clk: Introduce clk_has_parent()

This new function is similar to clk_set_parent(), except that it doesn't
actually change the parent. It merely checks that the given parent clock
can be a parent for the given clock.

A situation where this is useful is to check that a particular setup is
valid before switching to it. One specific use-case for this is atomic
modesetting in the DRM framework where setting a mode is divided into a
check phase where a given configuration is validated before applying
changes to the hardware.

Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>

include/linux/clk.h

@@ -301,6 +301,18 @@ long clk_round_rate(struct clk *clk, unsigned long rate);
  */
 int clk_set_rate(struct clk *clk, unsigned long rate);
 
+/**
+ * clk_has_parent - check if a clock is a possible parent for another
+ * @clk: clock source
+ * @parent: parent clock source
+ *
+ * This function can be used in drivers that need to check that a clock can be
+ * the parent of another without actually changing the parent.
+ *
+ * Returns true if @parent is a possible parent for @clk, false otherwise.
+ */
+bool clk_has_parent(struct clk *clk, struct clk *parent);
+
 /**
  * clk_set_parent - set the parent clock source for this clock
  * @clk: clock source
@@ -374,6 +386,11 @@ static inline long clk_round_rate(struct clk *clk, unsigned long rate)
 	return 0;
 }
 
+static inline bool clk_has_parent(struct clk *clk, struct clk *parent)
+{
+	return true;
+}
+
 static inline int clk_set_parent(struct clk *clk, struct clk *parent)
 {
 	return 0;