ALSA USB quirk mixer driver question

ALSA USB quirk mixer driver question

[Lena]

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`.

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

Re: ALSA USB quirk mixer driver question

[Takashi Iwai]

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

Re: ALSA USB quirk mixer driver question

[Lena]

Thanks for the reply!

So, if i understand correctly, instead of going through 
sound/usb/mixer_quirks.c, i need to create a standalone USBHID driver 
that exposes the ALSA kcontrols and then the ALSA userspace understands 
that it's the same interface and 'links' the UAC/isochronous audio part 
and the standalone ALSA mixer kcontrol part that has the volume knobs, 
mute, output select and input impedance/phantom power/headphone gain 
switches such that an userspace application (like alsamixer) treats it 
all as a single thing.

Does this driver then belong in sound/usb/ (given it imports the API in 
<sound/control.h>) or somewhere else (e.g. drivers/hid/usbhid/, 
drivers/usb/)?

Thanks again,

Lena

Re: ALSA USB quirk mixer driver question

[Takashi Iwai]

On Thu, 27 Feb 2025 00:15:01 +0100,
Lena wrote:
> 
> Thanks for the reply!
> 
> So, if i understand correctly, instead of going through
> sound/usb/mixer_quirks.c, i need to create a standalone USBHID driver
> that exposes the ALSA kcontrols and then the ALSA userspace
> understands that it's the same interface and 'links' the
> UAC/isochronous audio part and the standalone ALSA mixer kcontrol part
> that has the volume knobs, mute, output select and input
> impedance/phantom power/headphone gain switches such that an userspace
> application (like alsamixer) treats it all as a single thing.

Well, how to manage HID device is other question.  Usually a HID
device provides the controls via input device API.  It's not
prohibited to use a sound API (in this case you'd need to create your
own sound card object), though.

> Does this driver then belong in sound/usb/ (given it imports the API
> in <sound/control.h>) or somewhere else (e.g. drivers/hid/usbhid/,
> drivers/usb/)?

Not likely.  It'll be a pure HID driver.


HTH,

Takashi