ALSA: pcm: Skip ack callback without actual appl_ptr update

We call ack callback whenever appl_ptr gets updated via
pcm_lib_apply_appl_ptr().  There are various code paths to call this
function.  A part of them are for read/write/forward/rewind, where the
appl_ptr is always changed and thus the call of ack is mandatory.
OTOH, another part of code paths are from the explicit user call,
e.g. via SYNC_PTR ioctl.  There, we may receive the same appl_ptr
value, and in such a case, calling ack is obviously superfluous.

This patch adds the check of the given appl_ptr value, and returns
immediately if it's no real update.
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_lib.c

@@ -2112,6 +2112,9 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
 	snd_pcm_uframes_t old_appl_ptr = runtime->control->appl_ptr;
 	int ret;
 
+	if (old_appl_ptr == appl_ptr)
+		return 0;
+
 	runtime->control->appl_ptr = appl_ptr;
 	if (substream->ops->ack) {
 		ret = substream->ops->ack(substream);