Low-Latency 2.4.x with ALSA mini-HOWTO

by Paul Winkler 9/11/2000 Kernel config additions by Dave Phillips 10/07/2001 updated by Patrick Shirkey 10/07/2001 updated by Eric Rzewnicki 03/03/2002

Introduction
Downloading and Patching the Kernel
Configuring and Building the New Kernel
Update Modutils and Load Modules
Install ALSA
Necessary Upgrades

Introduction

This document is specifically aimed at those interested in using 2.4.x with Andrew Morton's low-latency patch for audio purposes.

It's not really hard when you know what to do, but there's a lot of stuff to upgrade before you can compile and install the kernel if you are moving from 2.2.x. So if you are, don't forget to read the section on updating your system files.

Downloading and Patching the Kernel

Download Andrew Morton's latest low-latency patch.
Download the 2.4.x sources of the right version for Andrew's patch. When I say x, I mean for example get linux-2.4.17 if you have a Morton patch for 2.4.17. Kernel sources can be found at ftp.kernel.org and various mirrors.
Remove the link /usr/src/linux (it points to your current kernel sources, probably something like /usr/src/linux-2.2.16).
```
rm /usr/src/linux
```
Unzip the 2.4.x source tarball and untar it in /usr/src/ . This will result in a /usr/src/linux directory. Rename this /usr/src/linux-2.4.x where x is the proper number.
```
mv linux linux-2.4.x
```
Re-create /usr/src/linux as a link to your new sources:
```
ln -s /usr/src/linux-2.4.x /usr/src/linux
```

Now apply Andrew's patches:

cd /usr/src/
patch -p0 < /dir/where/you/put/2.4.x-low-latency.patch

This should patch without any problems.

Configuring and Building the New Kernel

Configuration may be easier if you copy your old kernel configuration to your new source directory:
```
  cp /usr/src/linux-2.2.x/.config /usr/src/linux
  
```
To save a copy of your old kernel config as .config.old and create a .config updated for the current kernel:
```
 make oldconfig
  
```
In /usr/src/linux, do make config, make xconfig or make menuconfig (your choice). You'll want to read /usr/src/linux/Documentation/Changes first!
Options relevant to low-latency audio:
- If you have IDE drives, go to the "IDE, ATA and ATAPI Block devices" section and say Yes to "Generic PCI bus-master DMA support" and "Use PCI DMA by default when available". This will turn on DMA for your disks which is necessary for low-latency disk-intensive work. You may also want to browse this section for any options related to your specific motherboard/chipset.
- Next, go to the "Character devices" section and enable "Enhanced Real-time Clock Support" for access to your computer's hardware clock.
- Turn on sound support. If you want to use the ALSA drivers, enable sound support as a module but don't select anything else; otherwise, select the modules that work for your particular soundcard.
- Finally in the "Kernel Hacking" section, I suggest saying Yes to "Magic SysReq Key". This should give you a way to kill runaway processes, or reboot cleanly if all else fails. What can happen is that a DSP process is hogging the CPU with maximum realtime priority so you can't even interact with a shell to kill it. Well-designed applications should have safeguards against this, but you need ammunition for emergencies, especially when working with software in early development.
  Read /usr/src/linux/Documentation/ sysrq.txt for instructions; more importantly, PRINT IT since you won't be able to get at the file when you need it most!!!
  If you forget to print it Alt-SysRq-q might help you. q is currently not a valid sysrq key so using it causes sysrq to print a usage message with a list of the available commands. The appropriate letter for each is given in uppercase. This is a simple "cheat sheet" I've used in the past when I can't find my printed copy of sysrq.txt. This may not work in the future if q becomes used by sysrq.
  I've found that when something gets stuck, Alt-SysRq-e (which sends a "sigterm all" signal) will kill everything and drop me back to a login prompt so I don't even have to reboot.
(Optional step) Edit line 4 or your kernel Makefile to set an Extraversion. I find this useful for tracking various compiles of the same kernel version in case I make a mistake or want to test different configuration options. I usually use the hostname and a sequence number in addition to indicating any patches applied:
```
  vi Makefile
  ...
  EXTRAVERSION = -ll-box1-0
  
