ALSA: usb-mixer: Add support for Audio Class v2.0

USB Audio Class v2.0 compliant devices have different descriptors and a
different way of setting/getting min/max/res/cur properties. This patch
adds support for them.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

2 files changed, 245 insertions, 80 deletions

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index 994b0385235c..1deef623c081 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -33,6 +33,7 @@
 #include <linux/string.h>
 #include <linux/usb.h>
 #include <linux/usb/audio.h>
+#include <linux/usb/audio-v2.h>
 
 #include <sound/core.h>
 #include <sound/control.h>
@@ -197,6 +198,7 @@ static int check_mapped_selector_name(struct mixer_build *state, int unitid,
 
 /*
  * find an audio control unit with the given unit id
+ * this doesn't return any clock related units, so they need to be handled elsewhere
  */
 static void *find_audio_control_unit(struct mixer_build *state, unsigned char unit)
 {
@@ -205,7 +207,7 @@ static void *find_audio_control_unit(struct mixer_build *state, unsigned char un
         p = NULL;
         while ((p = snd_usb_find_desc(state->buffer, state->buflen, p,
                                       USB_DT_CS_INTERFACE)) != NULL) {
-                if (p[0] >= 4 && p[2] >= UAC_INPUT_TERMINAL && p[2] <= UAC_EXTENSION_UNIT_V1 && p[3] == unit)
+                if (p[0] >= 4 && p[2] >= UAC_INPUT_TERMINAL && p[2] <= UAC2_EXTENSION_UNIT_V2 && p[3] == unit)
                         return p;
         }
         return NULL;
@@ -302,7 +304,7 @@ static int get_abs_value(struct usb_mixer_elem_info *cval, int val)
  * retrieve a mixer value
  */
 
-static int get_ctl_value(struct usb_mixer_elem_info *cval, int request, int validx, int *value_ret)
+static int get_ctl_value_v1(struct usb_mixer_elem_info *cval, int request, int validx, int *value_ret)
 {
         unsigned char buf[2];
         int val_len = cval->val_type >= USB_MIXER_S16 ? 2 : 1;
@@ -324,6 +326,58 @@ static int get_ctl_value(struct usb_mixer_elem_info *cval, int request, int vali
         return -EINVAL;
 }
 
+static int get_ctl_value_v2(struct usb_mixer_elem_info *cval, int request, int validx, int *value_ret)
+{
+        unsigned char buf[14]; /* enough space for one range of 4 bytes */
+        unsigned char *val;
+        int ret;
+        __u8 bRequest;
+
+        bRequest = (request == UAC_GET_CUR) ?
+                UAC2_CS_CUR : UAC2_CS_RANGE;
+
+        ret = snd_usb_ctl_msg(cval->mixer->chip->dev,
+                              usb_rcvctrlpipe(cval->mixer->chip->dev, 0),
+                              bRequest,
+                              USB_RECIP_INTERFACE | USB_TYPE_CLASS | USB_DIR_IN,
+                              validx, cval->mixer->ctrlif | (cval->id << 8),
+                              buf, sizeof(buf), 1000);
+
+        if (ret < 0) {
+                snd_printdd(KERN_ERR "cannot get ctl value: req = %#x, wValue = %#x, wIndex = %#x, type = %d\n",
+                            request, validx, cval->mixer->ctrlif | (cval->id << 8), cval->val_type);
+                return ret;
+        }
+
+        switch (request) {
+        case UAC_GET_CUR:
+                val = buf;
+                break;
+        case UAC_GET_MIN:
+                val = buf + sizeof(__u16);
+                break;
+        case UAC_GET_MAX:
+                val = buf + sizeof(__u16) * 2;
+                break;
+        case UAC_GET_RES:
+                val = buf + sizeof(__u16) * 3;
+                break;
+        default:
+                return -EINVAL;
+        }
+
+        *value_ret = convert_signed_value(cval, snd_usb_combine_bytes(val, sizeof(__u16)));
+
+        return 0;
+}
+
+static int get_ctl_value(struct usb_mixer_elem_info *cval, int request, int validx, int *value_ret)
+{
+        return (cval->mixer->protocol == UAC_VERSION_1) ?
+                get_ctl_value_v1(cval, request, validx, value_ret) :
+                get_ctl_value_v2(cval, request, validx, value_ret);
+}
+
 static int get_cur_ctl_value(struct usb_mixer_elem_info *cval, int validx, int *value)
 {
         return get_ctl_value(cval, UAC_GET_CUR, validx, value);
@@ -348,8 +402,7 @@ static int get_cur_mix_value(struct usb_mixer_elem_info *cval,
         err = get_cur_mix_raw(cval, channel, value);
         if (err < 0) {
                 if (!cval->mixer->ignore_ctl_error)
-                        snd_printd(KERN_ERR "cannot get current value for "
-                                   "control %d ch %d: err = %d\n",
+                        snd_printd(KERN_ERR "cannot get current value for control %d ch %d: err = %d\n",
                                    cval->control, channel, err);
                 return err;
         }
@@ -367,8 +420,22 @@ int snd_usb_mixer_set_ctl_value(struct usb_mixer_elem_info *cval,
                                 int request, int validx, int value_set)
 {
         unsigned char buf[2];
-        int val_len = cval->val_type >= USB_MIXER_S16 ? 2 : 1;
-        int timeout = 10;
+        int val_len, timeout = 10;
+
+        if (cval->mixer->protocol == UAC_VERSION_1) {
+                val_len = cval->val_type >= USB_MIXER_S16 ? 2 : 1;
+        } else { /* UAC_VERSION_2 */
+                /* audio class v2 controls are always 2 bytes in size */
+                val_len = sizeof(__u16);
+
+                /* FIXME */
+                if (request != UAC_SET_CUR) {
+                        snd_printdd(KERN_WARNING "RANGE setting not yet supported\n");
+                        return -EINVAL;
+                }
+
+                request = UAC2_CS_CUR;
+        }
 
