[RFC PATCH] PM / devfreq: Add policy notifier

[Matthias Kaehlcke]

Policy notifiers are called before a frequency change and may adjust the
frequency, similar to CPUFREQ_ADJUST. The purpose of policy notifiers is
to support non-thermal throttling of devfreq devices.

As of now notifiers may only select a lower frequency, but not increase it.
The reason for this limitation is that devfreq currently doesn't have an
actual policy for frequencies and OPPs > freq might be disabled by a
devfreq cooling device (see drivers/thermal/devfreq_cooling.c).

Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
---
Marked as RFC since I'm aware that this isn't quite a 'policy' notifier,
but I also didn't want to make it too specific. Maybe devfreq devices should
have a policy similar to cpufreq?

Should devfreq core be in charge of disabling/enabling OPPs instead of
the thermal cooling device? Typically we don't want to use higher OPPs
than those whitelisted by thermal, so in practice the current situation
shouldn't be much of an issue.

 drivers/devfreq/devfreq.c | 20 ++++++++++++++++----
 include/linux/devfreq.h   |  6 ++++++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index fe2af6aa88fc..a7294c056f65 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -273,6 +273,9 @@ int update_devfreq(struct devfreq *devfreq)
 	if (err)
 		return err;
 
+	srcu_notifier_call_chain(&devfreq->policy_notifier_list,
+			DEVFREQ_ADJUST, &freq);
+
 	/*
 	 * Adjust the frequency with user freq, QoS and available freq.
 	 *
@@ -640,6 +643,7 @@ struct devfreq *devfreq_add_device(struct device *dev,
 	devfreq->last_stat_updated = jiffies;
 
 	srcu_init_notifier_head(&devfreq->transition_notifier_list);
+	srcu_init_notifier_head(&devfreq->policy_notifier_list);
 
 	mutex_unlock(&devfreq->lock);
 
@@ -1414,7 +1418,7 @@ EXPORT_SYMBOL(devm_devfreq_unregister_opp_notifier);
  * devfreq_register_notifier() - Register a driver with devfreq
  * @devfreq:	The devfreq object.
  * @nb:		The notifier block to register.
- * @list:	DEVFREQ_TRANSITION_NOTIFIER.
+ * @list:	DEVFREQ_TRANSITION_NOTIFIER or DEVFREQ_POLICY_NOTIFIER.
  */
 int devfreq_register_notifier(struct devfreq *devfreq,
 				struct notifier_block *nb,
@@ -1430,6 +1434,10 @@ int devfreq_register_notifier(struct devfreq *devfreq,
 		ret = srcu_notifier_chain_register(
 				&devfreq->transition_notifier_list, nb);
 		break;
+	case DEVFREQ_POLICY_NOTIFIER:
+		ret = srcu_notifier_chain_register(
+				&devfreq->policy_notifier_list, nb);
+		break;
 	default:
 		ret = -EINVAL;
 	}
@@ -1442,7 +1450,7 @@ EXPORT_SYMBOL(devfreq_register_notifier);
  * devfreq_unregister_notifier() - Unregister a driver with devfreq
  * @devfreq:	The devfreq object.
  * @nb:		The notifier block to be unregistered.
- * @list:	DEVFREQ_TRANSITION_NOTIFIER.
+ * @list:	DEVFREQ_TRANSITION_NOTIFIER or DEVFREQ_POLICY_NOTIFIER.
  */
 int devfreq_unregister_notifier(struct devfreq *devfreq,
 				struct notifier_block *nb,
@@ -1458,6 +1466,10 @@ int devfreq_unregister_notifier(struct devfreq *devfreq,
 		ret = srcu_notifier_chain_unregister(
 				&devfreq->transition_notifier_list, nb);
 		break;
+	case DEVFREQ_POLICY_NOTIFIER:
+		ret = srcu_notifier_chain_unregister(
+				&devfreq->policy_notifier_list, nb);
+		break;
 	default:
 		ret = -EINVAL;
 	}
@@ -1485,7 +1497,7 @@ static void devm_devfreq_notifier_release(struct device *dev, void *res)
  * @dev:	The devfreq user device. (parent of devfreq)
  * @devfreq:	The devfreq object.
  * @nb:		The notifier block to be unregistered.
- * @list:	DEVFREQ_TRANSITION_NOTIFIER.
+ * @list:	DEVFREQ_TRANSITION_NOTIFIER or DEVFREQ_POLICY_NOTIFIER.
  */
 int devm_devfreq_register_notifier(struct device *dev,
 				struct devfreq *devfreq,
@@ -1521,7 +1533,7 @@ EXPORT_SYMBOL(devm_devfreq_register_notifier);
  * @dev:	The devfreq user device. (parent of devfreq)
  * @devfreq:	The devfreq object.
  * @nb:		The notifier block to be unregistered.
- * @list:	DEVFREQ_TRANSITION_NOTIFIER.
+ * @list:	DEVFREQ_TRANSITION_NOTIFIER or DEVFREQ_POLICY_NOTIFIER.
  */
 void devm_devfreq_unregister_notifier(struct device *dev,
 				struct devfreq *devfreq,
diff --git a/include/linux/devfreq.h b/include/linux/devfreq.h
index 3aae5b3af87c..8edc538d78f7 100644
--- a/include/linux/devfreq.h
+++ b/include/linux/devfreq.h
@@ -33,6 +33,10 @@
 #define	DEVFREQ_PRECHANGE		(0)
 #define DEVFREQ_POSTCHANGE		(1)
 
+#define DEVFREQ_POLICY_NOTIFIER		1
+
+#define	DEVFREQ_ADJUST			0
+
 struct devfreq;
 struct devfreq_governor;
 
@@ -136,6 +140,7 @@ struct devfreq_dev_profile {
  * @time_in_state:	Statistics of devfreq states
  * @last_stat_updated:	The last time stat updated
  * @transition_notifier_list: list head of DEVFREQ_TRANSITION_NOTIFIER notifier
+ * @policy_notifier_list: list head of DEVFREQ_POLICY_NOTIFIER notifier
  *
  * This structure stores the devfreq information for a give device.
  *
@@ -174,6 +179,7 @@ struct devfreq {
 	unsigned long last_stat_updated;
 
 	struct srcu_notifier_head transition_notifier_list;
+	struct srcu_notifier_head policy_notifier_list;
 };
 
 struct devfreq_freqs {
-- 
2.17.0.441.gb46fe60e1d-goog