Patch "ALSA: pcm: Workaround for weird PulseAudio behavior on rewind error" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

ALSA: pcm: Workaround for weird PulseAudio behavior on rewind error

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
alsa-pcm-workaround-for-weird-pulseaudio-behavior-on-rewind-error.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Takashi Iwai <[email protected]>
Date: Wed, 3 Jan 2018 15:16:30 +0100
Subject: ALSA: pcm: Workaround for weird PulseAudio behavior on rewind error

From: Takashi Iwai <[email protected]>

commit fb51f1cd06f9ced7b7085a2a4636375d520431ca upstream.

The commit 9027c4639ef1 ("ALSA: pcm: Call ack() whenever appl_ptr is updated") introduced the possible error code returned from the PCM rewind ioctl. Basically the change was for handling the indirect PCM more correctly, but ironically, it caused rather a side-effect:
PulseAudio gets pissed off when receiving an error from rewind, throws everything away and stops processing further, resulting in the silence.

It's clearly a failure in the application side, so the best would be to fix that bug in PA. OTOH, PA is mostly the only user of the rewind feature, so it's not good to slap the sole customer.

This patch tries to mitigate the situation: instead of returning an error, now the rewind ioctl returns zero when the driver can't rewind. It indicates that no rewind was performed, so the behavior is consistent, at least.

---
 sound/core/pcm_native.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

@@ -2582,7 +2582,7 @@ static snd_pcm_sframes_t forward_appl_pt
 	return ret < 0 ? ret : frames;
 }
 
-/* decrease the appl_ptr; returns the processed frames or a negative error */
+/* decrease the appl_ptr; returns the processed frames or zero for error */
 static snd_pcm_sframes_t rewind_appl_ptr(struct snd_pcm_substream *substream,
 					 snd_pcm_uframes_t frames,
 					 snd_pcm_sframes_t avail)

@@ -2599,7 +2599,12 @@ static snd_pcm_sframes_t rewind_appl_ptr
 	if (appl_ptr < 0)
 		appl_ptr += runtime->boundary;
 	ret = pcm_lib_apply_appl_ptr(substream, appl_ptr);
-	return ret < 0 ? ret : frames;
+	/* NOTE: we return zero for errors because PulseAudio gets depressed
+	 * upon receiving an error from rewind ioctl and stops processing
+	 * any longer.  Returning zero means that no rewind is done, so
+	 * it's not absolutely wrong to answer like that.
+	 */
+	return ret < 0 ? 0 : frames;
 }
 
 static snd_pcm_sframes_t snd_pcm_playback_rewind(struct snd_pcm_substream *substream,

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/alsa-pcm-remove-incorrect-snd_bug_on-usages.patch queue-4.14/alsa-aloop-fix-inconsistent-format-due-to-incomplete-rule.patch queue-4.14/alsa-pcm-workaround-for-weird-pulseaudio-behavior-on-rewind-error.patch queue-4.14/alsa-pcm-add-missing-error-checks-in-oss-emulation-plugin-builder.patch queue-4.14/alsa-pcm-abort-properly-at-pending-signal-in-oss-read-write-loops.patch queue-4.14/alsa-pcm-allow-aborting-mutex-lock-at-oss-read-write-loops.patch queue-4.14/alsa-aloop-fix-racy-hw-constraints-adjustment.patch queue-4.14/alsa-aloop-release-cable-upon-open-error-path.patch