         value_set = convert_bytes_value(cval, value_set);
         buf[0] = value_set & 0xff;
@@ -564,46 +631,65 @@ static int get_term_name(struct mixer_build *state, struct usb_audio_term *iterm
  */
 static int check_input_term(struct mixer_build *state, int id, struct usb_audio_term *term)
 {
-        unsigned char *p1;
+        void *p1;
 
         memset(term, 0, sizeof(*term));
         while ((p1 = find_audio_control_unit(state, id)) != NULL) {
+                unsigned char *hdr = p1;
                 term->id = id;
-                switch (p1[2]) {
+                switch (hdr[2]) {
                 case UAC_INPUT_TERMINAL:
-                        term->type = combine_word(p1 + 4);
-                        term->channels = p1[7];
-                        term->chconfig = combine_word(p1 + 8);
-                        term->name = p1[11];
+                        if (state->mixer->protocol == UAC_VERSION_1) {
+                                struct uac_input_terminal_descriptor *d = p1;
+                                term->type = le16_to_cpu(d->wTerminalType);
+                                term->channels = d->bNrChannels;
+                                term->chconfig = le16_to_cpu(d->wChannelConfig);
+                                term->name = d->iTerminal;
+                        } else { /* UAC_VERSION_2 */
+                                struct uac2_input_terminal_descriptor *d = p1;
+                                term->type = le16_to_cpu(d->wTerminalType);
+                                term->channels = d->bNrChannels;
+                                term->chconfig = le32_to_cpu(d->bmChannelConfig);
+                                term->name = d->iTerminal;
+                        }
                         return 0;
-                case UAC_FEATURE_UNIT:
-                        id = p1[4];
+                case UAC_FEATURE_UNIT: {
+                        /* the header is the same for v1 and v2 */
+                        struct uac_feature_unit_descriptor *d = p1;
+                        id = d->bUnitID;
                         break; /* continue to parse */
-                case UAC_MIXER_UNIT:
-                        term->type = p1[2] << 16; /* virtual type */
-                        term->channels = p1[5 + p1[4]];
-                        term->chconfig = combine_word(p1 + 6 + p1[4]);
-                        term->name = p1[p1[0] - 1];
+                }
+                case UAC_MIXER_UNIT: {
+                        struct uac_mixer_unit_descriptor *d = p1;
+                        term->type = d->bDescriptorSubtype << 16; /* virtual type */
+                        term->channels = uac_mixer_unit_bNrChannels(d);
+                        term->chconfig = uac_mixer_unit_wChannelConfig(d, state->mixer->protocol);
+                        term->name = uac_mixer_unit_iMixer(d);
                         return 0;
-                case UAC_SELECTOR_UNIT:
+                }
+                case UAC_SELECTOR_UNIT: {
+                        struct uac_selector_unit_descriptor *d = p1;
                         /* call recursively to retrieve the channel info */
-                        if (check_input_term(state, p1[5], term) < 0)
+                        if (check_input_term(state, d->baSourceID[0], term) < 0)
                                 return -ENODEV;
-                        term->type = p1[2] << 16; /* virtual type */
+                        term->type = d->bDescriptorSubtype << 16; /* virtual type */
                         term->id = id;
-                        term->name = p1[9 + p1[0] - 1];
+                        term->name = uac_selector_unit_iSelector(d);
                         return 0;
+                }
                 case UAC_PROCESSING_UNIT_V1:
-                case UAC_EXTENSION_UNIT_V1:
-                        if (p1[6] == 1) {
-                                id = p1[7];
+                case UAC_EXTENSION_UNIT_V1: {
+                        struct uac_processing_unit_descriptor *d = p1;
+                        if (d->bNrInPins) {
+                                id = d->baSourceID[0];
                                 break; /* continue to parse */
                         }
-                        term->type = p1[2] << 16; /* virtual type */
-                        term->channels = p1[7 + p1[6]];
-                        term->chconfig = combine_word(p1 + 8 + p1[6]);
-                        term->name = p1[12 + p1[6] + p1[11 + p1[6]]];
+                        term->type = d->bDescriptorSubtype << 16; /* virtual type */
+                        term->channels = uac_processing_unit_bNrChannels(d);
+                        term->chconfig = uac_processing_unit_wChannelConfig(d, state->mixer->protocol);
+                        term->name = uac_processing_unit_iProcessing(d, state->mixer->protocol);
                         return 0;
+                }
                 default:
                         return -ENODEV;
                 }
@@ -850,6 +936,15 @@ static struct snd_kcontrol_new usb_feature_unit_ctl = {
         .put = mixer_ctl_feature_put,
 };
 
