SystemClock: use clock_gettime() on devices without /dev/alarm

On devices with an up-to-date kernel, the back-in-time bug affecting
clock_gettime() has been fixed and it can safely be used as an
alternative to the ANDROID_ALARM_GET_TIME ioctl.  To ensure consistent
behavior on existing devices, make clock_gettime() a fallback for when
the alarm driver isn't available.

Change-Id: I384af5e7ec9e73e0bad4b6b0f987a8ea4583cba6
Signed-off-by: Greg Hackmann <ghackmann@google.com>

libutils/SystemClock.cpp

@@ -119,22 +119,6 @@ int64_t elapsedRealtimeNano()
     static volatile int prevMethod;
 #endif
 
-#if 0
-    /*
-     * b/7100774
-     * clock_gettime appears to have clock skews and can sometimes return
-     * backwards values. Disable its use until we find out what's wrong.
-     */
-    result = clock_gettime(CLOCK_BOOTTIME, &ts);
-    if (result == 0) {
-        timestamp = seconds_to_nanoseconds(ts.tv_sec) + ts.tv_nsec;
-        checkTimeStamps(timestamp, &prevTimestamp, &prevMethod,
-                        METHOD_CLOCK_GETTIME);
-        return timestamp;
-    }
-#endif
-
-    // CLOCK_BOOTTIME doesn't exist, fallback to /dev/alarm
     static int s_fd = -1;
 
     if (s_fd == -1) {
@@ -153,6 +137,15 @@ int64_t elapsedRealtimeNano()
         return timestamp;
     }
 
+    // /dev/alarm doesn't exist, fallback to CLOCK_BOOTTIME
+    result = clock_gettime(CLOCK_BOOTTIME, &ts);
+    if (result == 0) {
+        timestamp = seconds_to_nanoseconds(ts.tv_sec) + ts.tv_nsec;
+        checkTimeStamps(timestamp, &prevTimestamp, &prevMethod,
+                        METHOD_CLOCK_GETTIME);
+        return timestamp;
+    }
+
     // XXX: there was an error, probably because the driver didn't
     // exist ... this should return
     // a real error, like an exception!