From c3bde2beacbcb40790f0e3c3f909f9e16bbced9c Mon Sep 17 00:00:00 2001 From: Henrik Enquist 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. Fixes: 597603d615d2 ("ALSA: introduce the snd-aloop module for the PCM loopback") Cc: stable@vger.kernel.org Signed-off-by: Henrik Enquist Link: https://patch.msgid.link/20261021-aloop-slave-controls-v1-1-62297727177f@gmail.com Signed-off-by: Takashi Iwai --- 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 d6a059ab3747..824d320b665b 100644 --- a/sound/drivers/aloop.c +++ b/sound/drivers/aloop.c @@ -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;