ALSA: usb-audio: add Starr Labs USB MIDI support

Add support for Starr Labs USB MIDI devices such as the Z7S, which are
based on an FTDI serial UART chip.

Based on a patch by Daniel Mack.
Signed-off-by: Kristian Amlie <kristian@amlie.name>
Acked-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/midi.c

@@ -816,6 +816,22 @@ static struct usb_protocol_ops snd_usbmidi_raw_ops = {
 	.output = snd_usbmidi_raw_output,
 };
 
+/*
+ * FTDI protocol: raw MIDI bytes, but input packets have two modem status bytes.
+ */
+
+static void snd_usbmidi_ftdi_input(struct snd_usb_midi_in_endpoint* ep,
+				   uint8_t* buffer, int buffer_length)
+{
+	if (buffer_length > 2)
+		snd_usbmidi_input_data(ep, 0, buffer + 2, buffer_length - 2);
+}
+
+static struct usb_protocol_ops snd_usbmidi_ftdi_ops = {
+	.input = snd_usbmidi_ftdi_input,
+	.output = snd_usbmidi_raw_output,
+};
+
 static void snd_usbmidi_us122l_input(struct snd_usb_midi_in_endpoint *ep,
 				     uint8_t *buffer, int buffer_length)
 {
@@ -2163,6 +2179,17 @@ int snd_usbmidi_create(struct snd_card *card,
 		/* endpoint 1 is input-only */
 		endpoints[1].out_cables = 0;
 		break;
+	case QUIRK_MIDI_FTDI:
+		umidi->usb_protocol_ops = &snd_usbmidi_ftdi_ops;
+
+		/* set baud rate to 31250 (48 MHz / 16 / 96) */
+		err = usb_control_msg(umidi->dev, usb_sndctrlpipe(umidi->dev, 0),
+				      3, 0x40, 0x60, 0, NULL, 0, 1000);
+		if (err < 0)
+			break;
+
+		err = snd_usbmidi_detect_per_port_endpoints(umidi, endpoints);
+		break;
 	default:
 		snd_printd(KERN_ERR "invalid quirk type %d\n", quirk->type);
 		err = -ENXIO;

sound/usb/quirks-table.h

@@ -39,6 +39,17 @@
 	.idProduct = prod, \
 	.bInterfaceClass = USB_CLASS_VENDOR_SPEC
 
+/* FTDI devices */
+{
+	USB_DEVICE(0x0403, 0xb8d8),
+	.driver_info = (unsigned long) & (const struct snd_usb_audio_quirk) {
+		/* .vendor_name = "STARR LABS", */
+		/* .product_name = "Starr Labs MIDI USB device", */
+		.ifnum = 0,
+		.type = QUIRK_MIDI_FTDI
+	}
+},
+
 /* Creative/Toshiba Multimedia Center SB-0500 */
 {
 	USB_DEVICE(0x041e, 0x3048),

sound/usb/quirks.c

@@ -306,6 +306,7 @@ int snd_usb_create_quirk(struct snd_usb_audio *chip,
 		[QUIRK_MIDI_EMAGIC] = create_any_midi_quirk,
 		[QUIRK_MIDI_CME] = create_any_midi_quirk,
 		[QUIRK_MIDI_AKAI] = create_any_midi_quirk,
+		[QUIRK_MIDI_FTDI] = create_any_midi_quirk,
 		[QUIRK_AUDIO_STANDARD_INTERFACE] = create_standard_audio_quirk,
 		[QUIRK_AUDIO_FIXED_ENDPOINT] = create_fixed_stream_quirk,
 		[QUIRK_AUDIO_EDIROL_UAXX] = create_uaxx_quirk,

sound/usb/usbaudio.h

@@ -80,6 +80,7 @@ enum quirk_type {
 	QUIRK_MIDI_CME,
 	QUIRK_MIDI_AKAI,
 	QUIRK_MIDI_US122L,
+	QUIRK_MIDI_FTDI,
 	QUIRK_AUDIO_STANDARD_INTERFACE,
 	QUIRK_AUDIO_FIXED_ENDPOINT,
 	QUIRK_AUDIO_EDIROL_UAXX,