5.1 Selecting a Kernel

Though there is only one main repository for the kernel, http://www.kernel.org/, the versions available from that site aren't always appropriate for all the architectures supported by Linux. In fact, these versions will often not even build for, much less run on, some of the most popular architectures in embedded Linux systems. This is primarily because the development of Linux for these architectures isn't synchronized with the main kernel releases.

To have a working kernel for your target, you need to obtain one of the versions made available by the development team in charge of your target's underlying processor architecture. Since each architecture is maintained by a different team, the site of choice for a kernel varies accordingly. Table 5-1 provides a list of locations where you will find the most appropriate kernel for your architecture, along with the means of download available from that site.

Table 5-1. Most appropriate kernel location for each processor architecture

Processor architecture	Most appropriate kernel location	Available download means
x86	http://www.kernel.org/	ftp, http, rsync
ARM	http://www.arm.linux.org.uk/developer/	ftp, rsync
PowerPC	http://penguinppc.org/	ftp, http, rsync, bitkeeper
MIPS	http://www.linux-mips.org/	cvs
SuperH	http://linuxsh.sourceforge.net/	cvs
M68k	http://www.linux-m68k.org/	ftp, http

As you can see, most of these sites are the same ones I recommended for each architecture in Chapter 3. That said, these are not the only kernel locations for each target. Other locations may also provide versions for your target. To begin with, some of these sites have mirrors that provide the same content. Then there are the kernels provided by various individuals, companies, and organizations. Exercise caution if you intend to use the latter type of kernel, as these kernels may not be supported by the community^[1] and you may be forced to rely on the provider's support, if available, in case of problems.

^[1] This lack of support from the community won't necessarily be due to lack of code availability (which shouldn't happen since Linux is distributed under the terms of the GPL), but most likely because the modifications to the kernel's functionality made by that provider are understood only by her. It may also be that the kernel modifications are not considered mature enough, or even desirable, by the community to warrant inclusion in the main kernel tree.

Once you have found the download site that is most appropriate for you, you will need to select a kernel version from that site. This is a difficult decision, as some versions have broken features, even if the same features were fully functional in older versions. The best way to find this sort of information is to stay in touch with the community maintaining the kernel for your architecture. This doesn't mean sending any emails or contacting anyone, but it involves subscribing to the appropriate mailing lists and keeping watch of the important notices on that list and on the port's main web site.

Some of these sites, such as the ARM site, don't necessarily distribute full kernels. Rather, they distribute patches to the official kernel. To obtain the appropriate kernel for your architecture, you must then download the kernel from the main repository and apply to it the appropriate patch provided by your port's site.

Kernel Version Variations The versions distributed by the alternative repositories often use variations on the kernel's versioning scheme to identify their work. Russell King, the maintainer of the ARM tree, distributes his kernels with the -rmk extension. Other developers base their work on Russell's work and add their own extensions. Nicolas Pitre, another ARM Linux developer, adds the -np extension to his kernels, and the maintainers of the handhelds.org Familiar distribution add the -hh extension to their kernels. Hence, kernel 2.4.20-rmk3-hh24, which I mentioned in Chapter 1, is handhelds.org's Release 24 of Russell's Release 3, which is itself based on Marcelo Tosatti's 2.4.20. (Though Linus Torvalds is the usual maintainer of Linux releases, Linus passed the maintenance of the 2.4.x series on to Marcelo so he could concentrate on the 2.5.x development series.)

For our ARM-based user interfaces, we download plain 2.4.18 from http://www.kernel.org/ and the 2.4.18-rmk5 patch from the official ARM Linux site, http://www.arm.linux.org.uk/. By applying the rmk5 patch to the vanilla 2.4.18, we obtain the 2.4.18-rmk5 kernel, which contains all the features required for ARM-based systems.

Most of the time, the latest known-to-be-functional version is the best one to use. So if 2.4.17 and 2.4.18 are known to work on your target, 2.4.18 should be the preferable one. There are cases, however, in which this doesn't hold true. Most folks who follow the kernel's development are aware, for example, that Versions 2.4.10 to 2.4.15, inclusive, are to be avoided, because they were part of a period during which a lot of changes were being integrated into the kernel and are therefore sometimes unstable. Again, this is the sort of information you can obtain by keeping in touch with the appropriate mailing lists and web sites.

If you find it too time consuming to subscribe to your port's mailing list or to the main kernel mailing list, you owe it to yourself to at least visit your port's web site once a week and read the Kernel Traffic (http://kt.zork.net/kernel-traffic/) weekly newsletter. Kernel Traffic provides a summary of the most important discussions that occurred on the main kernel mailing list during the past week.

Once you have found the appropriate kernel version for your target, download it into the ${PRJROOT}/kernel directory, extract it, and rename it if necessary, as we have done in the previous chapter in Section 4.2.2. Renaming the kernel directory will avoid the mistake of overwriting it while extracting another kernel you might download in the future.

Whichever version you choose, do not refrain from trying a couple of different kernel versions for your target. In addition to the recommendations and bug reports seen on the Net, your evaluation of different versions will provide you with insight on your hardware's interaction with the kernel.

You may also want to try some of the various patches made available by some developers. Extra kernel functionality is often available as an independent patch before it is integrated into the mainstream kernel. Robert Love's kernel preemption patch, for instance, was maintained as a separate patch before it was integrated by Linus into the 2.5 development series. We will discuss a few kernel patches in Chapter 11. Have a look at Running Linux (O'Reilly) if you are not familiar with patches.