_switch(). Also, I believe is a good idea to leave the option for
drivers to return 0 (signaling error) from their implementation of
.fast_switch().

[1] https://elixir.bootlin.com/linux/v5.8-rc4/source/drivers/cpufreq/cpufreq.c#L2043
[2] https://elixir.bootlin.com/linux/v5.8-rc4/source/drivers/cpufreq/scmi-cpufreq.c#L76
[3] https://elixir.bootlin.com/linux/v5.8-rc4/source/kernel/sched/cpufreq_schedutil.c#L124

Thanks,
Ionela.

> -- 
> viresh