A recently merged commit to the CyanogenMod tree caught my eye recently - Change I5be9bd4b: bluetooth networking (PAN). Bluetooth Personal Area Networking permits tethering over bluetooth. Now this is something that will allow me to tether a Wifi Xoom to my phone when I'm someplace without wifi. Unfortunately, while this is in the CM tree - we need some additional kernel support for the Nexus S. The prebuilt kernel provided with CM 7.1.0 RC1 doesn't include BNEP support which we need to complete this feature. So lets build a kernel module.

The Nexus S kernel is maintained at android.git.kernel.org but I notice there is also a cyanogenMod fork on github as well. So to begin we can clone these and see what differences exist between the stock kernel and the CM version. To setup the repository: 

mkdir kernel && cd kernel
git clone git://android.git.kernel.org/kernel/samsung.git samsung
cd samsung
git remote add cm git://github.com/CyanogenMod/samsung-kernel-crespo.git
git remote update
git log --oneline origin/android-samsung-2.6.35..cm/android-samsung-2.6.35 
f288739 update herring_defconfig

The only changes between the samsung repository and the CM repository are a single commit changing the configuration. This has evidently been done using the kernel configuration utility so its a bit hard to work out the changes by comparing the config files directly. However, if I use each config in turn and regenerate a new one via the kernel configuration utility I can then extract just the changes. 

git cat-file blob cm/android-samsung-2.6.35^:arch/arm/configs/herring_defconfig > x_prev.config
git cat-file blob cm/android-samsung-2.6.35:arch/arm/configs/herring_defconfig > x_cm.config
Then make gconfig and load each one, saving to a new file.
diff -u z_prev.config z_cm.config
and we can see that the new settings are just: 
CONFIG_SLOW_WORK=y
CONFIG_TUN=y
CONFIG_CIFS=y
CONFIG_CIFS_STATS=y
CONFIG_CIFS_STATS2=y
CONFIG_CIFS_WEAK_PW_HASH=y
CONFIG_CIFS_XATTR=y
CONFIG_CIFS_POSIX=y

In current versions of Linux the modules retain version information that includes the git commit id of the kernel source used to build them. This is also present in the kernel and the kernel will reject a module with the wrong id. So to make a module that will load into the currently running kernel I need to checkout that version - simple enough as it is the current head-but-one of the cm-samsung-kernel repository (e382d80). Adding CONFIG_BNEP=m to the kernel config file enables building BNEP support as a module and taking the HEAD herring_defconfig and building HEAD^ results in a compatible bnep.ko module.

To test this I copied the module onto the device and restarted the PAN service. 

% adb push bnep.ko /data/local/tmp/bnep.ko
% adb shell
# insmod /data/local/tmp/bnep.ko
# ndc pan stop
# ndc pan start
# exit
With this done we can try it out. The Blueman app on ubuntu lets me rescan the services on a device and following the above changes the context menu for my device now shows Network Access Point. Selecting this results in a bluetooth tethering icon on the Nexus S and we are away. Further checking on the Xoom with Wifi disabled proves that it all works. Routing is properly configured and the Xoom can access the internet via the phone.

To make this permanent, I could just remount the phone system partition read-write and edit /system/etc/init.d/04modules to add my new module on restart. That works ok. However, I may as well add the configuration changes from above to the current samsung stock kernel and change the kernel in use when I re-build a CyanogenMod image. So that is what I am running now.