FAILED: patch "[PATCH] ALSA: aloop: Update the PCM Slave controls for a matching playback" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x c3bde2beacbcb40790f0e3c3f909f9e16bbced9c
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '[email protected]' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From c3bde2beacbcb40790f0e3c3f909f9e16bbced9c Mon Sep 17 00:00:00 2001
From: Henrik Enquist <[email protected]>
Date: Wed, 21 Oct 2026 09:46:45 +0200
Subject: [PATCH] ALSA: aloop: Update the PCM Slave controls for a matching
playback

The "PCM Slave" format, rate, channels and access controls report the parameters of the playback side of a cable. loopback_check_format() updates them when a playback starts, but returns early when a capture is open and its parameters match. The controls then keep the module defaults, or the parameters of an earlier stream.

A playback that joins an open capture with the same parameters is the normal case without notify, where it is constrained to the capture's parameters, and also with notify whenever the player happens to use them. An application that reads the controls to find the playback's parameters, for example to reopen the capture side at the right rate after a restart, then gets the wrong values.

Only skip the capture stop when the parameters match, and update the controls in every case. A value event is still sent only for a control whose value changes.

Reproducer:
arecord -D hw:Loopback,1,0 -r 44100 -f S16_LE -c 2 /dev/null & aplay -D hw:Loopback,0,0 -t raw -r 44100 -f S16_LE -c 2 /dev/zero & amixer -c Loopback cget "iface=PCM,name='PCM Slave Rate',device=1"
Before: the control reports 48000, the module default.
After: it reports 44100.

---
 sound/drivers/aloop.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

index d6a059ab37474..824d320b665bf 100644
@@ -412,11 +412,14 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
 			runtime->channels != cruntime->channels ||
 			is_access_interleaved(runtime->access) !=
 			is_access_interleaved(cruntime->access);
-			if (!check)
-				return 0;
-			if (stream == SNDRV_PCM_STREAM_CAPTURE)
+			/*
+			 * Matching params also go on to the update below, so that
+			 * the setup controls follow a playback that joins an open
+			 * capture.
+			 */
+			if (check && stream == SNDRV_PCM_STREAM_CAPTURE)
 				return -EIO;
-			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING) {
+			else if (check && cruntime->state == SNDRV_PCM_STATE_RUNNING) {
 				/* close must not free the peer runtime below */
 				snd_refcount_get(&cable->stop_count);
 				stop_capture = true;

2.53.0

From: Henrik Enquist <[email protected]>

[ Upstream commit c3bde2beacbcb40790f0e3c3f909f9e16bbced9c ]

The "PCM Slave" format, rate, channels and access controls report the parameters of the playback side of a cable. loopback_check_format() updates them when a playback starts, but returns early when a capture is open and its parameters match. The controls then keep the module defaults, or the parameters of an earlier stream.

A playback that joins an open capture with the same parameters is the normal case without notify, where it is constrained to the capture's parameters, and also with notify whenever the player happens to use them. An application that reads the controls to find the playback's parameters, for example to reopen the capture side at the right rate after a restart, then gets the wrong values.

Only skip the capture stop when the parameters match, and update the controls in every case. A value event is still sent only for a control whose value changes.

Reproducer:
arecord -D hw:Loopback,1,0 -r 44100 -f S16_LE -c 2 /dev/null & aplay -D hw:Loopback,0,0 -t raw -r 44100 -f S16_LE -c 2 /dev/zero & amixer -c Loopback cget "iface=PCM,name='PCM Slave Rate',device=1"
Before: the control reports 48000, the module default.
After: it reports 44100.

---
 sound/drivers/aloop.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

index fb507076e006f..b2df37d63ad73 100644
@@ -351,12 +351,15 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
 		check = runtime->format != cruntime->format ||
 			runtime->rate != cruntime->rate ||
 			runtime->channels != cruntime->channels;
-		if (!check)
-			goto unlock;
-		if (stream == SNDRV_PCM_STREAM_CAPTURE) {
+		/*
+		 * Matching params also go on to the update below, so that
+		 * the setup controls follow a playback that joins an open
+		 * capture.
+		 */
+		if (check && stream == SNDRV_PCM_STREAM_CAPTURE) {
 			err = -EIO;
 			goto unlock;
-		} else if (snd_pcm_running(dpcm_capt->substream)) {
+		} else if (check && snd_pcm_running(dpcm_capt->substream)) {
 			/* close must not free the peer runtime below */
 			atomic_inc(&cable->stop_count);
 			stop_capture = true;

2.53.0