[alsa-devel] Handling of the DXS controls in the via82xx driver

Raymond Yau superquad.vortex2 at gmail.com
Mon Mar 29 03:24:33 CEST 2010

2010/3/28 Soeren D. Schulze <soeren.d.schulze at gmx.de>

> Hi everyone,
> before my latest upgrade to Linux 2.6.32 (yes, sorry, it's Debian), I
> had four levers in my alsamixer: VIA DXS, VIA DXS 1, VIA DXS 2 and VIA
> DXS 3.  They controlled the hardware mixing when accessing the sound
> device multiple times.
> Now they have disappeared.  I think I can pretty certainly trace this
> down to commit 2fb930b53f513cbc4c102d415d2923a8a7091337 to Linux
> [
> http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.32.y.git;a=commit;h=2fb930b53f513cbc4c102d415d2923a8a7091337
> ],
> which "move[s] DXS volume controls to PCM interface".
> As I'm not an ALSA developer, I don't realize the intended effect of
> this change, but the actual effect is that I cannot change the DXS
> volumes any more.  mplayer still controls the global PCM volume.  Well,
> in fact, there is one way.  While an mplayer is running, I can do:
> amixer cset numid=47,iface=PCM,name='PCM Playback Volume' 24,24
> When restarting mplayer, the volume is reset to 31,31 (maximum).  Commit
> 3d00941371a765779c4e3509214c7e5793cce1fe
> [
> http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.33.y.git;a=commit;h=3d00941371a765779c4e3509214c7e5793cce1fe
> ]
> seems to be responsible for this.
> The bad thing about it is that 31,31 is a really bad setting for my
> poorly engineered sound card.  It distorts a lot.
> I suspect that the real *bug* is that mplayer does not use the
> per-channel DXS controls even though it's supposed to (which seems to be
> why the DXS controls are called "PCM Playback Volume").
> Personally, rather than fixing this bug, I would just revert to the
> former state.  I was always happy with all the DXS levers set to 75%.
> Thank you
> Sören

Those are per voice volume control , they can be used for the hardware
acclearation in wine if wine implmenet the real hardware acceleration

After this change , they are not used by the mixer any more , only those
application which open those subdevices can find the corresponding control
by the subdevice number

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