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>
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