ALSA: usb-audio: UAC2: auto clock selection module param

Add a module param to disable auto clock selection.
This is provided for users that expect the audio stream to
fail when the clock source is invalid (e.g., the word clock
was unintentionally disconnected).
Signed-off-by: Eldad Zack <eldad@fogrefinery.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.c

@@ -82,6 +82,7 @@ static int pid[SNDRV_CARDS] = { [0 ... (SNDRV_CARDS-1)] = -1 };
 static int nrpacks = 8;		/* max. number of packets per urb */
 static int device_setup[SNDRV_CARDS]; /* device parameter for this card */
 static bool ignore_ctl_error;
+static bool autoclock = true;
 
 module_param_array(index, int, NULL, 0444);
 MODULE_PARM_DESC(index, "Index value for the USB audio adapter.");
@@ -100,6 +101,8 @@ MODULE_PARM_DESC(device_setup, "Specific device setup (if needed).");
 module_param(ignore_ctl_error, bool, 0444);
 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).");
 
 /*
  * we keep the snd_usb_audio_t instances by ourselves for merging
@@ -354,6 +357,7 @@ static int snd_usb_audio_create(struct usb_device *dev, int idx,
 	chip->card = card;
 	chip->setup = device_setup[idx];
 	chip->nrpacks = nrpacks;
+	chip->autoclock = autoclock;
 	chip->probing = 1;
 
 	chip->usb_id = USB_ID(le16_to_cpu(dev->descriptor.idVendor),

sound/usb/clock.c

@@ -217,7 +217,7 @@ static int __uac_clock_find_source(struct snd_usb_audio *chip,
 		cur = ret;
 		ret = __uac_clock_find_source(chip, selector->baCSourceID[ret - 1],
 					       visited, validate);
-		if (!validate || ret > 0)
+		if (!validate || ret > 0 || !chip->autoclock)
 			return ret;
 
 		/* The current clock source is invalid, try others. */

sound/usb/usbaudio.h

@@ -56,6 +56,7 @@ struct snd_usb_audio {
 
 	int setup;			/* from the 'device_setup' module param */
 	int nrpacks;			/* from the 'nrpacks' module param */
+	bool autoclock;			/* from the 'autoclock' module param */
 
 	struct usb_host_interface *ctrl_intf;	/* the audio control interface */
 };