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

docs/devel/clocks.rst

@@ -235,6 +235,22 @@ object during device instance init. For example:
     /* set initial value to 10ns / 100MHz */
     clock_set_ns(clk, 10);
 
+To enforce that the clock is wired up by the board code, you can
+call ``clock_has_source()`` in your device's realize method:
+
+.. code-block:: c
+
+   if (!clock_has_source(s->clk)) {
+       error_setg(errp, "MyDevice: clk input must be connected");
+       return;
+   }
+
+Note that this only checks that the clock has been wired up; it is
+still possible that the output clock connected to it is disabled
+or has not yet been configured, in which case the period will be
+zero. You should use the clock callback to find out when the clock
+period changes.
+
 Fetching clock frequency/period
 -------------------------------
 

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.