Willem Jan Palenstijn wrote: > I recently investigated some trouble with sending MIDI via a USB MIDI cable > (15ca:1806) to a Roland MT-32. In some cases, commands were being dropped. > > (This is what is reported on > https://bugtrack.alsa-project.org/alsa-bug/view.php?id=3752 . I'm the colleague > mentioned by druon in note 0021737 there.) Apparently, the bug tracker drops some email notifications. > After we experimented a bit, it turned out that packets of 2 MIDI commands (8 > bytes total) were being sent to the USB MIDI cable, and that if the first of > these was a 2-byte MIDI command (such as a 0xC0 or 0xD0 command), the second > command in the packet would get dropped. > > A workaround is to limit the packet size to 4 bytes, as is already happening on > at least one other device (the ESI M4U referred to in usbmidi.c). > > I played with it some more this week, and it turns out that if the zero-padding > from the USB MIDI commands is removed (violating the specs, as far as I > understand), Indeed. > the second command does actually not get dropped. I'd guess a certain other OS never sends more than 4 bytes in one packet. > What do you think would be the right way to handle this problem? Don't zero-pad > the outgoing packets for this specific device (and maybe a few others mentioned > on bug #3752, if they end up suffering from the same bug)? This would be possible, but there is no existing driver that actually uses such a protocol, so I wouldn't want to rely on some implementation detail that is likely to be changed in some other firmware revision. > Limit the outgoing packets to 4 bytes like with the ESI M4U for these > devices? This is what the device expects. The driver can have multiple active packets, so this shouldn't reduce MIDI bandwidth. > Add an option to the snd-usb-audio module to toggle this behaviour? This might be useful if other such devices are found. Or we could use a whitelist of vendors whose devices work correctly. Regards, Clemens _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
