[ALSA] usb-audio: fix Emagic MIDI protocol handling

USB generic driver
Emagic devices pad their packets not with 0xff bytes but with a 0xff
byte followed by garbage, so we have to stop at the first such byte.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

1 files changed, 12 insertions, 5 deletions

diff --git a/sound/usb/usbmidi.c b/sound/usb/usbmidi.c
index d0d895df5375..5f19b494923e 100644
--- a/sound/usb/usbmidi.c
+++ b/sound/usb/usbmidi.c
@@ -594,17 +594,20 @@ static void snd_usbmidi_emagic_finish_out(snd_usb_midi_out_endpoint_t* ep)
 static void snd_usbmidi_emagic_input(snd_usb_midi_in_endpoint_t* ep,
                                      uint8_t* buffer, int buffer_length)
 {
-        /* ignore padding bytes at end of buffer */
-        while (buffer_length > 0 && buffer[buffer_length - 1] == 0xff)
-                --buffer_length;
+        int i;
+
+        /* FF indicates end of valid data */
+        for (i = 0; i < buffer_length; ++i)
+                if (buffer[i] == 0xff) {
+                        buffer_length = i;
+                        break;
+                }
 
         /* handle F5 at end of last buffer */
         if (ep->seen_f5)
                 goto switch_port;
 
         while (buffer_length > 0) {
-                int i;
-
                 /* determine size of data until next F5 */
                 for (i = 0; i < buffer_length; ++i)
                         if (buffer[i] == 0xf5)
@@ -671,6 +674,10 @@ static void snd_usbmidi_emagic_output(snd_usb_midi_out_endpoint_t* ep)
                                 break;
                 }
         }
+        if (buf_free < ep->max_transfer && buf_free > 0) {
+                *buf = 0xff;
+                --buf_free;
+        }
         ep->urb->transfer_buffer_length = ep->max_transfer - buf_free;
 }