99% of my reads are 14 bytes, a status record, but a nonce result is 7 bytes. Config data is 8 bytes, but I haven't been using that yet. The max size defined by the descriptor is 64 bytes. I'm thinking of just standardizing on 15 byte reads (which allows for a 1 byte queue flag in compact 16 bytes circular queues) and ignoring extra bytes. That way libusb always waits for 15 bytes regardless of what format is being replied. Right now I poll on 31 bytes (my buffer size) and ignore any timeouts due to less bytes being returned. But doing this sometimes results in a second reply appended on the first which seems to be because two packets come in too quickly and don't get read individually.

The usbutils appears to return what I request but always with a timeout as it loops trying to get the full requested size, and fails. It's probably designed to handle larger transfers but isn't very flexible with tiny status updates. By using a fixed size I avoid this. It always returns with what I request and not less. I can easily ignore the extra myself and the sizes are so small that there will be no performance difference.

Most of the writes are only 2 bytes except pushing new work which is 47 bytes. I don't appear to have issues with writing except now and then the device just stops reading even though it continues to write out nonce data. So another bug to work on there.

BTW I've updated the driver to 3.3.1 and pushed to github along with firmware. This is working for me now but rather unreliably. I'm working on improvements.