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Compiling and flashing custom kernels

prerequisites

  1. repo

In Ubuntu 24.04 repo can be installed with sudo apt-get install repo but not in 20.04.

Download source

Initialize repo and sync with the version that you want using instructions from building pixel kernels

For example:

repo init -u https://android.googlesource.com/kernel/manifest -b android-gs-raviole-5.10-android12-d1
repo sync -c --no-tags

extract vendor ramdisk

Download a facotry image with compatible vendor ramdisk. SD1A.210817.015.A4 for pixel 6 on Android 12.

after unzipping dowloaded factory image, run

PATH_TO_CUSTOM_KERNEL_ROOT/tools/mkbootimg/unpack_bootimg.py --boot_img vendor_boot.img --out vendor_boot_out

Then copy created vendor_boot_out/vendor-ramdisk-by-name/ramdisk_ file to the correct ramdisk path in your custom kernel (prebuilts/boot-artifacts/ramdisks/vendor_ramdisk-oriole.img for pixel 6).

Compiling source

Set the build options:

export BUILD_AOSP_KERNEL=1
export BUILD_KERNEL=1

Edit configs to include KGDB

To build source with KGDB, you must set export KMI_SYMBOL_LIST_STRICT_MODE=0 to prevent the build system from failing when trying to use different configs

In your build/build.config file look for the name of the build config used to generate your android kernel in the first line. It should look like this:

. ${ROOT_DIR}/${KERNEL_DIR}/build.config.gs101

In your project root, use find to locate this build config file (likely in private/gs-google/{FILENAME})

In this file, edit the PRE_DEFCONFIG_CMDS by adding a new fragment with your kgdb config options. Your PRE_DEFCONFIG_CMDS may look like the following:

PRE_DEFCONFIG_CMDS="KCONFIG_CONFIG=${ROOT_DIR}/${KERNEL_DIR}/arch/arm64/configs/${DEFCONFIG} ${ROOT_DIR}/${KERNEL_DIR}/scripts/kconfig/merge_config.sh -m -r ${ROOT_DIR}/${KERNEL_DIR}/arch/arm64/configs/gki_defconfig ${ROOT_DIR}/${KERNEL_DIR}/arch/arm64/configs/slider_gki.fragment ${ROOT_DIR}/${KERNEL_DIR}/arch/arm64/configs/kgdb.fragment"

To include kgdb with access to a serial console (as done in this article) the following configs are necessary:

CONFIG_KGDB=y
CONFIG_VT=y
CONFIG_HW_CONSOLE=y
CONFIG_KGDB_SERIAL_CONSOLE=y

Building kernel

Build kernel using the legacy build system:

./build/build.sh

(You should also be able to build using ./build_<devicename>.sh but that randomly fails sometimes)

The output of building should be: vendor boot image created at /home/<user>/research/kernel_build/out/mixed/dist/vendor_boot.img

There is more information about building kernels for virtual devices here

flashing

After building, use fastboot to flash the boot.img, dtbo.img, vendor_boot.img, and vendor_dlkm.img images:

fastboot flash boot        out/slider/dist/boot.img
fastboot flash dtbo        out/slider/dist/dtbo.img
fastboot flash vendor_boot out/slider/dist/vendor_boot.img
fastboot reboot fastboot
fastboot flash vendor_dlkm out/slider/dist/vendor_dlkm.img

These can be found in out/mixed/dist/