Re: ALSA USB quirk mixer driver question

[Takashi Iwai <tiwai@suse.de>]

On Fri, 14 Feb 2025 13:05:13 +0100,
Lena wrote:
> 
> Hi all!
> 
> I'm currently trying to develop an usb quirk mixer driver for a
> desktop USB audio interface.
> 
> The interface exposes the following:
> 
> - interface 0-2 are USB UAC compliant, bound to by snd-usb-audio, and
>   working flawlessly
> 
> - interface 3 is vendor specific, and we can ignore it (used for
>   firmware updates)
> 
> - interface 4 exposes a non-standard mixer with a HID interface exposing
>   2 interrupt endpoints, and is used by the vendor windows app.
> 
> I have studied the protocol with usbpcap and can successfully talk to
> the interface on the userspace, but i had a question regarding how to
> proceed in the kernel.
> 
> This is because, after creating a new entry in `sound/usb/mixer_quirks.c`
> with all the needed controls, userspace can interact with them, etc.
> 
> I can't figure out how to tell ALSA to
> bind to the correct interface for the mixer (and kick out usbhid).
> Looking at the other quirk mixer drivers it doesn't look (?) like any
> other quirk mixer driver is binding to a *different* interface, not
> even `sound/usb/mixer_scarlett.c`.

The HID device won't be bound with the sound driver, and the mixer
"quirks" found in USB-audio driver aren't for managing HID interfaces
but only about audio interfaces.  That said, what you'd need would be
to write rather a HID driver for the interface.  The plumbing of both
interfaces is done in the upper level, typically in user space.

HTH,

Takashi

> Generally, in a standalone USB driver you'd have tracking/teardown
> handled easily with a callback, but i'm having trouble with the
> dependencies here. (simply using `usb_sndintpipe()` or
> `usb_interrupt_msg()` triggers kernel oopses, so that's probably not it)
> 
> What i think i have to do is to use `usb_get_intf()`, then i have to
> somehow claim it (does `usb_driver_claim_interface()` work here?). But
> `usb_get_intf()` requires me to then release the interface with
> `usb_put_intf()`, and it doesn't look like there's anywhere for me to do
> that. (`snd_usb_mixer_elem_free()`? doesn't look like the order is
> correct)
> 
> `iface_ref_find()` would sorta do what i need, but i cannot use it here.
> 
> Could someone please point me in the right direction? I'm very stuck at
> the moment.
> 
> Thanks,
> 
> Lena