lategoodbye/mainline_guide.md

## mainline_guide.md

      
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    Raspberry Pi: How to cross-compile and use Mainline Kernel

Notes on how to set up a new Ubuntu LTS x64 environment, how to build a recent Mainline Kernel and place it on a Raspberry Pi OS SD card.
Procedure


Install tools needed:

$ apt install git make gcc g++ device-tree-compiler bc bison flex libssl-dev libncurses-dev python3-ply python3-git libgmp3-dev libmpc-dev


Install an up-to-date cross compiler and associated toolset. This may be obtained from https://snapshots.linaro.org/gnu-toolchain/. The directory arm-linux-gnueabihf contains the necessary compiler for ARM 32-bit and the directory aarch64-linux-gnu for ARM 64-bit.

Choose the version suited to your development machine's architecture. For example, at the present time, for use on 64-bit Ubuntu on an Intel-based development machine, the appropriate version is gcc-linaro-11.3.1-2022.06-x86_64_arm-linux-gnueabihf.tar.xz.
$ wget https://snapshots.linaro.org/gnu-toolchain/11.3-2022.06-1/arm-linux-gnueabihf/gcc-linaro-11.3.1-2022.06-x86_64_arm-linux-gnueabihf.tar.xz
$ sudo tar xf gcc-linaro-11.3.1-2022.06-x86_64_arm-linux-gnueabihf.tar.xz -C /opt


Get Mainline Linux Sources (try to avoid linux-next which tends to be unstable)

$ git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Move into the repository folder and setup cross-compiler (for ARM 64-bit replace ARCH=arm with ARCH=arm64):

$ cd linux
$ export ARCH=arm
$ export CROSS_COMPILE=/opt/gcc-linaro-11.3.1-2022.06-x86_64_arm-linux-gnueabihf/bin/arm-linux-gnueabihf-


Choose the kernel configuration. To do this, you need to know what model of System on a Chip (SoC) is used on your Pi. Please refer to the Raspberry Pi Wikipedia Section. Since we are building from an upstream source, then, by reference to Emma Anholt's pages we can deduce that we should select bcm2835_defconfig for BCM2835-based models, multi_v7_defconfig for BCM2836- and BCM2837-based models (ARM 32-bit), multi_v7_lpae_defconfig for BCM2711-based models (ARM 32-bit LPAE) or defconfig for BCM2837- and BCM2711-based models (ARM64). For the Raspberry Pi 3B Plus (ARM 32-bit), the following is appropriate:

$ make multi_v7_defconfig


At this point you can modify the configuration using, for instance, menuconfig:

$ make menuconfig


Build the kernel, the modules and the device tree blobs:

$ make

To speed up compilation on a multicore machine, add the argument -j <number_of_cores+1> to the above command.

Insert an existing Raspberry Pi OS SD Card to your development machine. Please keep in mind that kernel architecture must match accordingly (Raspberry Pi OS 32 bit for arm and Raspberry Pi OS 64 bit for arm64). In Ubuntu, it will appear at /media/<username>/bootfs and /media/<username>/rootfs, where <username> is the username you are logged in under.
Copy the kernel file to the card's boot directory (for ARM 64-bit the kernel file is arch/arm64/boot/Image ):

$ cp arch/arm/boot/zImage /media/<username>/bootfs/


Copy the device tree blob to the card's boot directory. The exact device tree blob file to use depends on the model. The following is for a Raspberry Pi 3B Plus, so the device tree blob's filename is bcm2837-rpi-3-b-plus.dtb (for ARM 64-bit the source directory is arch/arm64/boot/dts/broadcom):

$ cp arch/arm/boot/dts/broadcom/bcm2837-rpi-3-b-plus.dtb /media/<username>/bootfs/


Adjust the device tree information in /media/<username>/bootfs/config.txt:


Add the following lines to /media/<username>/bootfs/config.txt. The filename is that of the device tree blob copied over in Step 10. If /media/<username>/bootfs/config.txt already contains a device_tree entry, replace it with this. For ARM 64-bit the kernel is Image.

device_tree=bcm2837-rpi-3-b-plus.dtb
kernel=zImage


In case you need to debug kernel issues during early boot stage, try the following settings according to your setup:


Board
console= (cmdline.txt)
earlycon= (cmdline.txt)
enable_uart= (config.txt)


Raspberry Pi 3
ttyS1,115200
uart8250,mmio32,0x3f215040
0


Copy the kernel modules to the SD card. Note that you need to be in superuser mode for this (for ARM 64-bit replace ARCH=arm with ARCH=arm64):

$ sudo ARCH=arm make modules_install INSTALL_MOD_PATH=/media/<username>/rootfs


Safely dismount the SD card from the development machine, put it into the target machine and reboot.

Notes


Always use the DTB files from the mainline tree and not the downstream ones
Avoid using rpi-update, as it will overwrite your mainline binary. The downstream kernel typically has problems with the upstream device tree blob.
In some cases (e.g. the arm64), the kernel image tends to be very big (~ 16 MB) and may hit the limit of the bootfs partition.
The AUX UART has a different device name with mainline kernel: /dev/ttyS1 instead of /dev/ttyS0