[alsa-devel] USB asynchronous mode feedback format

Julian Scheel julian at jusst.de
Sat Oct 16 18:30:31 CEST 2010

Hi Alex,

Am Samstag, 16. Oktober 2010, 15:58:40 schrieb Alex Lee:
> On Sat, 2010-10-16 at 12:25 +0200, Julian Scheel wrote:
> > Thanks, I implemented them the way same you did. So for get_min/max/cur I
> > always return 48kHz and set_cur simply does nothing.
> > Now playback in general works again. But still the rate feedback seems to
> > have no impact on the data-rate. Also the frequency value shown in
> > proc-interface is unchanged. When rate feedback works, it would probably
> > be slightly different than 48.000kHz?!
> OK, you are getting close :-)

I hope so (c:
> One debug technique we have been using is to do a USB wire dump, using
> something like wireshark.  This way you can see exactly what goes on
> between the PC host and the device.

Yes, using wireshark is a good idea indeed. And it immediately turns out what 
the problem is.
The device sends ISOCHRONOUS out packets on the feedback endpoint (5, 
respectively 0x85) - but when I try to send 4 byte, the URB length is always 
0. If I only send 3 or less bytes the packets look fine.
I thought it might be a bug regarding max packet size in the at91lib and 
raised it to 5, but still only 3 bytes can be send. Any clues what might cause 
such an error?

> The other technique is to build a special debug kernel to have copious
> debug messages.
> You might want to double check on the EP limitations.  Is the feedback
> EP capable of ISO transfer?  Are you able to use 4 bytes as the max EP
> size?  Is the EP double buffered (required for ISO transfers)?  With the
> wireshark dump you will be able to see exactly what is transferred as
> the feedback rate.

Yep, using endpoint 5, which is able to do ISO and has dual bank. It allows up 
to 512byte packet size.

> Also, the Max EP size of the ISO OUT data EP should only be one sample
> frame (ie 4 bytes in your case - 16 bits x stereo) bigger than the
> "correct" frame size, ie 192 bytes ====> 196 bytes.

Ok, changed that.

> I looked through your updated USB descriptors and they appear to be
> correct.

Fine, thanks (c:


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