clock: Add new clock_has_source() function

Add a function for checking whether a clock has a source.  This is
useful for devices which have input clocks that must be wired up by
the board as it allows them to fail in realize rather than ploughing
on with a zero-period clock.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210128114145.20536-3-peter.maydell@linaro.org
Message-id: 20210121190622.22000-3-peter.maydell@linaro.org

include/hw/clock.h

@@ -139,6 +139,21 @@ void clock_clear_callback(Clock *clk);
  */
 void clock_set_source(Clock *clk, Clock *src);
 
+/**
+ * clock_has_source:
+ * @clk: the clock
+ *
+ * Returns true if the clock has a source clock connected to it.
+ * This is useful for devices which have input clocks which must
+ * be connected by the board/SoC code which creates them. The
+ * device code can use this to check in its realize method that
+ * the clock has been connected.
+ */
+static inline bool clock_has_source(const Clock *clk)
+{
+    return clk->source != NULL;
+}
+
 /**
  * clock_set:
  * @clk: the clock to initialize.