clk: add clk_round_rate()

It returns the rate which will be set if you ask clk_set_rate() to set
that rate. It provides a way to query exactly what rate you'll get if
you call clk_set_rate() with that same argument.
So essentially, clk_round_rate() and clk_set_rate() are equivalent
except the former does not modify the clock hardware in any way.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>

include/clk.h

@@ -366,6 +366,29 @@ struct clk *clk_get_parent(struct clk *clk);
  */
 long long clk_get_parent_rate(struct clk *clk);
 
+/**
+ * clk_round_rate() - Adjust a rate to the exact rate a clock can provide
+ *
+ * This answers the question "if I were to pass @rate to clk_set_rate(),
+ * what clock rate would I end up with?" without changing the hardware
+ * in any way.  In other words:
+ *
+ *   rate = clk_round_rate(clk, r);
+ *
+ * and:
+ *
+ *   rate = clk_set_rate(clk, r);
+ *
+ * are equivalent except the former does not modify the clock hardware
+ * in any way.
+ *
+ * @clk: A clock struct that was previously successfully requested by
+ *       clk_request/get_by_*().
+ * @rate: desired clock rate in Hz.
+ * @return rounded rate in Hz, or -ve error code.
+ */
+ulong clk_round_rate(struct clk *clk, ulong rate);
+
 /**
  * clk_set_rate() - Set current clock rate.
  *
@@ -482,6 +505,11 @@ static inline long long clk_get_parent_rate(struct clk *clk)
 	return -ENOSYS;
 }
 
+static inline ulong clk_round_rate(struct clk *clk, ulong rate)
+{
+	return -ENOSYS;
+}
+
 static inline ulong clk_set_rate(struct clk *clk, ulong rate)
 {
 	return -ENOSYS;