+/* the read-only variant */
+static struct snd_kcontrol_new usb_feature_unit_ctl_ro = {
+        .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+        .name = "", /* will be filled later manually */
+        .info = mixer_ctl_feature_info,
+        .get = mixer_ctl_feature_get,
+        .put = NULL,
+};
+
 
 /*
  * build a feature control
@@ -862,7 +957,8 @@ static size_t append_ctl_name(struct snd_kcontrol *kctl, const char *str)
 
 static void build_feature_ctl(struct mixer_build *state, void *raw_desc,
                               unsigned int ctl_mask, int control,
-                              struct usb_audio_term *iterm, int unitid)
+                              struct usb_audio_term *iterm, int unitid,
+                              int read_only)
 {
         struct uac_feature_unit_descriptor *desc = raw_desc;
         unsigned int len = 0;
@@ -906,7 +1002,11 @@ static void build_feature_ctl(struct mixer_build *state, void *raw_desc,
         /* get min/max values */
         get_min_max(cval, 0);
 
-        kctl = snd_ctl_new1(&usb_feature_unit_ctl, cval);
+        if (read_only)
+                kctl = snd_ctl_new1(&usb_feature_unit_ctl_ro, cval);
+        else
+                kctl = snd_ctl_new1(&usb_feature_unit_ctl, cval);
+
         if (! kctl) {
                 snd_printk(KERN_ERR "cannot malloc kcontrol\n");
                 kfree(cval);
@@ -1016,24 +1116,34 @@ static int parse_audio_feature_unit(struct mixer_build *state, int unitid, void
         struct usb_audio_term iterm;
         unsigned int master_bits, first_ch_bits;
         int err, csize;
-        struct uac_feature_unit_descriptor *ftr = _ftr;
+        struct uac_feature_unit_descriptor *hdr = _ftr;
+        __u8 *bmaControls;
+
+        if (state->mixer->protocol == UAC_VERSION_1) {
+                csize = hdr->bControlSize;
+                channels = (hdr->bLength - 7) / csize - 1;
+                bmaControls = hdr->bmaControls;
+        } else {
+                struct uac2_feature_unit_descriptor *ftr = _ftr;
+                csize = 4;
+                channels = (hdr->bLength - 6) / 4;
+                bmaControls = ftr->bmaControls;
+        }
 
-        if (ftr->bLength < 7 || ! (csize = ftr->bControlSize) || ftr->bLength < 7 + csize) {
+        if (hdr->bLength < 7 || !csize || hdr->bLength < 7 + csize) {
                 snd_printk(KERN_ERR "usbaudio: unit %u: invalid UAC_FEATURE_UNIT descriptor\n", unitid);
                 return -EINVAL;
         }
 
