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") Signed-off-by: Takashi Iwai tiwai@suse.de --- 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 cf8f3953f78f..fd570a42f043 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]; @@ -93,6 +94,8 @@ 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); @@ -623,6 +626,7 @@ 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 94261d19cceb..167834133b9b 100644 --- a/sound/usb/usbaudio.h +++ b/sound/usb/usbaudio.h @@ -55,6 +55,7 @@ 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;