In this page i'll try to outline all the steps necessary to compile an Ubuntu kernel for an android device, and flash it.

get the toolchain

sudo apt-get install fakeroot build-essential kexec-tools kernel-wedge
sudo apt-get install gcc-arm-linux-gnueabihf libncurses5 libncurses5-dev libelf-dev 
sudo apt-get install asciidoc binutils-dev
sudo apt-get build-dep linux

get the source code

[nexus 7]

git clone git://phablet.ubuntu.com/CyanogenMod/android_kernel_asus_grouper.git
cd android_device_asus_grouper
git checkout phablet-10.1
make ARCH=arm cyanogenmod_grouper_defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- zImage
...
[a lot of lines later]
...
  Kernel: arch/arm/boot/Image is ready
  SHIPPED arch/arm/boot/compressed/lib1funcs.S
  AS      arch/arm/boot/compressed/lib1funcs.o
  LD      arch/arm/boot/compressed/vmlinux
  OBJCOPY arch/arm/boot/zImage
  Kernel: arch/arm/boot/zImage is ready

[nexus 4]

git clone git://phablet.ubuntu.com/CyanogenMod/lge-kernel-mako.git
cd lge-kernel-mako
git checkout phablet-10.1
make ARCH=arm mako_defconfig
...
[rest is the same as above]

identify the boot partition on the target device

First login in the device:

[flag@luxor ~]$ adb root
adbd is already running as root
[flag@luxor ~]$ adb shell
root@android:/ # ubuntu_chroot shell                                           

after the chroot $PWD is /data/ubuntu.

[nexus 7]

root@localhost:/# ls -la /dev/block/platform/sdhci-tegra.3/by-name/
total 0
drwxr-xr-x 2 root root 220 Apr  3 09:21 .
drwxr-xr-x 4 root root 320 Apr  3 09:21 ..
lrwxrwxrwx 1 root root  20 Apr  3 09:21 APP -> /dev/block/mmcblk0p3
lrwxrwxrwx 1 root root  20 Apr  3 09:21 CAC -> /dev/block/mmcblk0p4
lrwxrwxrwx 1 root root  20 Apr  3 09:21 LNX -> /dev/block/mmcblk0p2
lrwxrwxrwx 1 root root  20 Apr  3 09:21 MDA -> /dev/block/mmcblk0p8
lrwxrwxrwx 1 root root  20 Apr  3 09:21 MSC -> /dev/block/mmcblk0p5
lrwxrwxrwx 1 root root  20 Apr  3 09:21 PER -> /dev/block/mmcblk0p7
lrwxrwxrwx 1 root root  20 Apr  3 09:21 SOS -> /dev/block/mmcblk0p1
lrwxrwxrwx 1 root root  20 Apr  3 09:21 UDA -> /dev/block/mmcblk0p9
lrwxrwxrwx 1 root root  20 Apr  3 09:21 USP -> /dev/block/mmcblk0p6

LNX is the boot partition (where vmlinux resides) SOS is another boot partition (the recovery one but we won't use it)

[nexus 4]

root@localhost:~# ls -al /dev/block/platform/msm_sdcc.1/by-name/
total 0
drwxr-xr-x 2 system root 540 Apr  3 10:05 .
drwxr-xr-x 4 root   root 600 Apr  3 10:05 ..
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 aboot -> /dev/block/mmcblk0p12
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 abootb -> /dev/block/mmcblk0p15
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 boot -> /dev/block/mmcblk0p6
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 cache -> /dev/block/mmcblk0p22
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 DDR -> /dev/block/mmcblk0p24
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 grow -> /dev/block/mmcblk0p25
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 m9kefs1 -> /dev/block/mmcblk0p8
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 m9kefs2 -> /dev/block/mmcblk0p9
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 m9kefs3 -> /dev/block/mmcblk0p10
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 metadata -> /dev/block/mmcblk0p18
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 misc -> /dev/block/mmcblk0p19
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 modem -> /dev/block/mmcblk0p1
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 persist -> /dev/block/mmcblk0p20
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 recovery -> /dev/block/mmcblk0p7
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 rpm -> /dev/block/mmcblk0p11
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 rpmb -> /dev/block/mmcblk0p16
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 sbl1 -> /dev/block/mmcblk0p2
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 sbl2 -> /dev/block/mmcblk0p3
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 sbl2b -> /dev/block/mmcblk0p13
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 sbl3 -> /dev/block/mmcblk0p4
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 sbl3b -> /dev/block/mmcblk0p14
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 system -> /dev/block/mmcblk0p21
lrwxrwxrwx 1 root   root  20 Apr  3 10:05 tz -> /dev/block/mmcblk0p5
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 tzb -> /dev/block/mmcblk0p17
lrwxrwxrwx 1 root   root  21 Apr  3 10:05 userdata -> /dev/block/mmcblk0p23

in this case the boot partition is /dev/block/mmcblk0p6

get the flashing tools

root@localhost:/# apt-get update && apt-get install abootimg

retrieve information about the boot partition

root@localhost:/# abootimg -i /dev/block/mmcblk0p2 

Android Boot Image Info:

* file name = /dev/block/mmcblk0p2 [block device]

* image size = 8388608 bytes (8.00 MB)
  page size  = 2048 bytes

* Boot Name = ""

* kernel size       = 4940276 bytes (4.71 MB)
  ramdisk size      = 186898 bytes (0.18 MB)

* load addresses:
  kernel:       0x10008000
  ramdisk:      0x11000000
  tags:         0x10000100

* empty cmdline
* id = 0x3474d135 0xea92c4df 0x1ba50966 0x61ac7815 0x254be794 0x00000000 0x00000000 0x00000000

this way we get some information about boot.img content, and we check if the target partition is actually a boot partition

move data from your desktop to your android device (and viceversa)

adb push arch/arm/boot/zImage /data/ubuntu/mykernel

adb pull /data/ubuntu/foobar

update the kernel

abootimg -u /dev/block/mmcblk0p2 -k mykernel
reading kernel from mykernel
Writing Boot Image /dev/block/mmcblk0p2

and there it is, your new kernel is in place just reboot to see if it works.

Some more commands you might find useful:

get a raw copy of the boot partition

dd if=/dev/block/mmcblk0p2 of=./boot.img

explode the content of a boot partition

abootimg -x ./boot.img

to retrieve the original zImage, initrd and the loading configuration

FAQ

• 1) can't talk to my device ("device offline"):
• unplug the usb cable
• adb kill-server
• remove ~/.android (mv ~/.android ~/.foobar)
• plug the usb cable
• adb root
• answer to the security question on the device's display
• adb shell

that should make it.

• 2) i can't get my device to boot after i flashed my recompiled kernel:

The repository mentioned above are the official Ubuntu one, so these kernels should work - did you do any modification to the config? In that case you are on your own.

Anyhow, let's rule out the tools:

• dump boot.img partition

dd if=/dev/block/mmcblk0p2 of=./boot.img

• explode the boot.img partition

abootimg -x boot.img 
writing boot image config in bootimg.cfg
extracting kernel in zImage
extracting ramdisk in initrd.img

• update boot.img with the same kernel and dump it again:

abootimg -u /dev/mmcblk0p2 -k zImage 
dd if=/dev/block/mmcblk0p2 of=./boot2.img

• calculate & compare the checksums

md5sum boot.img
9e2ef0de37718e8f9b6f3a6f83bedfa1  boot.img
md5sum boot2.img
9e2ef0de37718e8f9b6f3a6f83bedfa1  boot2.img

if checksums are different, then your tools can't operate on that device (check for new tools or ask on #ubuntu-touch)