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 > > <[email protected]> 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
