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Trusted Firmware-A for Raspberry Pi 3

The Raspberry Pi 3 is an inexpensive single-board computer that contains four Arm Cortex-A53 cores.

The following instructions explain how to use this port of the TF-A with the default distribution of Raspbian because that's the distribution officially supported by the Raspberry Pi Foundation. At the moment of writing this, the officially supported kernel is a AArch32 kernel. This doesn't mean that this port of TF-A can't boot a AArch64 kernel. The Linux tree fork maintained by the Foundation can be compiled for AArch64 by following the steps in AArch64 kernel build instructions.

IMPORTANT NOTE: This port isn't secure. All of the memory used is DRAM, which is available from both the Non-secure and Secure worlds. This port shouldn't be considered more than a prototype to play with and implement elements like PSCI to support the Linux kernel.

1.   Design

The SoC used by the Raspberry Pi 3 is the Broadcom BCM2837. It is a SoC with a VideoCore IV that acts as primary processor (and loads everything from the SD card) and is located between all Arm cores and the DRAM. Check the Raspberry Pi 3 documentation for more information.

This explains why it is possible to change the execution state (AArch64/AArch32) depending on a few files on the SD card. We only care about the cases in which the cores boot in AArch64 mode.

The rules are simple:

  • If a file called kernel8.img is located on the boot partition of the SD card, it will load it and execute in EL2 in AArch64. Basically, it executes a default AArch64 stub at address 0x0 that jumps to the kernel.
  • If there is also a file called armstub8.bin, it will load it at address 0x0 (instead of the default stub) and execute it in EL3 in AArch64. All the cores are powered on at the same time and start at address 0x0.

This means that we can use the default AArch32 kernel provided in the official Raspbian distribution by renaming it to kernel8.img, while TF-A and anything else we need is in armstub8.bin. This way we can forget about the default bootstrap code. When using a AArch64 kernel, it is only needed to make sure that the name on the SD card is kernel8.img.

Ideally, we want to load the kernel and have all cores available, which means that we need to make the secondary cores work in the way the kernel expects, as explained in Secondary cores. In practice, a small bootstrap is needed between TF-A and the kernel.

To get the most out of a AArch32 kernel, we want to boot it in Hypervisor mode in AArch32. This means that BL33 can't be in EL2 in AArch64 mode. The architecture specifies that AArch32 Hypervisor mode isn't present when AArch64 is used for EL2. When using a AArch64 kernel, it should simply start in EL2.

1.1.   Placement of images

The file armstub8.bin contains BL1 and the FIP. It is needed to add padding between them so that the addresses they are loaded to match the ones specified when compiling TF-A. This is done automatically by the build system.

The device tree block is loaded by the VideoCore loader from an appropriate file, but we can specify the address it is loaded to in config.txt.

The file kernel8.img contains a kernel image that is loaded to the address specified in config.txt. The Linux kernel tree has information about how a AArch32 Linux kernel image is loaded in Documentation/arm/Booting:

The zImage may also be placed in system RAM and called there.  The
kernel should be placed in the first 128MiB of RAM.  It is recommended
that it is loaded above 32MiB in order to avoid the need to relocate
prior to decompression, which will make the boot process slightly
faster.

There are no similar restrictions for AArch64 kernels, as specified in the file Documentation/arm64/booting.txt.

This means that we need to avoid the first 128 MiB of RAM when placing the TF-A images (and specially the first 32 MiB, as they are directly used to place the uncompressed AArch32 kernel image. This way, both AArch32 and AArch64 kernels can be placed at the same address.

In the end, the images look like the following diagram when placed in memory. All addresses are Physical Addresses from the point of view of the Arm cores. Again, note that this is all just part of the same DRAM that goes from 0x00000000 to 0x3F000000, it just has different names to simulate a real secure platform!

