linux-kernel - [PATCH 5.15 004/207] ALSA: usb-audio: Disable low-latency mode for implicit feedback sync

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Date:   Mon,  6 Dec 2021 15:54:18 +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 5.15 004/207] ALSA: usb-audio: Disable low-latency mode for implicit feedback sync

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

commit bceee75387554f682638e719d1ea60125ea78cea upstream.

When a playback stream runs in the implicit feedback mode, its
operation is passive and won't start unless the capture packet is
received.  This behavior contradicts with the low-latency playback
mode, and we should turn off lowlatency_playback flag accordingly.

In theory, we may take the low-latency mode when the playback-first
quirk is set, but it still conflicts with the later operation with the
fixed packet numbers, so it's disabled all together for now.

Link: https://lore.kernel.org/r/20210929080844.11583-6-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 sound/usb/pcm.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -595,6 +595,9 @@ static int lowlatency_playback_available
 	/* free-wheeling mode? (e.g. dmix) */
 	if (runtime->stop_threshold > runtime->buffer_size)
 		return false;
+	/* implicit feedback mode has own operation mode */
+	if (snd_usb_endpoint_implicit_feedback_sink(subs->data_endpoint))
+		return false;
 	/* too short periods? */
 	if (subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes)
 		return false;