Ignored USB-audio implicit feedback in kernel 5.8rc3

[Pavel Hofman]

Dne 29. 06. 20 v 17:08 Pavel Hofman napsal(a):
> Dne 29. 06. 20 v 13:28 Pavel Hofman napsal(a):
>>
>> Audio analyzer RTX6001 (XMOS-based) is using implicit feedback, yet the
>> feedback is not used by the latest kernel 5.8 rc3 (i.e. already with the
>> latest implicit-feedback patches).
>>
>> Two analyzers on two different PCs are getting clicks in duplex-mode
>> loobpack, one every 10 secs, the other one every 50 secs.
>>

I am very sure the problem is identical to that for MOTU M2/4, Solid
State Logic SSL2+, Fractal Audio Axe-Fx III etc.
https://github.com/torvalds/linux/commit/e7585db1b0b5b4e4eb1967bb1472df308f7ffcbf
. I will try the simple quirk, IMO it will work OK. BTW if I understand
correctly all these devices use the XMOS.

However, I would like to ask, why a quirk for these devices is required.
The comment explanation in this interesting patch
https://lore.kernel.org/patchwork/patch/1174179/ talks about the same EP
numbers, but in the different direction. 0x01 EP OUT + 0x81 EP IN
implicit feedback data. All the devices mentioned above have this
numbering, just the EP IN is in a different interface than the EP OUT.
But please are there actually any duplex implicit feedback devices
having EP OUT and implicit-feedback EP IN  in the same interface? The
explicit feedback EP IN is clear, they are in the same interface. E-MU
0404 USB has EP OUT + EP IN explicit feedback in interface 1, together,
EP IN implicit feedback data in interface 2. My other soundcards,
adaptive OUT and async IN again each in a different interface (though
there is no feedback to solve).

Please is the requirement that EP OUT + EP IN implicit feedback data
must be in the same interface really necessary? If such a requirement
was dropped, IMO many devices could be removed from the existing
set_sync_ep_implicit_fb_quirk and many devices would work out of the
box, as they require no other quirk (unlike the MOTUs, but that's a
different issue).

Thank you very much for comments. Very likely I am missing something
important, I just do not see what :-)

Best regards,

Pavel.