docs/plat/rpi3.rst - filogic/atf - Gitiles

 Trusted Firmware-A for Raspberry Pi 3
 =====================================

 .. section-numbering::
     :suffix: .

 .. contents::

 The `Raspberry Pi 3`_ is an inexpensive single-board computer that contains four
 Arm Cortex-A53 cores, which makes it possible to have a port of Trusted
 Firmware-A (TF-A).

 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.

 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.

 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.

 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
     0x00010000 +-----------------+
                |       FIP       |
     0x00200000 +-----------------+
                |                 |
                |       ...       |
                |                 |
     0x01000000 +-----------------+
                |     Kernel      |
                +-----------------+
                |                 |
                |       ...       |
                |                 |
     0x02000000 +-----------------+
                |       DTB       |
                +-----------------+
                |                 |
                |       ...       |
                |                 |
     0x10000000 +-----------------+
                |   Secure SRAM   | BL2, BL31
     0x10100000 +-----------------+
                |   Secure DRAM   |
     0x10300000 +-----------------+
                | Non-secure DRAM | BL33
     0x11000000 +-----------------+
                |                 |
                |       ...       |
                |                 |
     0x3F000000 +-----------------+
                |       I/O       |
     0x40000000 +-----------------+

 The area between **0x10000000** and **0x11000000** has to be protected so that
 the kernel doesn't use it. That is done by adding ``memmap=256M$16M`` to the
 command line passed to the kernel. See the `Setup SD card`_ instructions to see
 how to do it.

 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.

 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.

 Secondary cores
 ~~~~~~~~~~~~~~~

 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.

 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.

 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.

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

 Then compile TF-A. For a AArch32 kernel, use the following command line:

 .. code:: shell

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

 For a AArch64 kernel, use this other command line:

 .. code:: shell

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

 Then, join BL1 and the FIP with the following instructions (replace ``release``
 by ``debug`` if you set the build option ``DEBUG=1``):

 .. code:: shell

     cp build/rpi3/release/bl1.bin bl1.pad.bin
     truncate --size=65536 bl1.pad.bin
     cat bl1.pad.bin build/rpi3/release/fip.bin > armstub8.bin

 The resulting file, ``armstub8.bin``, contains BL1 and the FIP in the place they
 need to be for TF-A to boot correctly. Now, follow the instructions in
 `Setup SD card`_.

 The following build options are supported:

 - ``PRELOADED_BL33_BASE``: 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.

 - ``RESET_TO_BL31``: Set to 1 by default. If using a 32-bit kernel like
   `Raspbian`_, the space used by BL1 can overwritten by the kernel when it is
   being loaded. Even when using a AArch64 kernel the region used by
   BL1 isn't protected and the kernel could overwrite it. The space used by BL31
   is reserved by the command line passed to the kernel.

 - ``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.

 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.

 - ``LOAD_IMAGE_V2=0``: Only version 2 is supported.

 - ``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.

 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.

 .. code:: shell

     git clone --depth=1 -b rpi-4.14.y https://github.com/raspberrypi/linux
     cd linux

 2. 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.

 .. code:: shell

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

 3. 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.

 .. code:: shell

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

 4. Install the kernel modules. Replace the path by the corresponding path to the
    filesystem partition of the SD card on your computer.

 .. code:: shell

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

 5. Follow the instructions in `Setup SD card`_ except for the step of renaming
    the existing ``kernel7.img`` (we have already copied a AArch64 kernel).

 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. Open ``cmdline.txt`` and add ``memmap=256M$16M`` to prevent the kernel from
    using the memory needed by TF-A. If you want to enable the serial port
    "Mini UART", make sure that this file also contains
    ``console=serial0,115200 console=tty1``.

    Note that the 16 MiB reserved this way won't be available for Linux, the same
    way as the memory reserved in DRAM for the GPU isn't available.

 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=0x01000000
     device_tree_address=0x02000000

 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.

 .. _default Arm stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub7.S
 .. _default AArch64 stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub8.S
 .. _Linux kernel tree: https://github.com/torvalds/linux
 .. _Linux tree fork: https://github.com/raspberrypi/linux
 .. _Raspberry Pi 3: https://www.raspberrypi.org/products/raspberry-pi-3-model-b/
 .. _Raspberry Pi 3 TF-A bootstrap: https://github.com/AntonioND/rpi3-arm-tf-bootstrap
 .. _Raspberry Pi 3 documentation: https://www.raspberrypi.org/documentation/
 .. _Raspbian: https://www.raspberrypi.org/downloads/raspbian/
	Trusted Firmware-A for Raspberry Pi 3
	=====================================

	.. section-numbering::
	:suffix: .

	.. contents::

	The `Raspberry Pi 3`_ is an inexpensive single-board computer that contains four
	Arm Cortex-A53 cores, which makes it possible to have a port of Trusted
	Firmware-A (TF-A).

	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.

	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.

	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.

	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
	0x00010000 +-----------------+
	\| FIP \|
	0x00200000 +-----------------+
	\| \|
	\| ... \|
	\| \|
	0x01000000 +-----------------+
	\| Kernel \|
	+-----------------+
	\| \|
	\| ... \|
	\| \|
	0x02000000 +-----------------+
	\| DTB \|
	+-----------------+
	\| \|
	\| ... \|
	\| \|
	0x10000000 +-----------------+
	\| Secure SRAM \| BL2, BL31
	0x10100000 +-----------------+
	\| Secure DRAM \|
	0x10300000 +-----------------+
	\| Non-secure DRAM \| BL33
	0x11000000 +-----------------+
	\| \|
	\| ... \|
	\| \|
	0x3F000000 +-----------------+
	\| I/O \|
	0x40000000 +-----------------+

	The area between 0x10000000 and 0x11000000 has to be protected so that
	the kernel doesn't use it. That is done by adding ``memmap=256M$16M`` to the
	command line passed to the kernel. See the `Setup SD card`_ instructions to see
	how to do it.

	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.

	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.

	Secondary cores
	~~~~~~~~~~~~~~~

