-
- OSS Proxy - emulate OSS device using CUSE
-
- Copyright (C) 2008-2009 SUSE Linux Products GmbH
- Copyright (C) 2008-2009 Tejun Heo <tj@kernel.org>
-
-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
projects / osspd.git / blobdiff