mixer ioctls on sound devices

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I've been tracking down a problem with the Linux Citrix ICA client killing 
sound if the volume in changed in a Windows session. I traced this to it 
trying to alter mixer settings on /dev/dsp (a.k.a. /dev/sound0) instead of 
/dev/mixer0. It was getting EINVAL and then shutting down its sound. It 
therefore appears that Linux semantics are to pass through mixer ioctls 
if performed on /dev/sound0. We need to support this at some level.
A cheap workaround is:

--- sys/dev/audio.c     30 Jan 2008 12:28:35 -0000      1.1
+++ sys/dev/audio.c     30 Jan 2008 12:29:49 -0000      1.2
@@ -1067,9 +1067,10 @@
        case AUDIO_DEVICE:
        case AUDIOCTL_DEVICE:
                error = audio_ioctl(sc, cmd, addr, flag, l);
-               break;
+               if (error == EINVAL)
+                       /* FALLTHROUGH */
        case MIXER_DEVICE:
-               error = mixer_ioctl(sc, cmd, addr, flag, l);
+                       error = mixer_ioctl(sc, cmd, addr, flag, l);
                break;
        default:
                error = ENXIO;

However, it might be felt that this is not the right place to do this pass 
through (i.e. should we natively support mixer operations on 
/dev/sound0?). Perhaps it should be in compat/ossaudio/ossaudio.c or even 
compat/linux/common/linux_ioctl.c. Or perhaps audio_ioctl should return 
EPASSTHROUGH in such a situation.

Opinions?

--
Stephen