Oss2jack

Introduction

Oss2jack allows OSS applications to use the jack sound server with the advantage of being able to start several OSS applications at the same time. With giving jackd some additional options both oss2jack and jackd can also make use of your .asoundrc settings - including software mixing and plugins like Softvol.
Since oss2jack depends on the fusd-kor kernel module, which isn't in portage and versions from several sites don't seem to compile on newer kernels (>= 2.6.19), a few steps are required to get it working.
To get best latencies for your jackd and OSS based applications you should also install a realtime kernel as described below.

ALSA Configuration

To have better control over the ALSA devices and settings that jackd will use, I suggest to create an .asoundrc configuration file for ALSA with some basic settings:

pcm.!default {        ### this will be the standard device for any ALSA based application
	type plug
	slave.pcm "dmixer"
}
pcm.dmixer {          ### defines a new playback device with software mixing enabled
       type dmix
       ipc_key 1024
       ipc_perm 0666
       slave {
               pcm "hw:0"
               channels 2
               period_time 0
               period_size 1024
               buffer_size 4096
               rate 48000
       }
       bindings {
              0 0
              1 1
       }
}
pcm.dsnooper {       ### defines a new capturing device with software mixing enabled
       type dsnoop
       ipc_key 1024
       ipc_perm 0666
       slave {
               pcm "hw:0"   
               channels 2
               period_time 0
               period_size 1024
               buffer_size 4096
               rate 48000
       }
       bindings {
               0 0
               1 1
       }
}
ctl.!default {
	type hw
	card 0
}
ctl.dsnooper {
	type hw
	card 0
}
ctl.dmixer {
	type hw
	card 0
} 

If you need more information on setting up ALSA you could try this HOWTO.

Recommended versions

At the time of writing this (04/2008) I only got proper results when using a certain combination of driver and software versions. When using jack-audio-connection-kit-0.109.* for example jackd would crash if an oss2jack application causes xruns. Also using the kernel-alsa driver rather than the alsa-driver ebuild gave me best results. So here is my current list of versions that work fine:

1. Linux Kernel 2.6.22.1-rt9 (vanilla kernel with manually applied realtime patches)
2. media-libs/alsa-lib-1.0.16
3. media-sound/alsa-utils-1.0.16
4. media-sound/alsa-headers-1.0.16
5. media-sound/jack-audio-connection-kit-0.103.0
6. media-sound/oss2jack-0.25
7. media-sound/fusd-kor-1.10.11-r1

Instructions:

The following can be added to /etc/portage/package.keywords; then just follow the rest of this HOWTO:

=media-libs/alsa-lib-1.0.16*
=media-sound/alsa-utils-1.0.16*
=media-sound/alsa-headers-1.0.16*
=media-sound/jack-audio-connection-kit-0.103.0   # 0.101.1 or higher is needed for ardour-2.4
=media-sound/oss2jack-0.25                       # as found in my ebuild 
=media-sound/fusd-kor-1.10.11-r1                 # as found in my ebuild 

Use this command to update the ALSA packages:

emerge -uav alsa-lib alsa-utils alsa-headers

To install the kernel see this section below.

Preparation

First of all it's important that the standard oss-driver that alsa provides isn't loaded:

rmmod snd-pcm-oss
rmmod snd-mixer-oss

If you use the alsasound init script you can disable support for OSS by editing /etc/conf.d/alsasound:

ENABLE_OSS_EMUL="no" 

Next restart alsasound:

/etc/init.d/alsasound restart

There shouldn't be a /dev/dsp and /dev/mixer anymore after this step! Oss2jack will create its own devices with the same name.

Get a realtime-kernel

For best performance you should install a realtime-kernel or at least the deprecated realtime-lsm module (if it still works on newer kernels). You could use the rt-sources ebuilds from the pro-audio overlay (can be installed via layman) but some ebuild versions seem to be broken so it is probably easiest to install the kernel manually:

cd /usr/src
rm linux
wget ftp://ftp.kernel.org/pub/linux/kernel/v2.6/linux-2.6.22.1.tar.bz2
tar xjvf linux-2.6.22.1.tar.bz2
mv linux-2.6.22.1 linux-2.6.22.1-rt9
ln -s linux-2.6.22.1-rt9 linux
cd linux
wget ftp://ftp.kernel.org/pub/linux/kernel/projects/rt/older/patch-2.6.22.1-rt9
patch -p1 < patch-2.6.22.1-rt9
rm patch-2.6.22.1-rt9
rm ../linux-2.6.22.1.tar.bz2

If you have no kernel from the portage tree installed, you should do the following to inform portage that you provide an own kernel tree (important for packages that install kernel modules):

mkdir -p /etc/portage/profile
echo "sys-kernel/gentoo-sources-2.6.22" >> /etc/portage/profile/package.provided

