include/hw/irq.h: New function qemu_irq_is_connected()

Mostly devices don't need to care whether one of their output
qemu_irq lines is connected, because functions like qemu_set_irq()
silently do nothing if there is nothing on the other end.  However
sometimes a device might want to implement default behaviour for the
case where the machine hasn't wired the line up to anywhere.

Provide a function qemu_irq_is_connected() that devices can use for
this purpose.  (The test is trivial but encapsulating it in a
function makes it easier to see where we're doing it in case we need
to change the implementation later.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200728103744.6909-2-peter.maydell@linaro.org

include/hw/irq.h

@@ -55,4 +55,22 @@ qemu_irq qemu_irq_split(qemu_irq irq1, qemu_irq irq2);
    on an existing vector of qemu_irq.  */
 void qemu_irq_intercept_in(qemu_irq *gpio_in, qemu_irq_handler handler, int n);
 
+/**
+ * qemu_irq_is_connected: Return true if IRQ line is wired up
+ *
+ * If a qemu_irq has a device on the other (receiving) end of it,
+ * return true; otherwise return false.
+ *
+ * Usually device models don't need to care whether the machine model
+ * has wired up their outbound qemu_irq lines, because functions like
+ * qemu_set_irq() silently do nothing if there is nothing on the other
+ * end of the line. However occasionally a device model will want to
+ * provide default behaviour if its output is left floating, and
+ * it can use this function to identify when that is the case.
+ */
+static inline bool qemu_irq_is_connected(qemu_irq irq)
+{
+    return irq != NULL;
+}
+
 #endif