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: 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>

diff --git a/sound/usb/card.c b/sound/usb/card.c
index a1f8c3a..6abfc9d 100644 (file)
--- 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 f5cbf61..5dc9266 100644 (file)
--- 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 538831c..8b70c9e 100644 (file)
--- 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;