Re: Handling complex matrix mixers in ALSA

["Geoffrey D. Bennett" <g@b4.vu>]

Hi Lina,

On Mon, Jul 01, 2024 at 01:04:41AM +0900, Asahi Lina wrote:
> Hi,
> 
> I'm reverse engineering and implementing support for the RME Digiface
> USB, which is an ADAT interface with a non-class-compliant interface
> (probably similar to other RME interfaces like the MADIface, but I don't
> have any others to test). The basic audio streaming works fine with an
> entry in quirks-table.h and a format quirk to set the system sample rate
> in quirks.c. Now I need to figure out how to implement the mixer controls.
> 
> Currently I have the snd-usb-audio driver claiming only interface #0
> (streaming) and I use a Python script to control the mixer/settings
> directly with libusb (control transfers and interface #1). This works
> fine and there's some prior art for this in the firewire world (for
> example, snd-dice doesn't do any mixer control stuff and you have to use
> ffado-mixer to control everything from userspace) but I assume it's not
> really the best way to go?

I'm the developer of the Scarlett2 driver. Doing as much as possible
through ALSA mixer controls has worked well in my experience; as you
say, being able to do save/restore with alsactl is useful.

> The problem is that the device has a 66x34 matrix mixer, with up to 2048
> cross points enabled at once. Exposing each cross point as an ALSA mixer
> control (similar to how mixer_scarlett2.c does it) would mean 2244
> controls just for the mixer... which seems like a bit too much.

Note that mixer controls may have more than one value. So I think you
could have 66 controls with 34 values or 34 controls with 66 values or
1 control with 2244 values.

> On top of that there is also VU meter feedback for all the
> inputs/outputs, as well as general fader controls for each output and
> global output configs and status. I'm not sure about the VU meters, but
> everything else sounds like it would map fine to normal mixer controls.

I handle the VU meter feedback in the Scarlett2 driver with a
read-only volatile control that contains multiple values (see
scarlett2_meter_ctl).

Regards,
Geoffrey.

> Is there some recommended way to expose this kind of matrix mixer
> interface to userspace? I think for something like this you pretty much
> have to rely on device-specific tools to make the UX manageable, so
> maybe hwdep... but at least exposing everything as an ALSA control would
> have the advantage of supporting save/restore with something like
> alsactl. So I don't really know what's the way to go here.
> 
> System settings/general status/output faders go via control transfers,
> while interface #1 has an interrupt IN endpoint (streaming state
> feedback, not very useful) and two bulk endpoints (matrix mixer control
> out, VU meter data in). There's another pair of bulk endpoints in
> interface #2 which I'm guessing are for firmware updates (I haven't
> looked at that part). So in principle it's not crazy to expose all the
> system controls/output faders as mixer controls in ALSA and leave
> interface #1 entirely unclaimed so a userspace program can directly
> configure the matrix mixer and access VU meter levels. There is a global
> mixer enable bit (controlled via ctl transfer), so if that is exposed as
> an ALSA control and disabled by default the interface will operate as a
> 1:1 in/out interface without needing any custom userspace to configure
> the mixer.
> 
> There's one other quirky thing: it also needs a way to set the sample
> rate as a mixer control, because you need to be able to configure the
> rate even when the PCM device is not open (since that affects
> stand-alone mixer operation). I imagine the right logic here would be to
> have a selector control for the system sample rate, and automatically
> change it and lock it when the PCM is opened with a given rate?
> 
> Any thoughts welcome ^^
> 
> ~~ Lina