```

Now build your kernel:

  make dep
  make clean
  make bzImage
  make modules
  make modules_install
  cp arch/i386/boot/bzImage /boot/bzImage-4.x
  cp System.map /boot/System.map

Update LILO. Edit your /etc/lilo.conf file and put in a new image entry:

image=/boot/bzImage-2.4
        label=linux-2.4-test
        read-only
        root=/dev/whatever_you_use

Now run LILO so it finds the new image:
```
lilo
```
The 2.4 kernel uses a new device file for shared memory. It's a good idea to use this, so add this to /etc/fstab. Put this line in /etc/fstab:
```
none                /dev/shm              shm     defaults      0 0
```
And make the directory: mkdir /dev/shm
It won't affect booting your old kernel - you'll just get a complaint that the kernel does not support the shm filesystem.
Finally!! Shut down and boot your new kernel.

Update Modutils and Load your Modules

There's a few things you need to do before you can load modules with 2.4.

First, update modutils. Hopefully you've already downloaded the modutils package. Compile and install it now. Make sure your old modutils is gone!
If your module configuration is in /etc/conf.modules, be advised that this name is deprecated - you should rename it to /etc/modules.conf . Don't worry, this won't harm anything with your old 2.2 kernel.

Now you should build your kernel modules.

cd /usr/src/linux
make modules; make modules_install

Try to load a module using modprobe or insmod ... hopefully it should work!

Alternative Build and Install for Debian Users

The above directions will work just as well as the following, but this method using make-kpkg can save some work for Debian users. Starting after the kernel configuration:

Install kernel-package
```
apt-get install kernel-package
```
Build a kernel and pack it into a .deb file
```
make-kpkg clean
make-kpkg kernel-image
```
or, if you want to time the kernel build so you'll know how long it takes if you need to do it again:
```
time sh -c "make-kpkg clean; make-kpkg kernel_image"
```
Install the kernel .deb package
```
cd ..
dpkg -i linux-2.4.x-ll-box1-0.deb
```
The kernel package will take care of configuring and running LILO. It also handles the modules. For more information on the organization of kernel package management in Debian see the dpkg and make-kpkg man pages.
Shut down and boot your new kernel.

Install ALSA

Now you're finally ready to get your soundcard working again. I won't go into detail on this because ALSA installation and configuration is well covered elsewhere. Go to The ALSA howto. I didn't have to do anything differently with the 2.4 kernel; ALSA just worked fine!

Necessary Upgrades

There are a number of packages you must upgrade. These are listed in /usr/src/linux/Documentation/Changes which is available here, but I will list them below too.

The Changes document notes: "...keep in mind that this list assumes you are already functionally running a Linux 2.2 kernel. Also, not all tools are necessary on all systems; obviously, if you don't have any PCMCIA (PC Card) hardware, for example, you probably needn't concern yourself with pcmcia-cs."

You should be running AT LEAST the following versions of each listed package. Higher version numbers should be fine too. I've listed test commands that can help you figure out what version you have. This is all lifted from the Changes file.

