Behringer UMC202HD issues and a partial solution

[Nandakumar Edamana]

Hi Geraldo,
> Have you tried unloading the snd-usb-audio module and the reloading it
> with implicit_fb=1 as in "modprobe -r snd-usb-audio" and "modprobe
> snd-usb-audio implicit_fb=1" ?
>
> If not, try it. If it works and your playback issues are gone then
> your device may just need a quirk. If it doesn't work we still
> have dyndbg to go through.
It works! Had to switch the devices and options back and forth in 
pavucontrol
(sometimes needed even for the choppy playback), but it works.

So why did my quirks didn't work? Maybe I gave the wrong numbers? Or the 
code
was unreachable (I didn't put printk()s, sorry). I guessed the combos from
`lsusb -v` output, and even tried the combos that didn't make any sense
(because I didn't know what I was doing).

Here's what I tried, one after another (added to 
playback_implicit_fb_quirks in
/sound/usb/implicit.c):

IMPLICIT_FB_GENERIC_DEV(0x1397, 0x0507),

/* Didn't work */
IMPLICIT_FB_FIXED_DEV(0x1397, 0x0507, 0x84, 1),

/* Interface 2 is actually Capture as per `cat /proc/asound/card1/stream0`,
  * but "Implicit feedback Data" is found under that
  * (Interface 1 has explicit Feedback), and a Scarlett quirk seemed to use
  * a capture interface in playback_implicit_fb_quirks
  * (I know that it's a capture interface from `lsusb` output posted by 
someone
  * online)
  */
/* Didn't work */
IMPLICIT_FB_FIXED_DEV(0x1397, 0x0507, 0x88, 2),

/* Didn't work */
/* What am I doing with 0x08 ? */
IMPLICIT_FB_FIXED_DEV(0x1397, 0x0507, 0x08, 1),

/* Didn't work */
/* Why BOTH? But what's FIXED at first? Didn't find any doc. */
IMPLICIT_FB_BOTH_DEV(0x1397, 0x0507, 0x84, 1),

/* Didn't work */
IMPLICIT_FB_SKIP_DEV(0x1397, 0x0507),
> Last but not least, please use a vanilla kernel for these tests.
I built it using the source from kernel.org, not apt. But I've used 
oldconfig,
and I think patches are being applied while installing the kernel 
(currently I'm
using my daily driver installation for this). If it has to be more pure, 
I 'll
try. BTW, do you have any distro to recommend that has a near-vanilla 
kernel for
experiments like this?

Another question: assuming a quirk gets added for this device, is there any
chance that the quirk would break the experience for those who are already
using the device (possibly a different revision, but the same ID [1]) 
without
any issues?

[1] https://h-node.org/soundcards/view/en/2228/Behringer-UMC202HD

Thank you,

-- 
Nandakumar Edamana
https://nandakumar.org

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