         /* parse the source unit */
-        if ((err = parse_audio_unit(state, ftr->bSourceID)) < 0)
+        if ((err = parse_audio_unit(state, hdr->bSourceID)) < 0)
                 return err;
 
         /* determine the input source type and name */
-        if (check_input_term(state, ftr->bSourceID, &iterm) < 0)
+        if (check_input_term(state, hdr->bSourceID, &iterm) < 0)
                 return -EINVAL;
 
-        channels = (ftr->bLength - 7) / csize - 1;
-
-        master_bits = snd_usb_combine_bytes(ftr->bmaControls, csize);
+        master_bits = snd_usb_combine_bytes(bmaControls, csize);
         /* master configuration quirks */
         switch (state->chip->usb_id) {
         case USB_ID(0x08bb, 0x2702):
@@ -1044,21 +1154,54 @@ static int parse_audio_feature_unit(struct mixer_build *state, int unitid, void
                 break;
         }
         if (channels > 0)
-                first_ch_bits = snd_usb_combine_bytes(ftr->bmaControls + csize, csize);
+                first_ch_bits = snd_usb_combine_bytes(bmaControls + csize, csize);
         else
                 first_ch_bits = 0;
-        /* check all control types */
-        for (i = 0; i < 10; i++) {
-                unsigned int ch_bits = 0;
-                for (j = 0; j < channels; j++) {
-                        unsigned int mask = snd_usb_combine_bytes(ftr->bmaControls + csize * (j+1), csize);
-                        if (mask & (1 << i))
-                                ch_bits |= (1 << j);
+
+        if (state->mixer->protocol == UAC_VERSION_1) {
+                /* check all control types */
+                for (i = 0; i < 10; i++) {
+                        unsigned int ch_bits = 0;
+                        for (j = 0; j < channels; j++) {
+                                unsigned int mask = snd_usb_combine_bytes(bmaControls + csize * (j+1), csize);
+                                if (mask & (1 << i))
+                                        ch_bits |= (1 << j);
+                        }
+                        /* audio class v1 controls are never read-only */
+                        if (ch_bits & 1) /* the first channel must be set (for ease of programming) */
+                                build_feature_ctl(state, _ftr, ch_bits, i, &iterm, unitid, 0);
+                        if (master_bits & (1 << i))
+                                build_feature_ctl(state, _ftr, 0, i, &iterm, unitid, 0);
+                }
+        } else { /* UAC_VERSION_2 */
+                for (i = 0; i < 30/2; i++) {
+                        /* From the USB Audio spec v2.0:
+                           bmaControls() is a (ch+1)-element array of 4-byte bitmaps,
+                           each containing a set of bit pairs. If a Control is present,
+                           it must be Host readable. If a certain Control is not
+                           present then the bit pair must be set to 0b00.
+                           If a Control is present but read-only, the bit pair must be
+                           set to 0b01. If a Control is also Host programmable, the bit
+                           pair must be set to 0b11. The value 0b10 is not allowed. */
+                        unsigned int ch_bits = 0;
+                        unsigned int ch_read_only = 0;
+
+                        for (j = 0; j < channels; j++) {
+                                unsigned int mask = snd_usb_combine_bytes(bmaControls + csize * (j+1), csize);
+                                if (mask & (1 << (i * 2))) {
+                                        ch_bits |= (1 << j);
+                                        if (~mask & (1 << ((i * 2) + 1)))
+                                                ch_read_only |= (1 << j);
+                                }
+                        }
+
+                        /* FIXME: the whole unit is read-only if any of the channels is marked read-only */
+                        if (ch_bits & 1) /* the first channel must be set (for ease of programming) */
+                                build_feature_ctl(state, _ftr, ch_bits, i, &iterm, unitid, !!ch_read_only);
+                        if (master_bits & (1 << i * 2))
+                                build_feature_ctl(state, _ftr, 0, i, &iterm, unitid,
+                                                  ~master_bits & (1 << ((i * 2) + 1)));
                 }
-                if (ch_bits & 1) /* the first channel must be set (for ease of programming) */
-                        build_feature_ctl(state, _ftr, ch_bits, i, &iterm, unitid);
-                if (master_bits & (1 << i))
-                        build_feature_ctl(state, _ftr, 0, i, &iterm, unitid);
         }
 
         return 0;
@@ -1100,7 +1243,7 @@ static void build_mixer_unit_ctl(struct mixer_build *state,
         cval->control = in_ch + 1; /* based on 1 */
         cval->val_type = USB_MIXER_S16;
         for (i = 0; i < num_outs; i++) {
-                if (check_matrix_bitmap(uac_mixer_unit_bmControls(desc), in_ch, i, num_outs)) {
+                if (check_matrix_bitmap(uac_mixer_unit_bmControls(desc, state->mixer->protocol), in_ch, i, num_outs)) {
                         cval->cmask |= (1 << i);
                         cval->channels++;
                 }
@@ -1164,7 +1307,7 @@ static int parse_audio_mixer_unit(struct mixer_build *state, int unitid, void *r
                         int och, ich_has_controls = 0;
 