	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.

	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.

	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.

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

	Then compile TF-A. For a AArch32 kernel, use the following command line:

	.. code:: shell

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

	For a AArch64 kernel, use this other command line:

	.. code:: shell

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

	Then, join BL1 and the FIP with the following instructions (replace ``release``
	by ``debug`` if you set the build option ``DEBUG=1``):

	.. code:: shell

	cp build/rpi3/release/bl1.bin bl1.pad.bin
	truncate --size=65536 bl1.pad.bin
	cat bl1.pad.bin build/rpi3/release/fip.bin > armstub8.bin

	The resulting file, ``armstub8.bin``, contains BL1 and the FIP in the place they
	need to be for TF-A to boot correctly. Now, follow the instructions in
	`Setup SD card`_.

	The following build options are supported:

	- ``PRELOADED_BL33_BASE``: 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.

	- ``RESET_TO_BL31``: Set to 1 by default. If using a 32-bit kernel like
	`Raspbian`_, the space used by BL1 can overwritten by the kernel when it is
	being loaded. Even when using a AArch64 kernel the region used by
	BL1 isn't protected and the kernel could overwrite it. The space used by BL31
	is reserved by the command line passed to the kernel.

	- ``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.

	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.

	- ``LOAD_IMAGE_V2=0``: Only version 2 is supported.

	- ``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.

	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.

	.. code:: shell

	git clone --depth=1 -b rpi-4.14.y https://github.com/raspberrypi/linux
	cd linux

	2. 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.

	.. code:: shell

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

	3. 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.

	.. code:: shell

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

	4. Install the kernel modules. Replace the path by the corresponding path to the
	filesystem partition of the SD card on your computer.

	.. code:: shell

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

	5. Follow the instructions in `Setup SD card`_ except for the step of renaming
	the existing ``kernel7.img`` (we have already copied a AArch64 kernel).

	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. Open ``cmdline.txt`` and add ``memmap=256M$16M`` to prevent the kernel from
	using the memory needed by TF-A. If you want to enable the serial port
	"Mini UART", make sure that this file also contains
	``console=serial0,115200 console=tty1``.

	Note that the 16 MiB reserved this way won't be available for Linux, the same
	way as the memory reserved in DRAM for the GPU isn't available.

	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=0x01000000
	device_tree_address=0x02000000

	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.

	.. _default Arm stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub7.S
	.. _default AArch64 stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub8.S
	.. _Linux kernel tree: https://github.com/torvalds/linux
	.. _Linux tree fork: https://github.com/raspberrypi/linux
	.. _Raspberry Pi 3: https://www.raspberrypi.org/products/raspberry-pi-3-model-b/
	.. _Raspberry Pi 3 TF-A bootstrap: https://github.com/AntonioND/rpi3-arm-tf-bootstrap
	.. _Raspberry Pi 3 documentation: https://www.raspberrypi.org/documentation/
	.. _Raspbian: https://www.raspberrypi.org/downloads/raspbian/