Dear Alsa developers,

I've owned a MIDEX8 for some years now and wanted to do something with it in Linux , so I started some analysis and coding. The current result can be found at https://github.com/sgorpi/midex8

it looks a bit like the standard midi class driver, but it requires some periodic (i guess timer) messages on endpoint 0x82. Therefore, I've took quite some inspiration from the sound/usb/midi.c driver.

In general, it seems to work now for simple cases, but I'm having a few issues, that might be due to my lack of knowledge on the USB subsystem, or something else. In the (simplified) libusb test in the above repository, I do not seem to have these issues.

Issue 1: Not all (interrupt) urbs that are submitted will complete (within 25 ms at least). Wireshark doesn't show the packets being sent out, and the completeion handler never gets called for those urbs, but also no errors are given by usb_submit_urb. This happens mainly with the timer messages, and my current solution is to unlink the urbs that haven't completed within 25ms. However, I feel there is something going wrong there. If I do not send urbs on the midi-in endpoint, I do not see any missing urbs... Do I have to mutex anything for a shared urb queue or so?

Issue 2: Sometimes after sending a few midi messages, an empty urb shows up in wireshark. However, in code, there's an if-statement that should allow only messages of length > 0 to be submitted (line 568, if (num_read > 0)). Might this have to do with issue 1?

I intend to improve on the driver some more, but if you could give me any advice on the above issues that would be highly appreciated.

With kind regards, Hedde