• Clemens Ladisch's avatar
    ALSA: usb-audio: work around CH345 input SysEx corruption · a91e627e
    Clemens Ladisch authored
    One of the many faults of the QinHeng CH345 USB MIDI interface chip is
    that it does not handle received SysEx messages correctly -- every second
    event packet has a wrong code index number, which is the one from the last
    seen message, instead of 4.  For example, the two messages "FE F0 01 02 03
    04 05 06 07 08 09 0A 0B 0C 0D 0E F7" result in the following event
    packets:
    
    correct:       CH345:
    0F FE 00 00    0F FE 00 00
    04 F0 01 02    04 F0 01 02
    04 03 04 05    0F 03 04 05
    04 06 07 08    04 06 07 08
    04 09 0A 0B    0F 09 0A 0B
    04 0C 0D 0E    04 0C 0D 0E
    05 F7 00 00    05 F7 00 00
    
    A class-compliant driver must interpret an event packet with CIN 15 as
    having a single data byte, so the other two bytes would be ignored.  The
    message received by the host would then be missing two bytes out of six;
    in this example, "F0 01 02 03 06 07 08 09 0C 0D 0E F7".
    
    These corrupted SysEx event packages contain only data bytes, while the
    CH345 uses event packets with a correct CIN value only for messages with
    a status byte, so it is possible to distinguish between these two cases by
    checking for the presence of this status byte.
    
    (Other bugs in the CH345's input handling, such as the corruption resulting
    from running status, cannot be worked around.)
    Signed-off-by: default avatarClemens Ladisch <clemens@ladisch.de>
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
    a91e627e
midi.c 68.4 KB