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ALSA: usb-audio: Add lowlatency module option

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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: 307cc9baac ("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>
This commit is contained in:
Takashi Iwai 2021-08-29 09:38:30 +02:00
parent 4267c5a8f3
commit 4801bee7d5
3 changed files with 7 additions and 1 deletions
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4
sound/usb/card.c
View file

@ -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);

3
sound/usb/pcm.c
View file

@ -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);

1
sound/usb/usbaudio.h
View file

@ -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;