hw/ptimer: Support "on the fly" timer mode switch

Allow switching between periodic <-> oneshot modes while timer is running.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: f030be6.1464367869.git.digetx@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/core/ptimer.c

@@ -163,16 +163,17 @@ void ptimer_set_count(ptimer_state *s, uint64_t count)
 
 void ptimer_run(ptimer_state *s, int oneshot)
 {
-    if (s->enabled) {
-        return;
-    }
-    if (s->period == 0) {
+    bool was_disabled = !s->enabled;
+
+    if (was_disabled && s->period == 0) {
         fprintf(stderr, "Timer with period zero, disabling\n");
         return;
     }
     s->enabled = oneshot ? 2 : 1;
-    s->next_event = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
-    ptimer_reload(s);
+    if (was_disabled) {
+        s->next_event = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
+        ptimer_reload(s);
+    }
 }
 
 /* Pause a timer.  Note that this may cause it to "lose" time, even if it