cpufreq: add support for intermediate (stable) frequencies

Douglas Anderson, recently pointed out an interesting problem due to which
udelay() was expiring earlier than it should.

While transitioning between frequencies few platforms may temporarily switch to
a stable frequency, waiting for the main PLL to stabilize.

For example: When we transition between very low frequencies on exynos, like
between 200MHz and 300MHz, we may temporarily switch to a PLL running at 800MHz.
No CPUFREQ notification is sent for that. That means there's a period of time
when we're running at 800MHz but loops_per_jiffy is calibrated at between 200MHz
and 300MHz. And so udelay behaves badly.

To get this fixed in a generic way, introduce another set of callbacks
get_intermediate() and target_intermediate(), only for drivers with
target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.

get_intermediate() should return a stable intermediate frequency platform wants
to switch to, and target_intermediate() should set CPU to that frequency,
before jumping to the frequency corresponding to 'index'. Core will take care of
sending notifications and driver doesn't have to handle them in
target_intermediate() or target_index().

NOTE: ->target_index() should restore to policy->restore_freq in case of
failures as core would send notifications for that.

Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 3f458896d45c..ec4112d257bc 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -75,6 +75,7 @@ struct cpufreq_policy {
 	unsigned int		max;    /* in kHz */
 	unsigned int		cur;    /* in kHz, only needed if cpufreq
 					 * governors are used */
+	unsigned int		restore_freq; /* = policy->cur before transition */
 	unsigned int		suspend_freq; /* freq to set during suspend */
 
 	unsigned int		policy; /* see above */
@@ -221,11 +222,35 @@ struct cpufreq_driver {
 
 	/* define one out of two */
 	int	(*setpolicy)	(struct cpufreq_policy *policy);
+
+	/*
+	 * On failure, should always restore frequency to policy->restore_freq
+	 * (i.e. old freq).
+	 */
 	int	(*target)	(struct cpufreq_policy *policy,	/* Deprecated */
 				 unsigned int target_freq,
 				 unsigned int relation);
 	int	(*target_index)	(struct cpufreq_policy *policy,
 				 unsigned int index);
+	/*
+	 * Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION
+	 * unset.
+	 *
+	 * get_intermediate should return a stable intermediate frequency
+	 * platform wants to switch to and target_intermediate() should set CPU
+	 * to to that frequency, before jumping to the frequency corresponding
+	 * to 'index'. Core will take care of sending notifications and driver
+	 * doesn't have to handle them in target_intermediate() or
+	 * target_index().
+	 *
+	 * Drivers can return '0' from get_intermediate() in case they don't
+	 * wish to switch to intermediate frequency for some target frequency.
+	 * In that case core will directly call ->target_index().
+	 */
+	unsigned int (*get_intermediate)(struct cpufreq_policy *policy,
+					 unsigned int index);
+	int	(*target_intermediate)(struct cpufreq_policy *policy,
+				       unsigned int index);
 
 	/* should be defined, if possible */
 	unsigned int	(*get)	(unsigned int cpu);