linux-kernel - [PATCH 5.15 003/207] ALSA: usb-audio: Disable low-latency playback for free-wheel mode

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Message-Id: <20211206145610.297918781@linuxfoundation.org>
Date:   Mon,  6 Dec 2021 15:54:17 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Takashi Iwai <tiwai@...e.de>
Subject: [PATCH 5.15 003/207] ALSA: usb-audio: Disable low-latency playback for free-wheel mode

From: Takashi Iwai <tiwai@...e.de>

commit e581f1cec4f899f788f6c9477f805b1d5fef25e2 upstream.

The free-wheel stream operation like dmix may not update the appl_ptr
appropriately, and it doesn't fit with the low-latency playback mode.
Disable the low-latency playback operation when the stream is set up
in such a mode.

Link: https://lore.kernel.org/r/20210929080844.11583-5-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 sound/usb/pcm.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -582,7 +582,8 @@ static int snd_usb_hw_free(struct snd_pc
 }
 
 /* check whether early start is needed for playback stream */
-static int lowlatency_playback_available(struct snd_usb_substream *subs)
+static int lowlatency_playback_available(struct snd_pcm_runtime *runtime,
+					 struct snd_usb_substream *subs)
 {
 	struct snd_usb_audio *chip = subs->stream->chip;
 
@@ -591,6 +592,9 @@ static int lowlatency_playback_available
 	/* disabled via module option? */
 	if (!chip->lowlatency)
 		return false;
+	/* free-wheeling mode? (e.g. dmix) */
+	if (runtime->stop_threshold > runtime->buffer_size)
+		return false;
 	/* too short periods? */
 	if (subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes)
 		return false;
@@ -630,7 +634,7 @@ static int snd_usb_pcm_prepare(struct sn
 	subs->period_elapsed_pending = 0;
 	runtime->delay = 0;
 
-	subs->lowlatency_playback = lowlatency_playback_available(subs);
+	subs->lowlatency_playback = lowlatency_playback_available(runtime, subs);
 	if (!subs->lowlatency_playback)
 		ret = start_endpoints(subs);