[PATCH] ALSA: M-Audio C600 disable Output Gain Knob

[Simon Wood <simon@mungewell.org>]

The C400/C600 interfaces have a large output gain knob which attenuates
the outputs as stereo pairs (1/2, 3/4 and 5/6). The Windows driver/app
provides a control to disable this knob/behaviour on any/all outputs,
forcing maximum gain on each.

The 'disable behaviour' is desirable if any outputs are being used for
aux/effects sends, or if interface is being used as a live/matrix mixer.

This patch adds a control to select which output pairs are affected by
the Output Gain Knob. Default behaviour is to select all outputs.

Tested on the C600, likely also works for the C400.

Signed-off-by: Simon Wood <simon@mungewell.org>
---
 sound/usb/mixer_quirks.c | 46 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/sound/usb/mixer_quirks.c b/sound/usb/mixer_quirks.c
index 11e205da7964..16d8eb111d01 100644
--- a/sound/usb/mixer_quirks.c
+++ b/sound/usb/mixer_quirks.c
@@ -1282,7 +1282,7 @@ static int snd_ftu_eff_switch_init(struct usb_mixer_interface *mixer,
 
 	err = snd_usb_ctl_msg(dev, usb_rcvctrlpipe(dev, 0), UAC_GET_CUR,
 			      USB_RECIP_INTERFACE | USB_TYPE_CLASS | USB_DIR_IN,
-			      pval & 0xff00,
+			      (pval & 0xff00) | ((pval & 0xff0000) >> 16),
 			      snd_usb_ctrl_intf(mixer->hostif) | ((pval & 0xff) << 8),
 			      value, 2);
 	if (err < 0)
@@ -1315,7 +1315,7 @@ static int snd_ftu_eff_switch_update(struct usb_mixer_elem_list *list)
 			       usb_sndctrlpipe(chip->dev, 0),
 			       UAC_SET_CUR,
 			       USB_RECIP_INTERFACE | USB_TYPE_CLASS | USB_DIR_OUT,
-			       pval & 0xff00,
+			       (pval & 0xff00) | ((pval & 0xff0000) >> 16),
 			       snd_usb_ctrl_intf(list->mixer->hostif) | ((pval & 0xff) << 8),
 			       value, 2);
 }
@@ -1728,6 +1728,44 @@ static int snd_c400_create_effect_ret_vol_ctls(struct usb_mixer_interface *mixer
 	return 0;
 }
 
+/* output gain knob selectively adjusts outputs as stereo pairs */
+/* reuses functions from FTU effect switch */
+static int snd_c400_knob_switch_info(struct snd_kcontrol *kcontrol,
+					struct snd_ctl_elem_info *uinfo)
+{
+	static const char *const texts[8] = {
+		"None", "1/2", "3/4", "1/2 3/4",
+		"5/6", "1/2 5/6", "3/4 5/6", "1/2 3/4 5/6"
+	};
+
+	return snd_ctl_enum_info(uinfo, 1, ARRAY_SIZE(texts), texts);
+}
+
+static int snd_c400_create_knob_switch(struct usb_mixer_interface *mixer,
+	int validx, int bUnitID)
+{
+	static struct snd_kcontrol_new template = {
+		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+		.name = "Output Gain Knob",
+		.index = 0,
+		.access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
+		.info = snd_c400_knob_switch_info,
+		.get = snd_ftu_eff_switch_get,
+		.put = snd_ftu_eff_switch_put
+	};
+	struct usb_mixer_elem_list *list;
+	int err;
+
+	err = add_single_ctl_with_resume(mixer, bUnitID,
+					 snd_ftu_eff_switch_update,
+					 &template, &list);
+	if (err < 0)
+		return err;
+	list->kctl->private_value = (validx << 8) | bUnitID;
+	snd_ftu_eff_switch_init(mixer, list->kctl);
+	return 0;
+}
+
 static int snd_c400_create_mixer(struct usb_mixer_interface *mixer)
 {
 	int err;
@@ -1760,6 +1798,10 @@ static int snd_c400_create_mixer(struct usb_mixer_interface *mixer)
 	if (err < 0)
 		return err;
 
+	err = snd_c400_create_knob_switch(mixer, 0x0900, 0x20);
+	if (err < 0)
+		return err;
+
 	return 0;
 }
 
-- 
2.43.0