ALSA: usb-audio: Prevent starting of audio stream if in use

With USB audio offloading, an audio session is started from the ASoC
platform sound card and PCM devices.  Likewise, the USB SND path is still
readily available for use, in case the non-offload path is desired.  In
order to prevent the two entities from attempting to use the USB bus,
introduce a flag that determines when either paths are in use.

If a PCM device is already in use, the check will return an error to
userspace notifying that the stream is currently busy.  This ensures that
only one path is using the USB substream.

Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409194804.3773260-12-quic_wcheng@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 27 insertions, 3 deletions

diff --git a/sound/usb/card.h b/sound/usb/card.h
index b65163c60176..cdafc9e9cecd 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -165,6 +165,7 @@ struct snd_usb_substream {
 	unsigned int pkt_offset_adj;	/* Bytes to drop from beginning of packets (for non-compliant devices) */
 	unsigned int stream_offset_adj;	/* Bytes to drop from beginning of stream (for non-compliant devices) */
 
+	unsigned int opened:1;		/* pcm device opened */
 	unsigned int running: 1;	/* running status */
 	unsigned int period_elapsed_pending;	/* delay period handling */
 
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 18467da6fd9e..b24ee38fad72 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -1241,8 +1241,17 @@ static int snd_usb_pcm_open(struct snd_pcm_substream *substream)
 	struct snd_usb_stream *as = snd_pcm_substream_chip(substream);
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	struct snd_usb_substream *subs = &as->substream[direction];
+	struct snd_usb_audio *chip = subs->stream->chip;
 	int ret;
 
+	mutex_lock(&chip->mutex);
+	if (subs->opened) {
+		mutex_unlock(&chip->mutex);
+		return -EBUSY;
+	}
+	subs->opened = 1;
+	mutex_unlock(&chip->mutex);
+
 	runtime->hw = snd_usb_hardware;
 	/* need an explicit sync to catch applptr update in low-latency mode */
 	if (direction == SNDRV_PCM_STREAM_PLAYBACK &&
@@ -1259,13 +1268,23 @@ static int snd_usb_pcm_open(struct snd_pcm_substream *substream)
 
 	ret = setup_hw_info(runtime, subs);
 	if (ret < 0)
-		return ret;
+		goto err_open;
 	ret = snd_usb_autoresume(subs->stream->chip);
 	if (ret < 0)
-		return ret;
+		goto err_open;
 	ret = snd_media_stream_init(subs, as->pcm, direction);
 	if (ret < 0)
-		snd_usb_autosuspend(subs->stream->chip);
+		goto err_resume;
+
+	return 0;
+
+err_resume:
+	snd_usb_autosuspend(subs->stream->chip);
+err_open:
+	mutex_lock(&chip->mutex);
+	subs->opened = 0;
+	mutex_unlock(&chip->mutex);
+
 	return ret;
 }
 
@@ -1274,6 +1293,7 @@ static int snd_usb_pcm_close(struct snd_pcm_substream *substream)
 	int direction = substream->stream;
 	struct snd_usb_stream *as = snd_pcm_substream_chip(substream);
 	struct snd_usb_substream *subs = &as->substream[direction];
+	struct snd_usb_audio *chip = subs->stream->chip;
 	int ret;
 
 	snd_media_stop_pipeline(subs);
@@ -1287,6 +1307,9 @@ static int snd_usb_pcm_close(struct snd_pcm_substream *substream)
 
 	subs->pcm_substream = NULL;
 	snd_usb_autosuspend(subs->stream->chip);
+	mutex_lock(&chip->mutex);
+	subs->opened = 0;
+	mutex_unlock(&chip->mutex);
 
 	return 0;
 }