[Bug 219111] Xone:23C mixer not recognized as a 2in/2out device

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=219111

--- Comment #38 from Alan Stern (stern@rowland.harvard.edu) ---
It's more than just the driver knowing what configuration to use.  The device
has to be _told_ to use that configuration; until it is told it won't use any
configuration at all and won't function.  Your usbmon traces and Windows packet
captures all show the computer telling the device to use configuration 1.  (If
you made a usbmon trace with old_scheme_first=Y, presumably it would show the
computer telling the device to use configuration 2.)

The configuration matters because the data sent over the USB connection is
formatted differently depending on the number of channels.  No matter what the
driver may know about the possible configs, as long as the device is using
config 1, it will provide only two-channel stereo.  And as long as it is using
config 2, it will provide only four-channel stereo.

The difference between the old scheme and the new scheme is rather technical. 
It has to do with how the device is initialized when it is first plugged in
(and also whenever it gets reset).

With the old scheme, after performing an initial reset the computer asks the
device to send just the first 8 bytes of its device descriptor, then the
computer tells the device what address to use, and then it reads the complete
device descriptor and all the other descriptors.

With the new scheme, after performing the initial reset the computer asks the
device to send the first 64 bytes of its device descriptor.  Since the device
descriptor is only 18 bytes long, the response is always a short one.  The
computer then resets the device a second time, tells it what address to use,
and reads all the descriptors.  (That's why I thought skipping the second reset
might cause the device to make both configurations available, like it does with
the old scheme.)

The old scheme is what the USB 2.0 specification recommends.  The new scheme is
what Microsoft implemented early on, however, and as a result quite a few
devices would work with the new scheme but not the old one.  In theory any
device should work properly with either scheme, but in practice they don't
always do so.  As in this case.

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