ALSA: pcm: optimize xrun detection in no-period-wakeup mode

Add a lightweight condition on top of the xrun checking so that we can
avoid the division when the application is calling the update function
often enough.
Suggested-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_lib.c

@@ -380,6 +380,8 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
 		 * the elapsed time to detect xruns.
 		 */
 		jdelta = jiffies - runtime->hw_ptr_jiffies;
+		if (jdelta < runtime->hw_ptr_buffer_jiffies / 2)
+			goto no_delta_check;
 		hdelta = jdelta - delta * HZ / runtime->rate;
 		while (hdelta > runtime->hw_ptr_buffer_jiffies / 2 + 1) {
 			delta += runtime->buffer_size;