1Trusted Firmware-A for Raspberry Pi 3 2===================================== 3 4.. section-numbering:: 5 :suffix: . 6 7.. contents:: 8 9The `Raspberry Pi 3`_ is an inexpensive single-board computer that contains four 10Arm Cortex-A53 cores. 11 12The following instructions explain how to use this port of the TF-A with the 13default distribution of `Raspbian`_ because that's the distribution officially 14supported by the Raspberry Pi Foundation. At the moment of writing this, the 15officially supported kernel is a AArch32 kernel. This doesn't mean that this 16port of TF-A can't boot a AArch64 kernel. The `Linux tree fork`_ maintained by 17the Foundation can be compiled for AArch64 by following the steps in 18`AArch64 kernel build instructions`_. 19 20**IMPORTANT NOTE**: This port isn't secure. All of the memory used is DRAM, 21which is available from both the Non-secure and Secure worlds. This port 22shouldn't be considered more than a prototype to play with and implement 23elements like PSCI to support the Linux kernel. 24 25Design 26------ 27 28The SoC used by the Raspberry Pi 3 is the Broadcom BCM2837. It is a SoC with a 29VideoCore IV that acts as primary processor (and loads everything from the SD 30card) and is located between all Arm cores and the DRAM. Check the `Raspberry Pi 313 documentation`_ for more information. 32 33This explains why it is possible to change the execution state (AArch64/AArch32) 34depending on a few files on the SD card. We only care about the cases in which 35the cores boot in AArch64 mode. 36 37The rules are simple: 38 39- If a file called ``kernel8.img`` is located on the ``boot`` partition of the 40 SD card, it will load it and execute in EL2 in AArch64. Basically, it executes 41 a `default AArch64 stub`_ at address **0x0** that jumps to the kernel. 42 43- If there is also a file called ``armstub8.bin``, it will load it at address 44 **0x0** (instead of the default stub) and execute it in EL3 in AArch64. All 45 the cores are powered on at the same time and start at address **0x0**. 46 47This means that we can use the default AArch32 kernel provided in the official 48`Raspbian`_ distribution by renaming it to ``kernel8.img``, while TF-A and 49anything else we need is in ``armstub8.bin``. This way we can forget about the 50default bootstrap code. When using a AArch64 kernel, it is only needed to make 51sure that the name on the SD card is ``kernel8.img``. 52 53Ideally, we want to load the kernel and have all cores available, which means 54that we need to make the secondary cores work in the way the kernel expects, as 55explained in `Secondary cores`_. In practice, a small bootstrap is needed 56between TF-A and the kernel. 57 58To get the most out of a AArch32 kernel, we want to boot it in Hypervisor mode 59in AArch32. This means that BL33 can't be in EL2 in AArch64 mode. The 60architecture specifies that AArch32 Hypervisor mode isn't present when AArch64 61is used for EL2. When using a AArch64 kernel, it should simply start in EL2. 62 63Placement of images 64~~~~~~~~~~~~~~~~~~~ 65 66The file ``armstub8.bin`` contains BL1 and the FIP. It is needed to add padding 67between them so that the addresses they are loaded to match the ones specified 68when compiling TF-A. This is done automatically by the build system. 69 70The device tree block is loaded by the VideoCore loader from an appropriate 71file, but we can specify the address it is loaded to in ``config.txt``. 72 73The file ``kernel8.img`` contains a kernel image that is loaded to the address 74specified in ``config.txt``. The `Linux kernel tree`_ has information about how 75a AArch32 Linux kernel image is loaded in ``Documentation/arm/Booting``: 76 77:: 78 79 The zImage may also be placed in system RAM and called there. The 80 kernel should be placed in the first 128MiB of RAM. It is recommended 81 that it is loaded above 32MiB in order to avoid the need to relocate 82 prior to decompression, which will make the boot process slightly 83 faster. 84 85There are no similar restrictions for AArch64 kernels, as specified in the file 86``Documentation/arm64/booting.txt``. 87 88This means that we need to avoid the first 128 MiB of RAM when placing the 89TF-A images (and specially the first 32 MiB, as they are directly used to 90place the uncompressed AArch32 kernel image. This way, both AArch32 and 91AArch64 kernels can be placed at the same address. 92 93In the end, the images look like the following diagram when placed in memory. 94All addresses are Physical Addresses from the point of view of the Arm cores. 95Again, note that this is all just part of the same DRAM that goes from 96**0x00000000** to **0x3F000000**, it just has different names to simulate a real 97secure platform! 98 99:: 100 101 0x00000000 +-----------------+ 102 | ROM | BL1 103 0x00020000 +-----------------+ 104 | FIP | 105 0x00200000 +-----------------+ 106 | | 107 | ... | 108 | | 109 0x01000000 +-----------------+ 110 | DTB | (Loaded by the VideoCore) 111 +-----------------+ 112 | | 113 | ... | 114 | | 115 0x02000000 +-----------------+ 116 | Kernel | (Loaded by the VideoCore) 117 +-----------------+ 118 | | 119 | ... | 120 | | 121 0x10000000 +-----------------+ 122 | Secure SRAM | BL2, BL31 123 0x10100000 +-----------------+ 124 | Secure DRAM | BL32 (Secure payload) 125 0x11000000 +-----------------+ 126 | Non-secure DRAM | BL33 127 +-----------------+ 128 | | 129 | ... | 130 | | 131 0x3F000000 +-----------------+ 132 | I/O | 133 0x40000000 +-----------------+ 134 135The area between **0x10000000** and **0x11000000** has to be manually protected 136so that the kernel doesn't use it. That is done by adding ``memmap=16M$256M`` to 137the command line passed to the kernel. See the `Setup SD card`_ instructions to 138see how to do it. 139 140The last 16 MiB of DRAM can only be accessed by the VideoCore, that has 141different mappings than the Arm cores in which the I/O addresses don't overlap 142the DRAM. The memory reserved to be used by the VideoCore is always placed at 143the end of the DRAM, so this space isn't wasted. 