Hi,

On Mon, Jun 29, 2015 at 09:26:21AM -0700, Michael Turquette wrote:

Quoting Felipe Balbi (2015-06-26 17:48:31)

quoted

Hi,

On Fri, Jun 26, 2015 at 04:53:42PM -0700, Michael Turquette wrote:

quoted

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