usb: gadget: midi2: Add "Operation Mode" control

Add a new ALSA control element to watch the current operation mode
(MIDI 1.0 or MIDI 2.0). It's a read-only control that reflects the
current value of altsetting, and 0 means unused, 1 for MIDI 1.0
(altset 0) and 2 for MIDI 2.0 (altset 1).

Signed-off-by: Takashi Iwai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/usb/gadget-testing.rst

···
 The access to MIDI 1.0 on altset 0 on the host is supported, and it's
 translated from/to UMP packets on the gadget. It's bound to only
 Function Block 0.
+
+The current operation mode can be observed in ALSA control element
+"Operation Mode" for SND_CTL_IFACE_RAWMIDI.  For example::
+
+  $ amixer -c1 contents
+  numid=1,iface=RAWMIDI,name='Operation Mode'
+    ; type=INTEGER,access=r--v----,values=1,min=0,max=2,step=0
+    : values=2
+
+where 0 = unused, 1 = MIDI 1.0 (altset 0), 2 = MIDI 2.0 (altset 1).
+The example above shows it's running in 2, i.e. MIDI 2.0.

drivers/usb/gadget/function/f_midi2.c

···
 #include <linux/slab.h>
 
 #include <sound/core.h>
+#include <sound/control.h>
 #include <sound/ump.h>
 #include <sound/ump_msg.h>
 #include <sound/ump_convert.h>
···
 };
 
 /*
+ * "Operation Mode" control element
+ */
+static int f_midi2_operation_mode_info(struct snd_kcontrol *kcontrol,
+				       struct snd_ctl_elem_info *uinfo)
+{
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = MIDI_OP_MODE_UNSET;
+	uinfo->value.integer.max = MIDI_OP_MODE_MIDI2;
+	return 0;
+}
+
+static int f_midi2_operation_mode_get(struct snd_kcontrol *kcontrol,
+				      struct snd_ctl_elem_value *ucontrol)
+{
+	struct f_midi2 *midi2 = snd_kcontrol_chip(kcontrol);
+
+	ucontrol->value.integer.value[0] = midi2->operation_mode;
+	return 0;
+}
+
+static const struct snd_kcontrol_new operation_mode_ctl = {
+	.iface = SNDRV_CTL_ELEM_IFACE_RAWMIDI,
+	.name = "Operation Mode",
+	.access = SNDRV_CTL_ELEM_ACCESS_READ | SNDRV_CTL_ELEM_ACCESS_VOLATILE,
+	.info = f_midi2_operation_mode_info,
+	.get = f_midi2_operation_mode_get,
+};
+
+/*
  * ALSA UMP instance creation / deletion
  */
 static void f_midi2_free_card(struct f_midi2 *midi2)
···
 			goto error;
 		id++;
 	}
+
+	err = snd_ctl_add(card, snd_ctl_new1(&operation_mode_ctl, midi2));
+	if (err < 0)
+		goto error;
 
 	err = snd_card_register(card);
 	if (err < 0)