Commit 4801bee7 authored by Takashi Iwai's avatar Takashi Iwai
Browse files

ALSA: usb-audio: Add lowlatency module option 

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: 307cc9ba ("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: default avatarTakashi Iwai <tiwai@suse.de>
parent 4267c5a8

sound/usb/card.c

+4 −0
Original line number Diff line number Diff line
@@ -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);

sound/usb/pcm.c

+2 −1
Original line number Diff line number Diff line
@@ -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);

sound/usb/usbaudio.h

+1 −0
Original line number Diff line number Diff line
@@ -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;