Gnu C 2.7.2.3 - note that egcs 1.1.2 and above should also work. I used egcs-1.1.2-30 with no apparent problems.
- Test commands:
```
gcc --version
rpm -q egcs gcc
dpkg -l gcc
```
binutils 2.9.1.0.22
- Test commands:
```
ld -v
rpm -q binutils
dpkg -l binutils
```
- Where to Get It: RedHat 6.1 comes with binutils-2.9.1.0.23-6 so you should be OK. Otherwise, get a tarball from ftp://ftp.valinux.com/pub/support/hjl/binutils/binutils-2.9.5.0.xx.tar.gz which can, if desired, be built into an rpm by doing rpm -ta binutils-2.9.5.0.xx.tar.gz
- Notes from Changes: "If you can, upgrade to the latest 2.9.5 binutils release. Older releases such as 2.8, 2.8.xx, and the FSF's 2.9.1 should be avoided if at all possible. The later releases of 2.9.1.0.x (anything where x >= 22) can and do compile the kernel properly, but there are many benefits to upgrading to 2.9.5 if you're up to it."
- Note: there is a problem with later 2.4.x kernels and recent binutils versions. This bit me with kernel 2.4.16 and binutils 2.11.92.0.12.3. Depending on the kernel configuration building the kernel fails with this error:
```
undefined reference to 'local symbols in discarded section .text.exit'
```
  There are several workarounds to be found in various mailing list archives. Using binutils version 2.11.92.0.10 instead of 2.11.92.0.12.3 worked for me.
util-linux 2.10g
- Test commands:
```
      chsh -v
      rpm -q util-linux
      dpkg -l util-linux
```
- Where to Get It: http://www.il.kernel.org/pub/linux/utils/util-linux/
- I have not been able to install an RPM for any version above 2.10f-7. I did find an RPM for 2.10m here, but it depends on an upgrade of glibc-2.2 that I don't want to do (rpm complains of so many dependencies that I've given up).
  I was able to compile and boot 2.4.0-test6 with 2.10f-7, but you may want to compile and install the latest yourself. Note that this is dangerous - you MUST carefully read the MCONFIG file and make sure everything's right. I tried to compile it and didn't know what I was doing, and I made it impossible to log in to my system!! After some hairy battling with broken boot floppies, I managed to get my system working again. The 2.10f-7 rpm seems to work for me so I went back to that.
  I have had one bit of mysterious trouble - Netscape seems to have become immune to kill, even kill -9; kill is part of util-linux, so I suspect something's funny there.
modutils 2.3.13
- Test commands:
```
        insmod -V
        rpm -q modutils
        dpkg -l modutils
```
- modutils are now available in 2.4.x versions for which the following issues regarding 2.3.x probably no longer apply.
- Where to Find It: ftp://ftp.kernel.org/pub/linux/utils/kernel/modutils/v2.3/ I was unable to find RPMS for 2.3.13 so it's necessary to compile from source ... but read this next bit:
- Installation Notes: Wait until after you've compiled and booted your 2.4 kernel before you try to compile modutils! I was unable to compile modutils until after I'd booted 2.4; then it went fine. This means also that you should NOT compile anything ESSENTIAL to your 2.4 kernel as a module, because you won't be able to load the modules the first time you boot 2.4.
  I had modutils-2.1.121-4 previously, and this version WILL NOT load modules for a 2.4 kernel.
e2fsprogs 1.18
- Test commands:
```
        /sbin/tune2fs --version
        rpm -q e2fsprogs
	dpkg -l e2fsprogs
```
- Where to Find It: Tarballs are at http://web.mit.edu/tytso/www/linux/dist/ and RPMS are at ftp://ftp.valinux.com/html/6.1.2-up/all/e2fsprogs-1.18-0.1.i386.html
pcmcia-cs 3.1.13 (only if you have PCMCIA devices ... I don't, so I have no experience with this.)
- Test commands:
```
       cardmgr -V
```
- Where to Find It: ftp://sourceforge.org/pcmcia/
PPP 2.4.0b1
- Test commands:
```
       pppd --version
       rpm -q ppp
       dpkg -l ppp
```
- Where to Get It: I found an rpm at http://fr.rpmfind.net/linux/RPM/contrib/libc6/i386/ppp-2.4.0b2-2.i386.html ... Or you can find a tarball at ftp://linuxcare.com.au/pub/ppp/
isdn4k-utils 3.1beta7 (only if you need ISDN. I don't, so I have no experience with this.)
- Test commands:
```
      isdnctrl 2>&1|grep version
```
- Where to Find It: ftp://ftp.isdn4linux.de/pub/isdn4linux/utils/testing/

Questions, comments, corrections: Paul Winkler