0x00000000 +-----------------+
           |       ROM       | BL1
0x00020000 +-----------------+
           |       FIP       |
0x00200000 +-----------------+
           |                 |
           |       ...       |
           |                 |
0x01000000 +-----------------+
           |       DTB       | (Loaded by the VideoCore)
           +-----------------+
           |                 |
           |       ...       |
           |                 |
0x02000000 +-----------------+
           |     Kernel      | (Loaded by the VideoCore)
           +-----------------+
           |                 |
           |       ...       |
           |                 |
0x10000000 +-----------------+
           |   Secure SRAM   | BL2, BL31
0x10100000 +-----------------+
           |   Secure DRAM   | BL32 (Secure payload)
0x11000000 +-----------------+
           | Non-secure DRAM | BL33
           +-----------------+
           |                 |
           |       ...       |
           |                 |
0x3F000000 +-----------------+
           |       I/O       |
0x40000000 +-----------------+

The area between 0x10000000 and 0x11000000 has to be manually protected so that the kernel doesn't use it. The current port tries to modify the live DTB to add a memreserve region that reserves the previously mentioned area.

If this is not possible, the user may manually add memmap=16M$256M to the command line passed to the kernel in cmdline.txt. See the Setup SD card instructions to see how to do it. This system is strongly discouraged.

The last 16 MiB of DRAM can only be accessed by the VideoCore, that has different mappings than the Arm cores in which the I/O addresses don't overlap the DRAM. The memory reserved to be used by the VideoCore is always placed at the end of the DRAM, so this space isn't wasted.

Considering the 128 MiB allocated to the GPU and the 16 MiB allocated for TF-A, there are 880 MiB available for Linux.

1.2.   Boot sequence

The boot sequence of TF-A is the usual one except when booting an AArch32 kernel. In that case, BL33 is booted in AArch32 Hypervisor mode so that it can jump to the kernel in the same mode and let it take over that privilege level. If BL33 was running in EL2 in AArch64 (as in the default bootflow of TF-A) it could only jump to the kernel in AArch32 in Supervisor mode.

The Linux kernel tree has instructions on how to jump to the Linux kernel in Documentation/arm/Booting and Documentation/arm64/booting.txt. The bootstrap should take care of this.

This port support a direct boot of the Linux kernel from the firmware (as a BL33 image). Alternatively, U-Boot or other bootloaders may be used.

1.3.   Secondary cores

This port of the Trusted Firmware-A supports PSCI_CPU_ON, PSCI_SYSTEM_RESET` and PSCI_SYSTEM_OFF. The last one doesn't really turn the system off, it simply reboots it and asks the VideoCore firmware to keep it in a low power mode permanently.

The kernel used by Raspbian doesn't have support for PSCI, so it is needed to use mailboxes to trap the secondary cores until they are ready to jump to the kernel. This mailbox is located at a different address in the AArch32 default kernel than in the AArch64 kernel.

Kernels with PSCI support can use the PSCI calls instead for a cleaner boot.

Also, this port of TF-A has another Trusted Mailbox in Shared BL RAM. During cold boot, all secondary cores wait in a loop until they are given given an address to jump to in this Mailbox (bl31_warm_entrypoint).

Once BL31 has finished and the primary core has jumped to the BL33 payload, it has to call PSCI_CPU_ON to release the secondary CPUs from the wait loop. The payload then makes them wait in another waitloop listening from messages from the kernel. When the primary CPU jumps into the kernel, it will send an address to the mailbox so that the secondary CPUs jump to it and are recognised by the kernel.

2.   Build Instructions

To boot a AArch64 kernel, only the AArch64 toolchain is required.

To boot a AArch32 kernel, both AArch64 and AArch32 toolchains are required. The AArch32 toolchain is needed for the AArch32 bootstrap needed to load a 32-bit kernel.

The build system concatenates BL1 and the FIP so that the addresses match the ones in the memory map. The resulting file is armstub8.bin, located in the build folder (e.g. build/rpi3/debug/armstub8.bin). To know how to use this file, follow the instructions in Setup SD card.

The following build options are supported:

