[Bug 207065] C-media USB audio device stops working from 5.2.0-rc3 onwards

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=207065

--- Comment #6 from Alan Stern (stern@rowland.harvard.edu) ---
On Thu, 9 Apr 2020 bugzilla-daemon@bugzilla.kernel.org wrote:

> --- Comment #4 from Mathias Nyman (mathias.nyman@linux.intel.com) ---
> Thanks, traces show its related to Clearing TT buffer after a STALL on 
> endpoint 0.
> 
> The first stall looks like a protocol stall, not a function stall, meaning
> that
> endpoint isn't really halted, just that the device does not support the
> request in the control transfer. 
> 
> Anyway, xhci starts clearing what it assumes is a halted endpoint, 
> including clearing the hub TT buffer.
> 
> Specs are a bit unclear if TT should be cleared in this case, 
> or at least I couldn't find it.

TTs should be cleared when an error occurs in the protocol.  STALL
isn't an error; a real error would be a timeout or CRC mismatch or URB
cancellation, things like that -- things that might leave the TT in a
busy state (because it hasn't sent its final status back to the host)
when it ought to be idle.  Sending a STALL isn't a protocol error and
it does clear the TT status.

This is discussed (not as explicitly as one might want) in sections 
11.17.3 and 11.17.5 of the USB-2.0 specification.

Alan Stern

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