ALSA: usb-audio: Add implicit_fb module option

A new module option, implicit_fb, is added to specify the driver
looking for the implicit feedback sync.  This can be useful for a
device that could be working better in the implicit feed back mode and
user wants to test it quickly.  When this works, we can add the quirk
entry easily.
Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-40-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa-configuration.rst

@@ -2227,6 +2227,11 @@ quirk_alias
     Quirk alias list, pass strings like ``0123abcd:5678beef``, which
     applies the existing quirk for the device 5678:beef to a new
     device 0123:abcd.
+implicit_fb
+    Apply the generic implicit feedback sync mode.  When this is set
+    and the playback stream sync mode is ASYNC, the driver tries to
+    tie an adjacent ASYNC capture stream as the implicit feedback
+    source.
 use_vmalloc
     Use vmalloc() for allocations of the PCM buffers (default: yes).
     For architectures with non-coherent memory like ARM or MIPS, the

sound/usb/card.c

@@ -72,6 +72,7 @@ static bool ignore_ctl_error;
 static bool autoclock = true;
 static char *quirk_alias[SNDRV_CARDS];
 static char *delayed_register[SNDRV_CARDS];
+static bool implicit_fb[SNDRV_CARDS];
 
 bool snd_usb_use_vmalloc = true;
 bool snd_usb_skip_validation;
@@ -97,6 +98,8 @@ 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);
 MODULE_PARM_DESC(delayed_register, "Quirk for delayed registration, given by id:iface, e.g. 0123abcd:4.");
+module_param_array(implicit_fb, bool, NULL, 0444);
+MODULE_PARM_DESC(implicit_fb, "Apply generic implicit feedback sync mode.");
 module_param_named(use_vmalloc, snd_usb_use_vmalloc, bool, 0444);
 MODULE_PARM_DESC(use_vmalloc, "Use vmalloc for PCM intermediate buffers (default: yes).");
 module_param_named(skip_validation, snd_usb_skip_validation, bool, 0444);
@@ -596,6 +599,7 @@ static int snd_usb_audio_create(struct usb_interface *intf,
 	chip->dev = dev;
 	chip->card = card;
 	chip->setup = device_setup[idx];
+	chip->generic_implicit_fb = implicit_fb[idx];
 	chip->autoclock = autoclock;
 	atomic_set(&chip->active, 1); /* avoid autopm during probing */
 	atomic_set(&chip->usage_count, 0);

sound/usb/implicit.c

@@ -269,6 +269,10 @@ static int audioformat_implicit_fb_quirk(struct snd_usb_audio *chip,
 			return 1;
 	}
 
+	/* Try the generic implicit fb if available */
+	if (chip->generic_implicit_fb)
+		return add_generic_implicit_fb(chip, fmt, alts);
+
 	/* No quirk */
 	return 0;
 }

sound/usb/usbaudio.h

@@ -51,6 +51,7 @@ struct snd_usb_audio {
 	struct list_head mixer_list;	/* list of mixer interfaces */
 
 	int setup;			/* from the 'device_setup' module param */
+	bool generic_implicit_fb;	/* from the 'implicit_fb' module param */
 	bool autoclock;			/* from the 'autoclock' module param */
 
 	struct usb_host_interface *ctrl_intf;	/* the audio control interface */