Surround sound and volume control question

[Daniel Jacobowitz <drow@false.org>]

How is volume control supposed to work for surround sound codecs?  Is there
typically a register which affects every channel, or do you have to adjust
them separately?  I've been staring at the datasheets and prodding the
registers for mine (ICE1724, on a Shuttle SN25P).  There's three pieces:
the vt1720, the vt1617a, and a wm8728.  The vt1617a has ac97 volume control
registers, but the "master" volume doesn't affect the center/lfe or rear
surround channels.  I am guessing that the wm8728 is just for the last two
channels (I've got jacks for 7.1 but the vt1617a only does 5.1).

I cobbled together patches that add a stereo mute switch to the softvol
plugin, and set that in front of the card.  Then I can actually change
volume or mute the whole set.  But for various reasons this is
unsatisfactory, e.g. because it's not called "Master Playback", and because
it doesn't show up as the first mixer element - every application needs a
bit of configuration to find the right volume control.  And one of the
applications I wanted to configure, gnome-settings-daemon, is basically
unconfigurable.  It just grabs whatever gstreamer sees at the front of the
list.

(The mute patches available on request, btw, one alsa-lib patch and one
kernel driver bugfix for user-created boolean controls.)

Before I go any further with this I want to know how it works elsewhere.
I would think that having a single mixer control that affected all the
surround speakers would be a pretty standard thing to want!  Do other chips
have a single register for it?  Or does this one, and I'm just not seeing
it?

If I have no other way to get this to work than with the softvol plugin,
I think this is the way to go:
  - Rename the "Master" volume control/switch to "Front".
  - In userspace, have softvol create a control/switch named "Master".

Potential problems:
  - With a library older than kernel driver I think things would start
    picking up the Headphone channel as the master volume control if
    I made this change, yuck.
  - The user-created controls don't exist right away, so saving and
    restoring volume might not work.

All comments very welcome - I'm out of my depth here.  There's a bit of
related discussion in bug 1407.

-- 
Daniel Jacobowitz
CodeSourcery

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