[alsa-devel] Boss GT-001

[Ricard Wanderlof]

On Wed, 7 Oct 2015, Keith A. Milner wrote:

> The data I'm getting from Wireshark via USBMon seems OK from my initial
> viewing. Incidentally, Wireshark can sniff directly from usbmon interfaces and
> exposes them as capture interfaces (at least with the version I have).

Same here. I might have been unclear in my previous message, but that is 
what I meant.

> The isochronous data appears as follows:
> 
> "54","24.767751000","host","7.14","USB","192","URB_ISOCHRONOUS

Yes, these are the isochronous packet headers.

> in","eeffffff 00000000 70000000 00000000 eeffffff 70000000...",""
       -------- -------- -------- --------
         tag ?   offset   length   status  and again (next header)

My understanding is that the isochronous data is sent one packet per frame 
on the wire (interval: 1 ms for full speed, 125 µs (actually called a 
microframe) for high speed) but delivering the data one packet at a time 
to the USB application would result in a lot of packet transfers, so 
isochronous data is lumped together and delivered at some more or less 
regular interval to the higher layers. So at USB API (or Wireshark) layer 
you get these 'isochronous' packets which list a bunch of headers followed 
by the corresponding data as a single 'packet', even though it's been 
assembled from several distinct ones. The 'offset' fields are relative to 
the end of the headers in the resulting packet, where the actual 
isochronous data starts.

> Wireshark also appears to decode them quite nicely.  I've pasted one of the
> GET DESCRIPTOR responses below.
> ...
> DESCRIPTOR Response CONFIGURATION
> 
> Frame 9: 240 bytes on wire (1920 bits), 240 bytes captured (1920 bits) on
> interface 0
> ...
>     iInterface: 0
> UNKNOWN DESCRIPTOR
>     bLength: 6
>     bDescriptorType: 0x24 (unknown)
> INTERFACE DESCRIPTOR (1.1): class Vendor Specific
>     bLength: 9
>     bDescriptorType: 0x04 (INTERFACE)
> ...

Actually, you should be able to get the configuration information directly 
in Linux by using 'lsusb -v'. It lists the 'Unknown descriptors' in their 
entirety in hex (prefixed by 'UNRECOGNIZED:') so that you can decode them 
separately without having to examine the actual binary packet content in 
Wireshark.

/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