  • RPI3_BL33_IN_AARCH32: This port can load a AArch64 or AArch32 BL33 image. By default this option is 0, which means that TF-A will jump to BL33 in EL2 in AArch64 mode. If set to 1, it will jump to BL33 in Hypervisor in AArch32 mode.
  • PRELOADED_BL33_BASE: Used to specify the address of a BL33 binary that has been preloaded by any other system than using the firmware. BL33 isn't needed in the build command line if this option is used. Specially useful because the file kernel8.img can be loaded anywhere by modifying the file config.txt. It doesn't have to contain a kernel, it could have any arbitrary payload.
  • RPI3_DIRECT_LINUX_BOOT: Disabled by default. Set to 1 to enable the direct boot of the Linux kernel from the firmware. Option RPI3_PRELOADED_DTB_BASE is mandatory when the direct Linux kernel boot is used. Options PRELOADED_BL33_BASE will most likely be needed as well because it is unlikely that the kernel image will fit in the space reserved for BL33 images. This option can be combined with RPI3_BL33_IN_AARCH32 in order to boot a 32-bit kernel. The only thing this option does is to set the arguments in registers x0-x3 or r0-r2 as expected by the kernel.
  • RPI3_PRELOADED_DTB_BASE: Auxiliary build option needed when using RPI3_DIRECT_LINUX_BOOT=1. This option allows to specify the location of a DTB in memory.
  • RPI3_RUNTIME_UART: Indicates whether the UART should be used at runtime or disabled. -1 (default) disables the runtime UART. Any other value enables the default UART (currently UART1) for runtime messages.
  • RPI3_USE_UEFI_MAP: Set to 1 to build ATF with the altername memory mapping required for an UEFI firmware payload. These changes are needed to be able to run Windows on ARM64. This option, which is disabled by default, results in the following memory mappings:
0x00000000 +-----------------+
           |       ROM       | BL1
0x00010000 +-----------------+
           |       DTB       | (Loaded by the VideoCore)
0x00020000 +-----------------+
           |       FIP       |
0x00030000 +-----------------+
           |                 |
           |  UEFI PAYLOAD   |
           |                 |
0x00200000 +-----------------+
           |   Secure SRAM   | BL2, BL31
0x00300000 +-----------------+
           |   Secure DRAM   | BL32 (Secure payload)
0x00400000 +-----------------+
           |                 |
           |                 |
           | Non-secure DRAM | BL33
           |                 |
           |                 |
0x01000000 +-----------------+
           |                 |
           |       ...       |
           |                 |
0x3F000000 +-----------------+
           |       I/O       |
  • BL32: This port can load and run OP-TEE. The OP-TEE image is optional. Please use the code from here. Build the Trusted Firmware with option BL32=tee-header_v2.bin BL32_EXTRA1=tee-pager_v2.bin BL32_EXTRA2=tee-pageable_v2.bin to put the binaries into the FIP.

    Note: If OP-TEE is used it may be needed to add the following options to the Linux command line so that the USB driver doesn't use FIQs: dwc_otg.fiq_enable=0 dwc_otg.fiq_fsm_enable=0 dwc_otg.nak_holdoff=0. This will unfortunately reduce the performance of the USB driver. It is needed when using Raspbian, for example.

  • TRUSTED_BOARD_BOOT: This port supports TBB. Set this option to 1 to enable it. In order to use TBB, you might want to set GENERATE_COT=1 to let the contents of the FIP automatically signed by the build process. The ROT key will be generated and output to rot_key.pem in the build directory. It is able to set ROT_KEY to your own key in PEM format. Also in order to build, you need to clone mbed TLS from here. MBEDTLS_DIR must point at the mbed TLS source directory.

  • ENABLE_STACK_PROTECTOR: Disabled by default. It uses the hardware RNG of the board.

The following is not currently supported:

  • AArch32 for TF-A itself.
  • EL3_PAYLOAD_BASE: The reason is that you can already load anything to any address by changing the file armstub8.bin, so there's no point in using TF-A in this case.
  • MULTI_CONSOLE_API=0: The multi console API must be enabled. Note that the crash console uses the internal 16550 driver functions directly in order to be able to print error messages during early crashes before setting up the multi console API.

2.1.   Building the firmware for kernels that don't support PSCI

This is the case for the 32-bit image of Raspbian, for example. 64-bit kernels always support PSCI, but they may not know that the system understands PSCI due to an incorrect DTB file.

First, clone and compile the 32-bit version of the Raspberry Pi 3 TF-A bootstrap. Choose the one needed for the architecture of your kernel.

