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

[Zipdox]

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?