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How to cross-compile and use Mainline Kernel

How to cross-compile and use Mainline Kernel

Notes on how to set up a new Ubuntu 18.04 LTS x64 environment, how to build the Mainline Kernel and place it on a Raspbian SD card.

Procedure

  1. Install tools needed:
$ apt install git make gcc device-tree-compiler bison flex libssl-dev libncurses-dev
  1. Install an up-to-date cross compiler and associated toolset. This may be obtained from https://releases.linaro.org/components/toolchain/binaries/latest-7/. 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 18.04 on an Intel-based development machine, the appropriate version is gcc-linaro-7.3.1-2018.05-x86_64_arm-linux-gnueabihf.tar.xz.

$ wget https://releases.linaro.org/components/toolchain/binaries/latest-7/arm-linux-gnueabihf/gcc-linaro-7.3.1-2018.05-x86_64_arm-linux-gnueabihf.tar.xz
$ sudo tar xf gcc-linaro-7.3.1-2018.05-x86_64_arm-linux-gnueabihf.tar.xz -C /opt
  1. Get Raspberry Pi Linux Sources
$ git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
  1. 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-7.3.1-2018.05-x86_64_arm-linux-gnueabihf/bin/arm-linux-gnueabihf-
  1. 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 Eric Anholt's pages we can deduce that we should select bcm2835_defconfig for BCM2835-based models, multi_v7_defconfig for BCM2836-, BCM2837- and BCM2711-based models (ARM 32-bit) 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
  1. At this point you can modify the configuration using, for instance, menuconfig. This is at least necessary for Raspberry Pi 4 (32 bit) to gain USB support by enabling CONFIG_ARM_LPAE and CONFIG_PCIE_BRCMSTB:
$ make menuconfig
  1. 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.

  1. Insert an existing Raspbian SD Card to your development machine. In Ubuntu 18.04, it will appear at /media/<username>/boot and /media/<username>/rootfs, where <username> is the username you are logged in under.
  2. Identify the appropriate kernel file to replace. For some models, the operative kernel is in /media/<username>/boot/kernel.img, whereas in others including the Raspberry Pi 3B Plus used here, it is in the file /media/<username>/boot/kernel7.img. Refer to the official Kernel Building documentation to identify the correct kernel file. Copy the kernel file to the card's boot directory, renaming the existing kernel file beforehand to preserve it (for ARM 64-bit the kernel file is just Image):
$ cp arch/arm/boot/zImage /media/<username>/boot/
  1. 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:
$ cp arch/arm/boot/dts/bcm2837-rpi-3-b-plus.dtb /media/<username>/boot/
  1. Adjust the device tree information in /media/<username>/boot/config.txt:
  • Add the following lines to /media/<username>/boot/config.txt. The filename is that of the device tree blob copied over in Step 10. If /media/<username>/boot/config.txt already contains a device_tree entry, replace it with this.
device_tree=bcm2837-rpi-3-b-plus.dtb
kernel=zImage
  1. 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
  1. Safely dismount the SD card from the development machine, put it into the target machine and reboot.

Notes

  • 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 boot partition.
  • The AUX UART has a different device name with mainline kernel: /dev/ttyS1 instead of /dev/ttyS0
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