Linux Bootstrap for Raspberry Pi 3 with Trusted Firmware-A

This repository contains bootstraps that the Trusted Firmware-A needs to use as BL33 images in order to boot the Raspberry Pi Linux kernel tree. The reason for that is that this port doesn't support PSCI. However, this repository also contains a patch thatcan be applied to that tree in order to enable support for PSCI in the DTS files used by the Raspberry Pi (only for 64-bit kernels). This means that there are two options:

• Use the Linux kernel tree as it is and use the bootstraps. This is the only option for now for 32-bit kernels.
• Modify the DTS files to enable PSCI support. If you do this, there are two possibilities:
  • Use Das U-Boot. If so, compile the Trusted Firmware with it as BL33.
  • Compile the Trusted Firmware with direct-to-kernel boot.

The code of this repository (except for the patch for the Linux kernel) is licensed under the terms of the MIT license.

Bootstraps

It is needed to use a specific bootstrap because the kernel expects the secondary CPU cores to use a specific mailbox address to perform a warmboot instead of using PSCI as the Trusted Firmware would expect. Note that this mailbox is different depending on the architecture. Ultimately, it depends on the Device Tree, the bootstrap scripts have been created considering the same addresses used in the upstream kernel.

The kernel also needs to get the address of the DTB in a specific register. This last part can be done by the Trusted Firmware-A, but it is also done by the bootstrap.

The bootstraps are simple pieces of code that have to be executed in AArch32 Hypervisor mode or EL2 in AArch64.

Depending on the architecture the kernel was compiled for, you'll need to use the code in folder aarch32 or aarch64. For a default instalation of Raspbian, go to the folder aarch32 and do:

export CROSS_COMPILE_AARCH32=arm-linux-gnueabihf-
./build.sh

If the kernel was compiled for 64 bits, go to folder aarch64 and do:

export CROSS_COMPILE_AARCH64=aarch64-linux-gnu-
./build.sh

This will generate the file el2-bootstrap.bin that can be passed to the Trusted Firmware-A build system as BL33.

The code assumes the addresses of the device tree block (DTB) and the kernel. If they don't match the ones you are using, change them to the ones you need.

This bootstrap has been created by imitating the behaviour of the default AArch32 stub and AArch64 stub used by the VideoCore loader.

Fix DTS files

Apply the patch in the Linux tree:

git clone --depth=1 -b rpi-4.18.y https://github.com/raspberrypi/linux
cd linux
git am path/to/0001-rpi3-Enable-PSCI-support.patch

Now, compile the kernel as usual:

make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcmrpi3_defconfig
make -j 6 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-

cp arch/arm64/boot/Image /path/to/boot/kernel8.img
cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b.dtb /path/to/boot/
cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b-plus.dtb /path/to/boot/

make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
INSTALL_MOD_PATH=/path/to/filesystem modules_install

The Trusted Firmware has to be built with support for direct kernel boot. Follow the instructions in that repository. Copy the resulting armstub8.bin to the boot partition of the SD card. With this you should have a working AArch64 kernel that uses PSCI.