Re: Assigning sound card to specific X display - the chroot+bind-mount way

[Helge H. <hel...@ai...>]

Aivils wrote:

>On Thursday 02 June 2005 04:27, Ken McCord wrote:
>  
>
>>I'm running a dual-head setup using Debian unstable + the x.org packages
>>from Ubuntu.  I have two sound cards, and I'd like to assign each of
>>them
>>to a specific X display.  How can I do this?
>>
>>The first sound card is /dev/dsp and the second is /dev/dsp1.  I want
>>display :0 to use /dev/dsp and display :1 to use /dev/dsp1.
>>
>>I've tried the instructions available at
>>http://howtos.linux.com/howtos/XFree-Local-multi-user-HOWTO/automation_multy_snd-cards.shtml
>>but I haven't been able to get them to work with gdm/Gnome.
>>    
>>
>Under Linux access to sound subsystem  is little chaotic.
>Any application use hers own sound driver.
>
>Try out set up artsd under KDE or esd under GNOME . 
>
>Even if these software sound mixers like artsd or esd runs
>properly, then You cannot disable Java and Flash Player,
>which uses sound device directly.
>I open Java binary with hexedit and manulay replace
>/dev/dsp with fake non-existing device file for silence.
>  
>
There is a way.
One xserver can run in its own chroot.  The two servers can have
distinct /dev directories that way, where the two "dsp" files in the
distinct directories have different device numbers and so
refer to different sound devices.
Make a copy of /dev in /chroot/dev.  In there, make sure
"dsp" refers to the second soundcard instead of the first.

On a debian system "dsp" is a symlink to "dsp0".  Change it to
a link to "dsp1" instead.  On other systems you may have to
do something like "mv dsp1 dsp0".

Whatever the solution is, do the same for all other sound-related
files, such as "mixer" and so on.


Now the chroot has a customized /dev.  But what about all the
other directories needed for normal operation, such as
/bin, /usr, /var, /home and so on?

The answer is to bind-mount them.  Bind-mount every top-level
directory except /dev into the chroot, and you should be fine.
Then chroot into the /chroot directory, and start the second xserver.
(Details: man mount, search for "bind")

Having all those bind-mounts means you don't have to duplicate
a single file (except for the contents of /dev).  Both xservers will
access the same files, so they should work and the users shouldn't
be able to notice the difference. Except that they get distinct soundcards.

I use a different approach at home.  The machine happens to have
two soundcards, but I use only one of them currently.  It has a trident
chip, and that one support multiple users at the same time so no
problems there.  The two displays are side by side with the speakers
on each side of both of them, so both get correct stereo sound from the
same speaker set.

Helge Hafting