clk: add clk_rate_exclusive api

Using clock rate protection, we can now provide a way for clock consumer
to claim exclusive control over the rate of a producer

So far, rate change operations have been a "last write wins" affair. This
changes allows drivers to explicitly protect against this behavior, if
required.

Of course, if exclusivity over a producer is claimed more than once, the
rate is effectively locked as exclusivity cannot be preempted

Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20171201215200.23523-10-jbrunet@baylibre.com

include/linux/clk.h

@@ -331,6 +331,38 @@ struct clk *devm_clk_get(struct device *dev, const char *id);
  */
 struct clk *devm_get_clk_from_child(struct device *dev,
 				    struct device_node *np, const char *con_id);
+/**
+ * clk_rate_exclusive_get - get exclusivity over the rate control of a
+ *                          producer
+ * @clk: clock source
+ *
+ * This function allows drivers to get exclusive control over the rate of a
+ * provider. It prevents any other consumer to execute, even indirectly,
+ * opereation which could alter the rate of the provider or cause glitches
+ *
+ * If exlusivity is claimed more than once on clock, even by the same driver,
+ * the rate effectively gets locked as exclusivity can't be preempted.
+ *
+ * Must not be called from within atomic context.
+ *
+ * Returns success (0) or negative errno.
+ */
+int clk_rate_exclusive_get(struct clk *clk);
+
+/**
+ * clk_rate_exclusive_put - release exclusivity over the rate control of a
+ *                          producer
+ * @clk: clock source
+ *
+ * This function allows drivers to release the exclusivity it previously got
+ * from clk_rate_exclusive_get()
+ *
+ * The caller must balance the number of clk_rate_exclusive_get() and
+ * clk_rate_exclusive_put() calls.
+ *
+ * Must not be called from within atomic context.
+ */
+void clk_rate_exclusive_put(struct clk *clk);
 
 /**
  * clk_enable - inform the system when the clock source should be running.
@@ -472,6 +504,23 @@ long clk_round_rate(struct clk *clk, unsigned long rate);
  */
 int clk_set_rate(struct clk *clk, unsigned long rate);
 
+/**
+ * clk_set_rate_exclusive- set the clock rate and claim exclusivity over
+ *                         clock source
+ * @clk: clock source
+ * @rate: desired clock rate in Hz
+ *
+ * This helper function allows drivers to atomically set the rate of a producer
+ * and claim exclusivity over the rate control of the producer.
+ *
+ * It is essentially a combination of clk_set_rate() and
+ * clk_rate_exclusite_get(). Caller must balance this call with a call to
+ * clk_rate_exclusive_put()
+ *
+ * Returns success (0) or negative errno.
+ */
+int clk_set_rate_exclusive(struct clk *clk, unsigned long rate);
+
 /**
  * clk_has_parent - check if a clock is a possible parent for another
  * @clk: clock source
@@ -583,6 +632,14 @@ static inline void clk_bulk_put(int num_clks, struct clk_bulk_data *clks) {}
 
 static inline void devm_clk_put(struct device *dev, struct clk *clk) {}
 
+
+static inline int clk_rate_exclusive_get(struct clk *clk)
+{
+	return 0;
+}
+
+static inline void clk_rate_exclusive_put(struct clk *clk) {}
+
 static inline int clk_enable(struct clk *clk)
 {
 	return 0;
@@ -609,6 +666,11 @@ static inline int clk_set_rate(struct clk *clk, unsigned long rate)
 	return 0;
 }
 
+static inline int clk_set_rate_exclusive(struct clk *clk, unsigned long rate)
+{
+	return 0;
+}
+
 static inline long clk_round_rate(struct clk *clk, unsigned long rate)
 {
 	return 0;