[PATCH 5.14 280/334] ALSA: usb-audio: Add lowlatency module option

Greg Kroah-Hartman
2021-09-13T14:49:38Z

From: Takashi Iwai <tiwai@suse.de>

[ Upstream commit 4801bee7d5a36c199b734a28cde5259183aff822 ]

For making user to switch back to the old playback mode, this patch
adds a new module option 'lowlatency' to snd-usb-audio driver.
When user face a regression due to the recent low-latency playback
support, they can test easily by passing lowlatency=0 option without
rebuilding the kernel.

Fixes: 307cc9baac5c ("ALSA: usb-audio: Reduce latency at playback start, take#2")
Link: https://lore.kernel.org/r/20210829073830.22686-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
sound/usb/card.c | 4 ++++
sound/usb/pcm.c | 3 ++-
sound/usb/usbaudio.h | 1 +
3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/sound/usb/card.c b/sound/usb/card.c
index a1f8c3a026f5..6abfc9d079e7 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -68,6 +68,7 @@ static int pid[SNDRV_CARDS] = { [0 ... (SNDRV_CARDS-1)] = -1 };
static int device_setup[SNDRV_CARDS]; /* device parameter for this card */
static bool ignore_ctl_error;
static bool autoclock = true;
+static bool lowlatency = true;
static char *quirk_alias[SNDRV_CARDS];
static char *delayed_register[SNDRV_CARDS];
static bool implicit_fb[SNDRV_CARDS];
@@ -92,6 +93,8 @@ MODULE_PARM_DESC(ignore_ctl_error,
"Ignore errors from USB controller for mixer interfaces.");
module_param(autoclock, bool, 0444);
MODULE_PARM_DESC(autoclock, "Enable auto-clock selection for UAC2 devices (default: yes).");
+module_param(lowlatency, bool, 0444);
+MODULE_PARM_DESC(lowlatency, "Enable low latency playback (default: yes).");
module_param_array(quirk_alias, charp, NULL, 0444);
MODULE_PARM_DESC(quirk_alias, "Quirk aliases, e.g. 0123abcd:5678beef.");
module_param_array(delayed_register, charp, NULL, 0444);
@@ -599,6 +602,7 @@ static int snd_usb_audio_create(struct usb_interface *intf,
chip->setup = device_setup[idx];
chip->generic_implicit_fb = implicit_fb[idx];
chip->autoclock = autoclock;
+ chip->lowlatency = lowlatency;
atomic_set(&chip->active, 1); /* avoid autopm during probing */
atomic_set(&chip->usage_count, 0);
atomic_set(&chip->shutdown, 0);
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index f5cbf61ac366..5dc9266180e3 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -617,7 +617,8 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
/* check whether early start is needed for playback stream */
subs->early_playback_start =
subs->direction == SNDRV_PCM_STREAM_PLAYBACK &&
- subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes;
+ (!chip->lowlatency ||
+ (subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes));

if (subs->early_playback_start)
ret = start_endpoints(subs);
diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index 538831cbd925..8b70c9ea91b9 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -57,6 +57,7 @@ struct snd_usb_audio {
bool generic_implicit_fb; /* from the 'implicit_fb' module param */
bool autoclock; /* from the 'autoclock' module param */

+ bool lowlatency; /* from the 'lowlatency' module param */
struct usb_host_interface *ctrl_intf; /* the audio control interface */
struct media_device *media_dev;
struct media_intf_devnode *ctl_intf_media_devnode;
--
2.30.2