[ALSA] usb-audio: fix parsing of SysEx messages from CME keyboards

When CME keyboards send a SysEx message (e.g. master volume), the USB
packet uses a format different from the standard format.  Parsing this
packet according to the specification corrupts the SysEx message itself
and can cause the following MIDI messages to be misinterpreted, too.
This patch adds a workaround for this case.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

sound/usb/usbmidi.c

@@ -406,6 +406,20 @@ static void snd_usbmidi_maudio_broken_running_status_input(
 		}
 }
 
+/*
+ * CME protocol: like the standard protocol, but SysEx commands are sent as a
+ * single USB packet preceded by a 0x0F byte.
+ */
+static void snd_usbmidi_cme_input(struct snd_usb_midi_in_endpoint *ep,
+				  uint8_t *buffer, int buffer_length)
+{
+	if (buffer_length < 2 || (buffer[0] & 0x0f) != 0x0f)
+		snd_usbmidi_standard_input(ep, buffer, buffer_length);
+	else
+		snd_usbmidi_input_data(ep, buffer[0] >> 4,
+				       &buffer[1], buffer_length - 1);
+}
+
 /*
  * Adds one USB MIDI packet to the output buffer.
  */
@@ -572,6 +586,12 @@ static struct usb_protocol_ops snd_usbmidi_maudio_broken_running_status_ops = {
 	.output_packet = snd_usbmidi_output_standard_packet,
 };
 
+static struct usb_protocol_ops snd_usbmidi_cme_ops = {
+	.input = snd_usbmidi_cme_input,
+	.output = snd_usbmidi_standard_output,
+	.output_packet = snd_usbmidi_output_standard_packet,
+};
+
 /*
  * Novation USB MIDI protocol: number of data bytes is in the first byte
  * (when receiving) (+1!) or in the second byte (when sending); data begins
@@ -1690,6 +1710,7 @@ int snd_usb_create_midi_interface(struct snd_usb_audio* chip,
 		err = snd_usbmidi_detect_endpoints(umidi, &endpoints[0], 1);
 		break;
 	case QUIRK_MIDI_CME:
+		umidi->usb_protocol_ops = &snd_usbmidi_cme_ops;
 		err = snd_usbmidi_detect_per_port_endpoints(umidi, endpoints);
 		break;
 	default: