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Pixel C kernel build scripts

This is a set of scripts to generate a working kernel for the Pixel C.

TL;DR

git clone https://github.com/pixelc-linux/linux.git
git clone https://github.com/pixelc-linux/pixelc-kernel-scripts.git
git clone https://github.com/pixelc-linux/pixelc-mkinitramfs.sh.git
cd pixelc-kernel-scripts
./defconfig.sh ../linux
./build.sh ../linux
./make_zimage.sh
cd ../pixelc-mkinitramfs.sh
./mkinitramfs.sh -o ../pixelc-kernel-scripts/initramfs.cpio
cd ../pixelc-kernel-scripts
cp output/Image.fit .
# if you want to just boot
fastboot boot Image.fit initramfs.cpio.gz
# if you want to flash
./make_image.sh Image.fit initramfs.cpio.gz
./sign_image.sh
fastboot flash boot boot.img
fastboot boot boot.img

Long version

Kernel build

A basic build consists of two steps.

Configuration:

./defconfig.sh path_to_linux_kernel_tree

The configuration used for defconfig is tegra21_defconfig. You can override that with the KERN_DEFCONFIG environment variable.

Building:

./build.sh path_to_linux_kernel_tree -j5

You can also configure the kernel before building:

./config.sh path_to_linux_kernel_tree

That by default runs menuconfig, you can set the config tool to use with the KERN_CONFIGTOOL environment variable (e.g. gconfig or oldconfig).

The kernel tree must be patched for Pixel C, an upstream kernel tree won't function correctly.

Replace -j5 with how many threads you usually use for building. The scripts by default build out-of-tree in the build directory. You can change that directory with the KERN_BUILDDIR variable; you can also make it build in the kernel tree by specifying KERN_BUILD_IN_TREE.

Other accepted variables are:

  • KERN_CROSS_PREFIX - by default aarch64-linux-gnu-, the kernel cross toolchain prefix.
  • KERN_NO_CROSS - set to any value if you want to use the host compiler.

The build/arch/arm64/boot directory will afterwards contain an Image as well as device tree blobs.

Bootable image creation

After the kernel is built, this is not enough to boot it. You need to bundle the kernel and a corresponding Flattened Device Tree blob into a U-boot .fit image. For this, you use the make_zimage.sh script:

./make_zimage.sh

By default, it will attempt to take the files from build, as is default for the other scripts. If you changed the build directory, pass it as the first argument to the script.

The result will be in output. If you don't want that, pass your own path as the second argument to the script.

Booting

Now you only need a ramdisk. You can create one or obtain a pre-built one from another repository:

https://github.com/pixelc-linux/pixelc-mkinitramfs.sh

You can then boot the kernel + the ramdisk using fastboot:

fastboot boot Image.fit initramfs.cpio.gz

Once you have verified it successfully boots, you might want to flash it onto the device. However, the device won't boot unsigned images, therefore you need to sign it using Chrome OS keys and the futility tool.

Signing

Generating a complete unsigned image

First, you will need to create a single unsigned boot image using your kernel and ramdisk. For that, you will use the make_image.sh script.

./make_image.sh output/Image.fit path/to/initramfs.cpio.gz

This script uses the Android mkbootimg tool. If it's not present in PATH, you can point the script to it using the MKBOOTIMG environment variable.

This creates a boot.img.unsigned file. If you don't like that name for some reason, you can pass your own name using the third parameter to the script.

Signing the image

The final script here is sign_image.sh. It requires the futility tool as well as ChromeOS developer keys. You can typically find both in a package named vboot-kernel-utils or so.

If you don't have futility in PATH, you can point the script to it using the FUTILITY environment variable. Also, if /usr/share/vboot is not the default path to the keys, you can specify VBOOT_PATH as well. This path is checked for the presence of devkeys/kernel_data_key.{vpubk,vprivk}.

If you satisfy both conditions, run the script:

./sign_image.sh

This assumes there is a boot.img.unsigned. If you changed the filename, pass it as the first argument to the script. It will emit a boot.img by default; you can change that by passing your desired filename as the second argument to the script.

Flashing

Once you have a signed image, you can flash it onto your Pixel C. You do that normally using fastboot:

fastboot flash boot boot.img
fastboot boot boot.img

The device should boot, assuming your kernel, ramdisk and root filesystem are correctly set up.