                         for (och = 0; och < num_outs; ++och) {
-                                if (check_matrix_bitmap(uac_mixer_unit_bmControls(desc),
+                                if (check_matrix_bitmap(uac_mixer_unit_bmControls(desc, state->mixer->protocol),
                                                         ich, och, num_outs)) {
                                         ich_has_controls = 1;
                                         break;
@@ -1343,7 +1486,8 @@ static int build_audio_procunit(struct mixer_build *state, int unitid, void *raw
                 0, NULL, default_value_info
         };
 
-        if (desc->bLength < 13 || desc->bLength < 13 + num_ins || desc->bLength < num_ins + uac_processing_unit_bControlSize(desc)) {
+        if (desc->bLength < 13 || desc->bLength < 13 + num_ins ||
+            desc->bLength < num_ins + uac_processing_unit_bControlSize(desc, state->mixer->protocol)) {
                 snd_printk(KERN_ERR "invalid %s descriptor (id %d)\n", name, unitid);
                 return -EINVAL;
         }
@@ -1361,7 +1505,7 @@ static int build_audio_procunit(struct mixer_build *state, int unitid, void *raw
                 info = &default_info;
 
         for (valinfo = info->values; valinfo->control; valinfo++) {
-                __u8 *controls = uac_processing_unit_bmControls(desc);
+                __u8 *controls = uac_processing_unit_bmControls(desc, state->mixer->protocol);
 
                 if (! (controls[valinfo->control / 8] & (1 << ((valinfo->control % 8) - 1))))
                         continue;
@@ -1381,7 +1525,7 @@ static int build_audio_procunit(struct mixer_build *state, int unitid, void *raw
 
                 /* get min/max values */
                 if (type == USB_PROC_UPDOWN && cval->control == USB_PROC_UPDOWN_MODE_SEL) {
-                        __u8 *control_spec = uac_processing_unit_specific(desc);
+                        __u8 *control_spec = uac_processing_unit_specific(desc, state->mixer->protocol);
                         /* FIXME: hard-coded */
                         cval->min = 1;
                         cval->max = control_spec[0];
@@ -1414,7 +1558,7 @@ static int build_audio_procunit(struct mixer_build *state, int unitid, void *raw
                 else if (info->name)
                         strlcpy(kctl->id.name, info->name, sizeof(kctl->id.name));
                 else {
-                        nameid = uac_processing_unit_iProcessing(desc);
+                        nameid = uac_processing_unit_iProcessing(desc, state->mixer->protocol);
                         len = 0;
                         if (nameid)
                                 len = snd_usb_copy_string_desc(state, nameid, kctl->id.name, sizeof(kctl->id.name));
@@ -1676,9 +1820,17 @@ static int parse_audio_unit(struct mixer_build *state, int unitid)
         case UAC_FEATURE_UNIT:
                 return parse_audio_feature_unit(state, unitid, p1);
         case UAC_PROCESSING_UNIT_V1:
-                return parse_audio_processing_unit(state, unitid, p1);
+        /*   UAC2_EFFECT_UNIT has the same value */
+                if (state->mixer->protocol == UAC_VERSION_1)
+                        return parse_audio_processing_unit(state, unitid, p1);
+                else
+                        return 0; /* FIXME - effect units not implemented yet */
         case UAC_EXTENSION_UNIT_V1:
-                return parse_audio_extension_unit(state, unitid, p1);
+        /*   UAC2_PROCESSING_UNIT_V2 has the same value */
+                if (state->mixer->protocol == UAC_VERSION_1)
+                        return parse_audio_extension_unit(state, unitid, p1);
+                else /* UAC_VERSION_2 */
+                        return parse_audio_processing_unit(state, unitid, p1);
         default:
                 snd_printk(KERN_ERR "usbaudio: unit %u: unexpected type 0x%02x\n", unitid, p1[2]);
                 return -EINVAL;
@@ -1711,11 +1863,11 @@ static int snd_usb_mixer_dev_free(struct snd_device *device)
  */
 static int snd_usb_mixer_controls(struct usb_mixer_interface *mixer)
 {
-        struct uac_output_terminal_descriptor_v1 *desc;
         struct mixer_build state;
         int err;
         const struct usbmix_ctl_map *map;
         struct usb_host_interface *hostif;
+        void *p;
 
         hostif = &usb_ifnum_to_if(mixer->chip->dev, mixer->ctrlif)->altsetting[0];
         memset(&state, 0, sizeof(state));
@@ -1734,18 +1886,35 @@ static int snd_usb_mixer_controls(struct usb_mixer_interface *mixer)
                 }
         }
 
