Re: [PATCH] USB Audio Class 2 Mixer unit support for GET_CUR, SET_CUR and RANGE

[Takashi Iwai <tiwai@suse.de>]

On Fri, 18 Sep 2026 10:35:28 +0200,
Zipdox wrote:
> 
> 
> On 9/17/26 6:40 PM, Takashi Iwai wrote:
> >> Another thing on my mind is the following section from section
> >> 5.2.5.5.1 from the spec:
> >>> A Mixer Unit consists of a number of Mixer Controls, either programmable or fixed. A Mixer Control must
> >>> support the CUR and RANGE(MIN, MAX, RES) attributes. The settings for the CUR, MIN, and MAX
> >>> attributes can range from +127.9961 dB (0x7FFF) down to -127.9961 dB (0x8001) in steps of 1/256 dB or
> >>> 0.00390625 dB (0x0001). The settings for the RES attribute can only have positive values and range from
> >>> 1/256 dB (0x0001) to +127.9961 dB (0x7FFF).
> >>> In addition, code 0x8000, representing silence (i.e., -∞ dB), must always be implemented. However, it must
> >>> never be reported as the MIN attribute value.
> >> 
> >> Is this presently handled? For reference, 0x8000 is -32768. When I use
> >> qasmixer or alsamixer, for example, it only goes down to -32767. This
> >> is the same in UAC1 by the way. Should we add a mute control to mixer
> >> nodes? Or perhaps extend the reported range downward by one, and
> >> overwrite the lowest value to -32768 when writing to a mixer unit. The
> >> latter is my preferred solution.
> > 
> > I guess it's not; there is cval->min_mute flag, but it's for another
> > case (the minimal volume = mute) for the statically mapped mixer
> > elements, and not for this mixer unit case.
> > 
> > Maybe they should be implemented by additional mute switches.
> > 
> > Overall, the mixer unit controls have been rarely used by the devices,
> > so far, and that's the reason we didn't hit this over years :)
> > 
> > 
> > Takashi
> > 
> 
> From a UX perspective, I would prefer if the lowest setting on the
> control equals mute. This makes the most sense for a "mixer". Like I
> mentioned before, extending the range down by 1 and making that
> translate to -32768 seems like a convenient solution. When setting,
> simply check if it's a mixer unit (UAC1 or UAC2, both do this) and if
> the set value is equal to the minimum, and set the value to
> -32768. For getting, check if it's a mixer unit and check if the
> retrieved value is -32768, and then set the value to the minimum. The
> only caveat is that the resolution might not align anymore with the
> range. What do you think of this idea?

I thought of it, too.  OTOH, it'll be a bit tricky, because the range
isn't usually covering -32767 but only a part.  So -32768 won't be
reached continuously.

I'm inclined to have a simple mute switch in addition.


thanks,

Takashi