OSS Proxy - emulate OSS device using CUSE

1. What is it?
--------------

Well, first, OSS refers to Open Sound System.  If it still doesn't
ring a bell, think /dev/dsp, /dev/adsp and /dev/mixer.

Currently, Linux supports two audio programming interface - ALSA and
OSS.  The latter one is deprecated and has been that way for a long
time but there still are applications which still use them including
UML (usermode Linux) host sound support.

ALSA contains OSS emulation but sadly the emulation is behind
multiplexing layer (which is in userland) which means that if your
sound card doesn't support multiple audio streams, only either one of
ALSA or OSS interface would be usable at any given moment.

There have been also attempts to emulate OSS in userland using dynamic
library preloading - aoss and more recently padsp.  This works for
many applications but it's just not easy to emulate everything using
the technique.  Things like polling, signals, forking, privilege
changes make it very difficult to emulate things reliably.

OSS Proxy uses CUSE (extension of FUSE allowing character devices to
be implemented in userspace) to implement OSS interface - /dev/dsp,
/dev/adsp and /dev/mixer.  From the POV of the applications, these
devices are proper character devices and behave exactly the same way
so it can be made quite versatile.


2. Hmmm... So, how does the whole thing work?
---------------------------------------------

The OSS Proxy daemon - osspd - should be started first.  Note that
osspd will fail to start if sound device number regions are already
occupied.  You'll need to turn off OSS or its emulation[1].

On startup, osspd creates /dev/dsp, /dev/adsp and /dev/mixer using
CUSE.  When an application access one of the devices, all IOs are
redirected to osspd via CUSE.  Upon receiving a new DSP open request,
osspd creates a slave process which drops the root privilege and
assumes the opening process's credentials.  After handshaking, osspd
forwards all relevant IOs to the slave which is responsible for
actually playing the sound.

Currently there's only one slave implemented - ossp-padsp, which as
the name suggests forwards (again) the sound to pulseaudio.  To sum
up, the whole pipe looks like the following.

 App <-> /dev/dsp <-> CUSE <-> osspd <-> ossp-padsp <-> pulseaudio

Which is a lot of forwarding, but on modern machines, it won't be too
noticeable.


3. What works?
--------------

Well, MIDI part isn't implemented and I doubt it will be in any near
future but except that everything should work.  Playing, recording,
5.1ch, A-V syncing, all should work.  If not, it's a bug, so please
report.

The mixer behaves a bit differently tho.  In the original OSS,
/dev/mixer is the hardware mixer, so adjusting volumes there affects
all audio streams.  When using ossp, each process group gets its own
mixer and the mixer always contains only two knobs - PCM and IGAIN.
Combined with per-stream volume control of pulseaudio, this scheme
works quite well for applications with embedded volume control
although it makes standalone OSS mixer programs virtually useless[2].


4. How do I use it?
-------------------

First you need CUSE support in kernel which might land on 2.6.28 with
sufficient luck[3] and then you also need libfuse which supports
CUSE[4].  Once you have both, it should be easy.  First build it by
running `make'.  You can set OSSPD_CFLAGS, OSSPD_LDFLAGS,
OSSP_PADSP_CFLAGS and OSSP_PADSP_LDFLAGS if you have stuff at
non-default locations.

After build completes, there will be two executables - `osspd' and
`ossp-padsp'.  Just copy them to where other system executables live.
Specific location doesn't matter as long as both files end up in the
same directory.

Execute `osspd'.  It will create the device files and you're all set.
`osspd' uses syslog with LOG_DAEMON facility, so if something doesn't
work take a look at what osspd complains about.


[1] As of this writing, turning on any sound support makes the
    soundcore module claim OSS device regions.  Patch to make it claim
    OSS device regions only when OSS support or emulation is enabled
    is scheduled for 2.6.28.  Even with the patch, soundcore will
    claim OSS device regions if OSS support or ALSA OSS emulation is
    enabled.  Make sure they're turned off.

[2] If you have a strong reason to use standalone OSS mixer program,
    you can play some shell tricks to put it into the same process
    group as the target audio application.  e.g. To use aumix with
    mpg123 - `(mpg123 asdf.mp3 > /dev/null 2>&1 & aumix)', but
    seriously, just use PA or ALSA one.

[3] For the time being, here's the git tree with all the necessary
    changes.  This tree is base on top of 2.6.27-rc3.

    http://git.kernel.org/?p=linux/kernel/git/tj/misc.git;a=shortlog;h=cuse
    git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git cuse

[4] And libfuse with the modifications can be found at...

    http://userweb.kernel.org/~tj/ossp/fuse-cuse.tar.gz