-        desc = NULL;
-        while ((desc = snd_usb_find_csint_desc(hostif->extra, hostif->extralen, desc, UAC_OUTPUT_TERMINAL)) != NULL) {
-                if (desc->bLength < 9)
-                        continue; /* invalid descriptor? */
-                set_bit(desc->bTerminalID, state.unitbitmap);  /* mark terminal ID as visited */
-                state.oterm.id = desc->bTerminalID;
-                state.oterm.type = le16_to_cpu(desc->wTerminalType);
-                state.oterm.name = desc->iTerminal;
-                err = parse_audio_unit(&state, desc->bSourceID);
-                if (err < 0)
-                        return err;
+        p = NULL;
+        while ((p = snd_usb_find_csint_desc(hostif->extra, hostif->extralen, p, UAC_OUTPUT_TERMINAL)) != NULL) {
+                if (mixer->protocol == UAC_VERSION_1) {
+                        struct uac_output_terminal_descriptor_v1 *desc = p;
+
+                        if (desc->bLength < sizeof(*desc))
+                                continue; /* invalid descriptor? */
+                        set_bit(desc->bTerminalID, state.unitbitmap);  /* mark terminal ID as visited */
+                        state.oterm.id = desc->bTerminalID;
+                        state.oterm.type = le16_to_cpu(desc->wTerminalType);
+                        state.oterm.name = desc->iTerminal;
+                        err = parse_audio_unit(&state, desc->bSourceID);
+                        if (err < 0)
+                                return err;
+                } else { /* UAC_VERSION_2 */
+                        struct uac2_output_terminal_descriptor *desc = p;
+
+                        if (desc->bLength < sizeof(*desc))
+                                continue; /* invalid descriptor? */
+                        set_bit(desc->bTerminalID, state.unitbitmap);  /* mark terminal ID as visited */
+                        state.oterm.id = desc->bTerminalID;
+                        state.oterm.type = le16_to_cpu(desc->wTerminalType);
+                        state.oterm.name = desc->iTerminal;
+                        err = parse_audio_unit(&state, desc->bSourceID);
+                        if (err < 0)
+                                return err;
+                }
         }
+
         return 0;
 }
 
@@ -1868,7 +2037,7 @@ int snd_usb_create_mixer(struct snd_usb_audio *chip, int ctrlif,
         struct usb_mixer_interface *mixer;
         struct snd_info_entry *entry;
         struct usb_host_interface *host_iface;
-        int err, protocol;
+        int err;
 
         strcpy(chip->card->mixername, "USB Mixer");
 
@@ -1886,14 +2055,7 @@ int snd_usb_create_mixer(struct snd_usb_audio *chip, int ctrlif,
         }
 
         host_iface = &usb_ifnum_to_if(chip->dev, ctrlif)->altsetting[0];
-        protocol = host_iface->desc.bInterfaceProtocol;
-
-        /* FIXME! */
-        if (protocol != UAC_VERSION_1) {
-                snd_printk(KERN_WARNING "mixer interface protocol 0x%02x not yet supported\n",
-                                        protocol);
-                return 0;
-        }
+        mixer->protocol = host_iface->desc.bInterfaceProtocol;
 
         if ((err = snd_usb_mixer_controls(mixer)) < 0 ||
             (err = snd_usb_mixer_status_create(mixer)) < 0)
diff --git a/sound/usb/mixer.h b/sound/usb/mixer.h
index 63101ae201cc..130123854a6c 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -10,6 +10,9 @@ struct usb_mixer_interface {
         /* array[MAX_ID_ELEMS], indexed by unit id */
         struct usb_mixer_elem_info **id_elems;
 
+        /* the usb audio specification version this interface complies to */
+        int protocol;
+
         /* Sound Blaster remote control stuff */
         const struct rc_config *rc_cfg;
         u32 rc_code;