As I mentioned above for me the kernel alsa driver worked better than the alsa-driver ebuild so you might want to enable ALSA support directly in the kernel (i suggest to compile all sound related options as modules where it's possible - the OSS part of ALSA in particular, which would be best to deactivate completely or it could collide with oss2jack). In this case unmerge alsa-driver:

emerge -Ca alsa-driver

Here are some links that might help you with configuring your kernel:
Configuring the Kernel (Gentoo Handbook)

HOWTO Compile a Kernel Manually (Gentoo Wiki) 

HOWTO Compile Kernel with ALSA (Gentoo Wiki) 

See this and this Howto for more specialized details on setting up a realtime kernel. Don't forget to alter your /etc/security/limits.conf as described on these sites. For example:

@audio - rtprio 80
@audio - nice -10
@audio - memlock 500000 

Install the ebuilds

First get jackd from portage:

emerge jack-audio-connection-kit

Now you need to install fusd-kor and oss2jack. Please use the following file. It contains both ebuilds with patches for newer kernels (tested with 2.6.22 - 2.6.25; I found a working patch in a debian source package):

Gentoo ebuilds for fusd-kor and oss2jack (not a direct link)

To install them, go to your portage overlay dir and extract them there. For example:

cd /usr/local/portage
tar xfvz $HOME/downloads/fusd-kor_oss2jack-ebuilds.tar.gz

media-sound/fusd-kor
media-sound/oss2jack 

now emerge them:

emerge -av fusd-kor oss2jack

A new rule for udev will be added so you either have to reboot or run

udevcontrol reload_rules 

to reload the rules. After this do

modprobe kfusd

Start jackd and oss2jack

Now run jackd. I use the following configuration, which allows me to use jackd and still be able to run normal alsa-apps (always start jackd and oss2jack as the user your sound-applications are run with - not as root!):

/usr/bin/jackd -R -dalsa -r48000 -p1024 -Pdmixer -Cdsnooper &

Please replace dmixer with an appropriate defined device from your .asoundrc. If you use the Softvol plugin for example the command is sth. like:

/usr/bin/jackd -R -dalsa -r48000 -p1024 -Psoftvol -Cdsnooper &

Don't use -Pdefault or -Cdefault for playback and capturing. Jackd will complain about this and will possibly not run at all.

explanations:

-R      make use of realtime 
-dalsa  set alsa as the device 
-r      the sample rate 
-p      frames per period. this must be the same value as you use for period_size in your .asoundrc 
-P      force jackd to use one of your defined devices instead of hw:0 for playback. 
-C      force jackd to use one of your defined devices instead of hw:0 for recording. 

Next start oss2jack.

oss2jack &

Now you should have two new devices: /dev/dsp and /dev/mixer

Finally start an OSS-application of your choice - you should now be able to run several OSS applications at the same time. After all this it's even possible for me to start an alsa player, an oss player and record with ardour2 without any xruns (= sound buffer overruns or underruns).
However, if you still get xruns you can easily shut down jackd/oss2jack and start jackd the normal way like

/usr/bin/jackd -R -dalsa -r48000 -p1024 &

while you're working with your jackd application. When you're done, just restart jackd and oss2jack:

stopjack; /usr/bin/jackd -R -dalsa -r48000 -p1024 & ardour2; stopjack; startjack &

I assumed here that you use my scripts below.

If everything works as expected you should make kfusd being loaded at startup:

echo kfusd >> /etc/modules.autoload.d/kernel-2.6

And of course remove anything with oss in the same file.

Autostart scripts

If you like to automatically start (and restart in case of crashes) jackd and oss2jack you could use the following simple bash scripts:

#!/bin/bash
while true; do
        if [ "`ps x | grep "jackd -R -dalsa" | grep -v grep`" == "" ]; then
                jackd -R -dalsa -r 48000 -Pdmixer -Cdsnooper -p1024 1>/dev/null 2>/dev/null &
        fi
        sleep 1
        if [ "`ps x | grep "oss2jack -q" | grep -v grep`" == "" ]; then
                oss2jack -q 2>/dev/null 1>/dev/null 
        fi
        sleep 2
done 

#!/bin/sh
killall -q startjack
killall -q oss2jack
killall -q jackd 

Make them executable:

chmod +x /usr/local/bin/*

Put the following at the end of $HOME/.bashrc (to start jackd and oss2jack with your first console login)

if [ "`ps x | grep "/bin/bash /usr/local/bin/startjack" | grep -v grep`" == "" ];then
        startjack 1>/dev/null 2>/dev/null &
fi 

Or create a new executable file in your window managers autostart directory (e.g. ~/.kde/env/startjack.sh):

#!/bin/bash
if [ "`ps x | grep "/bin/bash /usr/local/bin/startjack" | grep -v grep`" == "" ];then
        startjack 1>/dev/null 2>/dev/null &
fi 

Now jackd and oss2jack will be started the first time you log in. Use stopjack to kill the jackd processes manually.