[PATCH 6.6.y] ALSA: aloop: Update the PCM Slave controls for a matching playback

[Sasha Levin <sashal@kernel.org>]

From: Henrik Enquist <henrik.enquist@gmail.com>

[ 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.

Fixes: 597603d615d2 ("ALSA: introduce the snd-aloop module for the PCM loopback")
Cc: stable@vger.kernel.org
Signed-off-by: Henrik Enquist <henrik.enquist@gmail.com>
Link: https://patch.msgid.link/20261021-aloop-slave-controls-v1-1-62297727177f@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[ sashal: Adjusted loopback_check_format() context to account for missing access-mode comparisons. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 sound/drivers/aloop.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
index 9c7671f034d54..29959455fe072 100644
--- a/sound/drivers/aloop.c
+++ b/sound/drivers/aloop.c
@@ -346,11 +346,14 @@ 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)
-				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 */
 				atomic_inc(&cable->stop_count);
 				stop_capture = true;
-- 
2.53.0