ALSA: usb-audio: support partially write-protected UAC2 controls

So far, UAC2 controls are marked read-only if any of the channels are
marked read-only in the descriptors. Change this behaviour and

 - mark them writeable unless all channels are read-only
 - store the read-only mask in usb_mixer_elem_info and
 - check the mask again in set_cur_mix_value(), and bail out for
   write-protected channels.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

2 files changed, 26 insertions, 6 deletions

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index 43d6417b811e..9149a84c716f 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -462,6 +462,16 @@ static int set_cur_mix_value(struct usb_mixer_elem_info *cval, int channel,
                              int index, int value)
 {
         int err;
+        unsigned int read_only = (channel == 0) ?
+                cval->master_readonly :
+                cval->ch_readonly & (1 << (channel - 1));
+
+        if (read_only) {
+                snd_printdd(KERN_INFO "%s(): channel %d of control %d is read_only\n",
+                            __func__, channel, cval->control);
+                return 0;
+        }
+
         err = snd_usb_mixer_set_ctl_value(cval, UAC_SET_CUR, (cval->control << 8) | channel,
                             value);
         if (err < 0)
@@ -958,7 +968,7 @@ 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,
-                              int read_only)
+                              int readonly_mask)
 {
         struct uac_feature_unit_descriptor *desc = raw_desc;
         unsigned int len = 0;
@@ -989,20 +999,25 @@ static void build_feature_ctl(struct mixer_build *state, void *raw_desc,
         cval->control = control;
         cval->cmask = ctl_mask;
         cval->val_type = audio_feature_info[control-1].type;
-        if (ctl_mask == 0)
+        if (ctl_mask == 0) {
                 cval->channels = 1;     /* master channel */
-        else {
+                cval->master_readonly = readonly_mask;
+        } else {
                 int i, c = 0;
                 for (i = 0; i < 16; i++)
                         if (ctl_mask & (1 << i))
                                 c++;
                 cval->channels = c;
+                cval->ch_readonly = readonly_mask;
         }
 
         /* get min/max values */
         get_min_max(cval, 0);
 
-        if (read_only)
+        /* if all channels in the mask are marked read-only, make the control
+         * read-only. set_cur_mix_value() will check the mask again and won't
+         * issue write commands to read-only channels. */
+        if (cval->channels == readonly_mask)
                 kctl = snd_ctl_new1(&usb_feature_unit_ctl_ro, cval);
         else
                 kctl = snd_ctl_new1(&usb_feature_unit_ctl, cval);
@@ -1195,9 +1210,12 @@ static int parse_audio_feature_unit(struct mixer_build *state, int unitid, void
                                 }
                         }
 
-                        /* FIXME: the whole unit is read-only if any of the channels is marked read-only */
+                        /* NOTE: build_feature_ctl() will mark the control read-only if all channels
+                         * are marked read-only in the descriptors. Otherwise, the control will be
+                         * reported as writeable, but the driver will not actually issue a write
+                         * command for read-only channels */
                         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);
+                                build_feature_ctl(state, _ftr, ch_bits, i, &iterm, unitid, ch_read_only);
                         if (uac2_control_is_readable(master_bits, i))
                                 build_feature_ctl(state, _ftr, 0, i, &iterm, unitid,
                                                   !uac2_control_is_writeable(master_bits, i));
diff --git a/sound/usb/mixer.h b/sound/usb/mixer.h
index 130123854a6c..a7cf1007fbb0 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -34,6 +34,8 @@ struct usb_mixer_elem_info {
         unsigned int id;
         unsigned int control;   /* CS or ICN (high byte) */
         unsigned int cmask; /* channel mask bitmap: 0 = master */
+        unsigned int ch_readonly;
+        unsigned int master_readonly;
         int channels;
         int val_type;
         int min, max, res;