[PATCH v2] ALSA: usb-audio: Add quirk for Tascam Model 12

[Takashi Iwai]

On Tue, 29 Nov 2022 14:00:59 +0100,
John Keeping wrote:
> 
> Tascam's Model 12 is a mixer which can also operate as a USB audio
> interface.  The audio interface uses explicit feedback but it seems that
> it does not correctly handle missing isochronous frames.
> 
> When injecting an xrun (or doing anything else that pauses the playback
> stream) the feedback rate climbs (for example, at 44,100Hz nominal, I
> see a stable rate around 44,099 but xrun injection sees this peak at
> around 44,135 in most cases) and glitches are heard in the audio stream
> for several seconds - this is significantly worse than the single glitch
> expected for an underrun.
> 
> While the stream does normally recover and the feedback rate returns to
> a stable value, I have seen some occurrences where this does not happen
> and the rate continues to increase while no audio is heard from the
> output.  I have not found a solid reproduction for this.
> 
> This misbehaviour can be avoided by totally resetting the stream state
> by switching the interface to alt 0 and back before restarting the
> playback stream.
> 
> Add a new quirk flag which forces the endpoint and interface to be
> reconfigured whenever the stream is stopped, and use this for the Tascam
> Model 12.
> 
> Separate interfaces are used for the playback and capture endpoints, so
> resetting the playback interface here will not affect the capture stream
> if it is running.  While there are two endpoints on the interface,
> these are the OUT data endpoint and the IN explicit feedback endpoint
> corresponding to it and these are always stopped and started together.
> 
> Signed-off-by: John Keeping <john at metanate.com>
> ---
> v2:
> - Set ep->need_prepare not ep->need_setup; this was missed when forward
>   porting the patch as need_prepare is new in 6.1
> - Add more details to the commit message about why it is safe to reset
>   the interface here

Thanks, applied now.


Takashi