Then compile TF-A. For a 32-bit kernel, use the following command line:

CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3             \
RPI3_BL33_IN_AARCH32=1                                      \
BL33=../rpi3-arm-tf-bootstrap/aarch32/el2-bootstrap.bin

For a 64-bit kernel, use this other command line:

CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3             \
BL33=../rpi3-arm-tf-bootstrap/aarch64/el2-bootstrap.bin

However, enabling PSCI support in a 64-bit kernel is really easy. In the repository Raspberry Pi 3 TF-A bootstrap there is a patch that can be applied to the Linux kernel tree maintained by the Raspberry Pi foundation. It modifes the DTS to tell the kernel to use PSCI. Once this patch is applied, follow the instructions in AArch64 kernel build instructions to get a working 64-bit kernel image and supporting files.

2.2.   Building the firmware for kernels that support PSCI

For a 64-bit kernel:

CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3             \
PRELOADED_BL33_BASE=0x02000000                              \
RPI3_PRELOADED_DTB_BASE=0x01000000                          \
RPI3_DIRECT_LINUX_BOOT=1

For a 32-bit kernel:

CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3             \
PRELOADED_BL33_BASE=0x02000000                              \
RPI3_PRELOADED_DTB_BASE=0x01000000                          \
RPI3_DIRECT_LINUX_BOOT=1                                    \
RPI3_BL33_IN_AARCH32=1

3.   AArch64 kernel build instructions

The following instructions show how to install and run a AArch64 kernel by using a SD card with the default Raspbian install as base. Skip them if you want to use the default 32-bit kernel.

Note that this system won't be fully 64-bit because all the tools in the filesystem are 32-bit binaries, but it's a quick way to get it working, and it allows the user to run 64-bit binaries in addition to 32-bit binaries.

  1. Clone the Linux tree fork maintained by the Raspberry Pi Foundation. To speed things up, do a shallow clone of the desired branch.
git clone --depth=1 -b rpi-4.18.y https://github.com/raspberrypi/linux
cd linux
  1. Configure and compile the kernel. Adapt the number after -j so that it is 1.5 times the number of CPUs in your computer. This may take some time to finish.
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcmrpi3_defconfig
make -j 6 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-
  1. Copy the kernel image and the device tree to the SD card. Replace the path by the corresponding path in your computers to the boot partition of the SD card.
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/
  1. Install the kernel modules. Replace the path by the corresponding path to the filesystem partition of the SD card on your computer.
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
INSTALL_MOD_PATH=/path/to/filesystem modules_install
  1. Follow the instructions in Setup SD card except for the step of renaming the existing kernel7.img (we have already copied a AArch64 kernel).

4.   Setup SD card

The instructions assume that you have an SD card with a fresh install of Raspbian (or that, at least, the boot partition is untouched, or nearly untouched). They have been tested with the image available in 2018-03-13.

  1. Insert the SD card and open the boot partition.
  2. Rename kernel7.img to kernel8.img. This tricks the VideoCore bootloader into booting the Arm cores in AArch64 mode, like TF-A needs, even though the kernel is not compiled for AArch64.
  3. Copy armstub8.bin here. When kernel8.img is available, The VideoCore bootloader will look for a file called armstub8.bin and load it at address 0x0 instead of a predefined one.
  4. To enable the serial port "Mini UART" in Linux, open cmdline.txt and add console=serial0,115200 console=tty1.
  5. Open config.txt and add the following lines at the end (enable_uart=1 is only needed to enable debugging through the Mini UART):
enable_uart=1
kernel_address=0x02000000
device_tree_address=0x01000000

If you connect a serial cable to the Mini UART and your computer, and connect to it (for example, with screen /dev/ttyUSB0 115200) you should see some text. In the case of an AArch32 kernel, you should see something like this:

NOTICE:  Booting Trusted Firmware
NOTICE:  BL1: v1.4(release):v1.4-329-g61e94684-dirty
NOTICE:  BL1: Built : 00:09:25, Nov  6 2017
NOTICE:  BL1: Booting BL2
NOTICE:  BL2: v1.4(release):v1.4-329-g61e94684-dirty
NOTICE:  BL2: Built : 00:09:25, Nov  6 2017
NOTICE:  BL1: Booting BL31
NOTICE:  BL31: v1.4(release):v1.4-329-g61e94684-dirty
NOTICE:  BL31: Built : 00:09:25, Nov  6 2017
[    0.266484] bcm2835-aux-uart 3f215040.serial: could not get clk: -517

Raspbian GNU/Linux 9 raspberrypi ttyS0
raspberrypi login:

Just enter your credentials, everything should work as expected. Note that the HDMI output won't show any text during boot.