When working on embedded devices, the build system doesn’t work out of the box for the latest Linux distributions. This leads the developer to either choose to keep their whole system on the older version or attempt to patch all the problems that have crept in. It shouldn’t be that way, you can have your cake and eat it too.

The way you get around this conundrum is by creating a chroot environment to do your building in. The basic idea is you have a folder that contains all the Linux system file inside it and they you “change your root folder” to be that folder. You’re running the same kernel just a different filesystem. For Ubuntu, there is a nice tool called “schroot” to manage this process. See the Ubuntu wiki page for details.

Here some tips on how to get the source for a given Linux kernel that you would like to tweak. I needed to do this to play with the Android emulator. So when the kernel boots after decompression you’ll see a line like this:


Linux version 2.6.29-00177-g24ee4d2 (veenstra@jackv.mtv.corp.google.com) (gcc version 4.4.0 (GCC) ) #21 Thu Aug 6 20:39:45 PDT 2009


I’ve highlighted in bold the bits we need. The first tells us the kernel version so we know what branch to look for, but really all we need is the second bit which is the SHA1 git hash for the tree containing the kernel. So if we go to the right kernel repository (See my previous entry about that) all we have to do is checkout that tree:

git checkout 24ee4d2

Lastly, we need the configuration for the kernel. Luckily the Android kernel usually has a copy in the /proc directory called /proc/config.gz. All we have to do is copy that to the kernel directory, gunzip it, rename it .config and run:

make oldconfig

This command will prompt for any new options that are not defined in the .config file so if we found the right kernel tree and config file, there will be no prompts. If there are prompts, keep searching because you haven’t found it yet. I’m not sure exactly what happens when you patch the kernel, build it, and don’t commit it in because there wouldn’t be a SHA1 generated. Let’s just hope the Android developers don’t do that… seriously, that’s bad…

So say you want to grab the Android linux kernel along with the rest of the project. By default the kernels are not included in the manifest to save bandwidth for the vast majority of developers that don’t need it. I like to add them back for convience and to track new code dumps automatically. There are a few kernel projects listed at the bottom of the AOSP page based on the SoC used. I’ve made a short list for reference:

• common – Collects changes from each the platform variants
• goldfish – Emulator
• msm – Qualcomm
• omap – Texas Instruments
• samsung – Samsung
• tegra – NVidia

So once you know which kernel(s) you want to add to your repo manifest. Here’s how you can do it (Note: $AOSP is the top directory of your Android tree):


$ cd $AOSP/.repo/manifests # Goto manifests directory
$ git checkout -b with_kernel # Make a new branch to add kernel(s)


Then you need to edit the default.xml manifest to add your kernel(s) between the tags:

By default you’ll get the master branch, which is empty. To find the one you really want, sync with repo then list all the branches available:

$ repo sync
$ cd $AOSP/kernel/$KERNEL_NAME
$ git branch -la


Then you need to edit the default.xml manifest again with the correct branch (called revision in manifest):

Now when you sync you’ll get the kernel tree. Since you just did a sync remotely, you can just a do a local sync to avoid going out to the internet to check for changes and speed it up:

$ repo sync -l