[alsa-devel] USB audio device startup question

[Ricard Wanderlof]

(This is in reference to getting the Zoom R16 to work, but the main query 
in this post is of a more general nature).

In trying to get a Zoom R16 in audio interface mode to work in Linux I'm 
trying to understand how the USB-audio (snd-usb-audio module) subsystem 
works in Linux. I've looked in the USB audio version 1 and 2 documents, as 
well as the USB 2.0 standard, but there are some things that don't seem to 
be well defined there, like exactly how the audio data is packaged in the 
isochronous transfer packets.

I've been using Wireshark to visualize the USB communication, and also 
comparing with the corresponding transfers on a Windows box by dumping the 
traffic with USBPcap.

The only other USB audio interface I have access to here is an Edirol 
UA-5. I've noted that when plugging in the UA-5, there's period of about 
15 seconds during which capture and playback are set up and audio data 
transferred, after which the interface is shut down again. When enabling 
the R16, during the corresponding initial period I see playback data but 
no capture data during this phase. I don't know if that is significant or 
not, for instance, if the lack of capture data during this phase is 
because the playback is not working as it should.

My question is: what governs this startup phase, can someone point to a 
specific part of the code or some documentation describing what is 
supposed to go on here? Is it some sort of device test period to determine 
that the device actually works?

The problem with the R16 is that although capture works fine, playback 
doesn't result in any audio. With the correct quirk set up (most 
importantly, using a 24 bit format), aplay appears to work fine with no 
errors, neither are there any complaints from dmesg, and looking at the 
communication I can see outgoing isochronous data from the host being 
transferred fine.

More work needs to be done, but I've looked at the startup of the Windows 
driver which is deceptively simple: it basically just sets the 
appropriate capture and playback interfaces, together with sending off 
some sample rate USB control commands, the isochronous link comes up and 
that seems to be it. I've tried to emulate the specific USB control 
commands in Linux, as well as add some delays that there appear to be in 
the Windows driver, but so far it hasn't made a difference.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30