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: 307cc9b ("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>

Author: tiwai

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);

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);

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;