Kernel 101

Prashant Somashekar

Kernel 101

• What will we discuss?

-Brief overview of a kernel

-How a kernel fits into the hierarchy

-Android versus Linux kernel

-Building an Android kernel from source

-Flashing a kernel to bare metal (mobile)

-Practical experience; pitfalls; methodology

Kernel 101

• What is a kernel?

-Heart of the system

-Hardware initialization

• Hierarchy of a mobile system

-Bootloader -> Kernel -> OS

-Kernel provides low level abstraction layer

Kernel 101

• What is a kernel’s function?

-Provide a layer for OS/App to speak with hardware

-CPU/Memory management

-Resource allocation

-Input/Output from/to device’s components

Kernel 101

• Difference between Android and Linux kernel?

-Based off Linux

-Wakelock/Power management implementation

-No paging to disk for virtual memory

-Lightweight libc standard library (Bionic)

-Video framebuffer instead of X11

-Low memory killer

Kernel 101

• How can I acquire kernel source code?

-Use of github as a source code repository

-Provides timeline of changes

-Eases kernel development

-Facilitates code sharing and dissemination

Kernel 101

• Will be using ubuntu (linux)

-Install git: apt-get install git-core• Pulling source code from a github repo:

Kernel 101

• Use Codesourcery’s toolchain

-ARM-EABI 2010.09 (GCC4.5)• Use and setup Codesourcery’s toolchain:

-Unpack to a folder (ex: toolchain in home)

-export CCOMPILER=~/toolchain/arm-2010.09/bin/arm-none-eabi-

-export PATH=$PATH:~/toolchain/arm-2010.09/bin

Kernel 101

• Go to kernel directory and generate a defconfig

-cd mykernel

-make –j2 ARCH=arm CROSS_COMPILE=$CCOMPILER pershoot_mahimahi_defconfig

-sample configurations in arch/arm/configs

• Compile a kernel:

-make –j2 ARCH=arm CROSS_COMPILE=$CCOMPILER

Kernel 101• Put device in bootloader; fastboot mode

• Boot device with new kernel

-fastboot boot zImage

-kernel image will be in arch/arm/boot

-zImage is a compressed kernel image

• Copy wireless driver to device

-adb remount

-adb push drivers/net/wireless/bcm4329.ko /system/lib/modules

• Make more configuration changes

-make –j2 ARCH=arm CROSS_COMPILE=$CCOMPILER menuconfig (must have lib-ncurses installed)

• Clean and rebuild:

-make –j2 ARCH=arm CROSS_COMPILE=$CCOMPILER clean && make –j2 ARCH=arm CROSS_COMPILE=$CCOMPILER

Kernel 101• Not all devices will have fastboot

-Pull existing boot.img from device

-Use split_bootimg.pl to separate ramdisk and kernel

-Use mkbootimg to merge your new kernel with existing ramdisk

-mkbootimg --cmdline 'no_console_suspend=1 console=null' --kernel zImage --ramdisk ramdisk.gz --base 0x20000000 -o boot.img

-Base listed is specific to Nexus One (Passion)

• Ramdisk (initrd) is a temporary root file system

-Bare minimum needed for boot until the real system’s filesystem is available

-May contain utilities, drivers, folder layout, etc.

• Flash boot.img to device via updater-script

-write_raw_image("/tmp/boot.img", "boot")

• Alternative method

-use AnyKernel (koush)

-Only need to supply a zImage and modules (optional)

-Resign update.zip (with a test key) or toggle signature checking off (Clockwork Mod), prior to flashing

Kernel 101

• Feel free to play with the source!

-Adjust frequency clocks (overclock)

-Under volt some frequency ranges to save some power

-Implement GCC optimizations

-Enable further functionality via loadable kernel modules

Kernel 101

• Not all devices are created equal

-Hardware variances makes a ‘one size fits all’ mantra difficult to maintain at times

-Different ROM bases may interact poorly

-Remote troubleshooting can make resolution times lengthy

• It is important to adhere to the GPL

-Keeping things open

-Engineering for the sake of making things faster and more fine-tuned

-Contribute to Social Coding

-Fit to a specific environment’s form

-Fix bugs

-Add and enhance functionality

Kernel 101

• CyanogenMod

-Distribution based off AOSP/CodeAurora/Nvidia/Omapzoom

-Aims to provide a unified experience across multiple devices

-Custom ROM and Kernel development

-Open Platform which encourages social coding and development

Kernel 101

• Q&A