Re: [PATCH v3 2/4] cpufreq: introduce cpufreq_driver_might_sleep

[Felipe Balbi]

Hi,

On Mon, Jun 29, 2015 at 09:26:21AM -0700, Michael Turquette wrote:
> Quoting Felipe Balbi (2015-06-26 17:48:31)
> > Hi,
> > 
> > On Fri, Jun 26, 2015 at 04:53:42PM -0700, Michael Turquette wrote:
> > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > > index 28e59a4..e5296c0 100644
> > > --- a/drivers/cpufreq/cpufreq.c
> > > +++ b/drivers/cpufreq/cpufreq.c
> > > @@ -112,6 +112,12 @@ bool have_governor_per_policy(void)
> > >  }
> > >  EXPORT_SYMBOL_GPL(have_governor_per_policy);
> > >  
> > > +bool cpufreq_driver_might_sleep(void)
> > > +{
> > > +     return !(cpufreq_driver->flags & CPUFREQ_DRIVER_WILL_NOT_SLEEP);
> > > +}
> > > +EXPORT_SYMBOL_GPL(cpufreq_driver_might_sleep);
> > > +
> > >  struct kobject *get_governor_parent_kobj(struct cpufreq_policy *policy)
> > >  {
> > >       if (have_governor_per_policy())
> > > diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> > > index 2ee4888..1f2c9a1 100644
> > > --- a/include/linux/cpufreq.h
> > > +++ b/include/linux/cpufreq.h
> > > @@ -157,6 +157,7 @@ u64 get_cpu_idle_time(unsigned int cpu, u64 *wall, int io_busy);
> > >  int cpufreq_get_policy(struct cpufreq_policy *policy, unsigned int cpu);
> > >  int cpufreq_update_policy(unsigned int cpu);
> > >  bool have_governor_per_policy(void);
> > > +bool cpufreq_driver_might_sleep(void);
> > >  struct kobject *get_governor_parent_kobj(struct cpufreq_policy *policy);
> > >  #else
> > >  static inline unsigned int cpufreq_get(unsigned int cpu)
> > > @@ -314,6 +315,14 @@ struct cpufreq_driver {
> > >   */
> > >  #define CPUFREQ_NEED_INITIAL_FREQ_CHECK      (1 << 5)
> > >  
> > > +/*
> > > + * Set by drivers that will never block or sleep during their frequency
> > > + * transition. Used to indicate when it is safe to call cpufreq_driver_target
> > > + * from non-interruptable context. Drivers must opt-in to this flag, as the
> > > + * safe default is that they might sleep.
> > > + */
> > > +#define CPUFREQ_DRIVER_WILL_NOT_SLEEP        (1 << 6)
> > 
> > don't you need to update current drivers and pass this flag where
> > necessary ?
> 
> Felipe,
> 
> Thanks for the review.
> 
> Setting the flag can be done, but it is an opt-in feature. First, none
> of the legacy cpufreq governors would actually make use of this flag.
> Everything they do is in process context. The first potential user of it
> is in patch #3.
> 
> Secondly, the governor in patch #3 will work without this flag set for a
> cpufreq driver. It will just defer the dvfs transition to a kthread
> instead of performing it in the hot path of the scheduler.
> 
> Finally, the only hardware I am aware of that can make use of this flag
> is Intel hardware. I know nothing about it and am happy for someone more
> knowledgeable than myself submit a patch enabling this flag for that
> architecture.

the follow-up question would be: then why introduce the flag at all ?
:-p

-- 
balbi

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