Re: [PATCH] Bluetooth: btusb: check SCO MTU before enabling USB ALT 3 for WBS

[Pauli Virtanen <pav@iki.fi>]

Hi Michał,

to, 2021-07-22 kello 15:29 +0200, Michał Kępień kirjoitti:
> Thank you for your work on this patch!
> 
> > Some USB BT adapters don't satisfy the MTU requirement mentioned in
> > commit e848dbd364ac ("Bluetooth: btusb: Add support USB ALT 3 for WBS")
> > and have ALT 3 setting that produces no/garbled audio. Check that the MTU
> > condition is satisfied, and fall back to ALT 1 if not.
> > 
> > Tested with USB adapters (mtu<72, produce sound only with ALT1)
> > BCM20702A1 0b05:17cb, CSR8510A10 0a12:0001, and (mtu>=72, ALT3)
> > RTL8761BU 0bda:8771, Intel AX200 8087:0029 (after disabling ALT6).
> 
> Commit e848dbd364aca44c9d23c04bef964fab79e2b34f causes garbled mSBC
> audio on my Bluetooth adapter (8087:0a2b), so I gave your patch a try.
> 
> Unfortunately, the patch in its current shape does not seem to be enough
> to fix things for this adapter, which reports an SCO MTU of 96 bytes and
> produces garbled mSBC audio with ALT 3.
> 
> For the record, reverting e848dbd364aca44c9d23c04bef964fab79e2b34f on
> top of v5.14-rc2 makes mSBC work again with my adapter.
> 
> I am no Bluetooth expert, but I will be happy to perform any additional
> debugging and/or testing.

Thanks for testing! What userspace software did you use? Pulseaudio at
least I think had assumption that mtu <= 60, which breaks here
(Pipewire works, don't know about bluez-alsa). If otherwise, maybe it
then needs also the manufacturer flag.

Best,
Pauli