Re: New meaning oss_sysinfo.nummixers

Hello Alfonso,

Alfonso Sabato Siciliano wrote:
> I was inactive for months, one week ago I reinstalled CURRENT on my main
> laptop (and others). The meaning of "struct oss_sysinfo.nummixers"
> changed. Let's say:
> 
> [...]
> 
> Output "nummixers: 7", But
> 
> % sysctl dev.pcm
> ... prints 6 devices dev.pcm.[0-5].*
> 
> % ls /dev/mixer*
> /dev/mixer0 /dev/mixer1 /dev/mixer2 /dev/mixer3 /dev/mixer4 /dev/mixer5
> 
> 6 mixers from 0 to 5. But now oss_sysinfo.nummixers is 7, it was 6 until
> some months ago.
> 
> I thought the new value could be the max index, in the case some mixers
> are closed, example /dev/mixer0 /dev/mixer2 /dev/mixer5. But in this
> case 'nummixers' should be 5 not 7.
> 
> Maybe the hidden /dev/mixer is in the count (6 + 1 = 7).
> 
> Same situation on 3 laptops. So, what is the new meaning for
> oss_sysinfo.nummixers?

The behavior changed in the following commit of mine: 5d980fadf73d
("sound: Handle unavailable devices in various OSS IOCTLs") [1].
The commit message explains the rationale behind changing this
behavior. I am aware it's a bit quite unintuitive, but it solves quite a
few bugs we have with people using nummixers to loop through mixer
devices, which I suppose is the most typical use.

oss_sysinfo.nummixers now is simply populated with whatever
devclass_get_maxunit(pcm_devclass) returns, which should be the _next_
maximum unit number that will be allocated for an audio device, hence
the 7 you get (this is how devclass_get_maxunit() works in general).

Although a bit tedious, you can count the actual number of mixers in the
system by issuing open(2) for each /dev/mixer<n> and seeing which ones
succeed. This way you know the total number, but also which mixers
specifically are present.

Christos

[1] https://cgit.freebsd.org/src/commit/?id=5d980fadf73df64a1e0eda40a93170ed76ce6f14