clk: Add clk_get_first_to_set_rate

The purpose of this function is to be used along with the notifier
mechanism.

When a parent clock can see its rate externally changed at any moment,
and a driver needs a specific clock rate to function, it can register a
notifier on the parent clock, and call clk_set_rate() on the base clock
to adjust its frequency according to the new parent clock.

This works fine, until the base clock has the CLK_SET_RATE_PARENT flag
set. In that case, calling clk_set_rate() on the base clock will call
clk_set_rate() on the parent clock, which will trigger the notifier
again, and we're in a loop.

For that reason, we need to register the notifier on the parent clock of
the first ancestor of the base clock that will effectively modify its
rate when clk_set_rate() is called, which we can now obtain with
clk_get_first_to_set_rate().

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

drivers/clk/clk.c

@@ -2429,6 +2429,15 @@ struct clk *clk_get_parent(struct clk *clk)
 }
 EXPORT_SYMBOL_GPL(clk_get_parent);
 
+struct clk *clk_get_first_to_set_rate(struct clk *clk)
+{
+	while (clk && (clk->core->flags & CLK_SET_RATE_PARENT))
+		clk = clk_get_parent(clk);
+
+	return clk;
+}
+
+
 static struct clk_core *__clk_init_parent(struct clk_core *core)
 {
 	u8 index = 0;

include/linux/clk.h

@@ -744,6 +744,17 @@ struct clk *clk_get_parent(struct clk *clk);
  */
 struct clk *clk_get_sys(const char *dev_id, const char *con_id);
 
+/**
+ * clk_get_first_to_set_rate - get a pointer to the clock that will
+ *   effectively modify its rate when clk_set_rate(clk) is called
+ *   (might be clk itself, or any ancestor)
+ * @clk: clock source
+ *
+ * Returns struct clk corresponding to the matched clock source, or
+ * NULL on error.
+ */
+struct clk *clk_get_first_to_set_rate(struct clk *clk);
+
 /**
  * clk_save_context - save clock context for poweroff
  *
@@ -906,6 +917,11 @@ static inline struct clk *clk_get_parent(struct clk *clk)
 	return NULL;
 }
 
+static inline struct clk *clk_get_first_to_set_rate(struct clk *clk)
+{
+	return NULL;
+}
+
 static inline struct clk *clk_get_sys(const char *dev_id, const char *con_id)
 {
 	return NULL;