144 145Considering the 128 MiB allocated to the GPU and the 16 MiB allocated for 146TF-A, there are 880 MiB available for Linux. 147 148Boot sequence 149~~~~~~~~~~~~~ 150 151The boot sequence of TF-A is the usual one except when booting an AArch32 152kernel. In that case, BL33 is booted in AArch32 Hypervisor mode so that it 153can jump to the kernel in the same mode and let it take over that privilege 154level. If BL33 was running in EL2 in AArch64 (as in the default bootflow of 155TF-A) it could only jump to the kernel in AArch32 in Supervisor mode. 156 157The `Linux kernel tree`_ has instructions on how to jump to the Linux kernel 158in ``Documentation/arm/Booting`` and ``Documentation/arm64/booting.txt``. The 159bootstrap should take care of this. 160 161This port support a direct boot of the Linux kernel from the firmware (as a BL33 162image). Alternatively, U-Boot or other bootloaders may be used. 163 164Secondary cores 165~~~~~~~~~~~~~~~ 166 167This port of the Trusted Firmware-A supports ``PSCI_CPU_ON``, 168`PSCI_SYSTEM_RESET`` and ``PSCI_SYSTEM_OFF``. The last one doesn't really turn 169the system off, it simply reboots it and asks the VideoCore firmware to keep it 170in a low power mode permanently. 171 172The kernel used by `Raspbian`_ doesn't have support for PSCI, so it is needed to 173use mailboxes to trap the secondary cores until they are ready to jump to the 174kernel. This mailbox is located at a different address in the AArch32 default 175kernel than in the AArch64 kernel. 176 177Kernels with PSCI support can use the PSCI calls instead for a cleaner boot. 178 179Also, this port of TF-A has another Trusted Mailbox in Shared BL RAM. During 180cold boot, all secondary cores wait in a loop until they are given given an 181address to jump to in this Mailbox (``bl31_warm_entrypoint``). 182 183Once BL31 has finished and the primary core has jumped to the BL33 payload, it 184has to call ``PSCI_CPU_ON`` to release the secondary CPUs from the wait loop. 185The payload then makes them wait in another waitloop listening from messages 186from the kernel. When the primary CPU jumps into the kernel, it will send an 187address to the mailbox so that the secondary CPUs jump to it and are recognised 188by the kernel. 189 190Build Instructions 191------------------ 192 193To boot a AArch64 kernel, only the AArch64 toolchain is required. 194 195To boot a AArch32 kernel, both AArch64 and AArch32 toolchains are required. The 196AArch32 toolchain is needed for the AArch32 bootstrap needed to load a 32-bit 197kernel. 198 199The build system concatenates BL1 and the FIP so that the addresses match the 200ones in the memory map. The resulting file is ``armstub8.bin``, located in the 201build folder (e.g. ``build/rpi3/debug/armstub8.bin``). To know how to use this 202file, follow the instructions in `Setup SD card`_. 203 204The following build options are supported: 205 206- ``RPI3_BL33_IN_AARCH32``: This port can load a AArch64 or AArch32 BL33 image. 207 By default this option is 0, which means that TF-A will jump to BL33 in EL2 208 in AArch64 mode. If set to 1, it will jump to BL33 in Hypervisor in AArch32 209 mode. 210 211- ``PRELOADED_BL33_BASE``: Used to specify the address of a BL33 binary that has 212 been preloaded by any other system than using the firmware. ``BL33`` isn't 213 needed in the build command line if this option is used. Specially useful 214 because the file ``kernel8.img`` can be loaded anywhere by modifying the file 215 ``config.txt``. It doesn't have to contain a kernel, it could have any 216 arbitrary payload. 217 218- ``RPI3_DIRECT_LINUX_BOOT``: Disabled by default. Set to 1 to enable the direct 219 boot of the Linux kernel from the firmware. Option ``RPI3_PRELOADED_DTB_BASE`` 220 is mandatory when the direct Linux kernel boot is used. Options 221 ``PRELOADED_BL33_BASE`` will most likely be needed as well because it is 222 unlikely that the kernel image will fit in the space reserved for BL33 images. 223 This option can be combined with ``RPI3_BL33_IN_AARCH32`` in order to boot a 224 32-bit kernel. The only thing this option does is to set the arguments in 225 registers x0-x3 or r0-r2 as expected by the kernel. 226 227- ``RPI3_PRELOADED_DTB_BASE``: Auxiliary build option needed when using 228 ``RPI3_DIRECT_LINUX_BOOT=1``. This option allows to specify the location of a 229 DTB in memory. 230 231- ``BL32``: This port can load and run OP-TEE. The OP-TEE image is optional. 232 Please use the code from `here <https://github.com/OP-TEE/optee_os>`__. 233 Build the Trusted Firmware with option ``BL32=tee-header_v2.bin 234 BL32_EXTRA1=tee-pager_v2.bin BL32_EXTRA2=tee-pageable_v2.bin`` 235 to put the binaries into the FIP. 236 237 Note: If OP-TEE is used it may be needed to add the following options to the 238 Linux command line so that the USB driver doesn't use FIQs: 239 ``dwc_otg.fiq_enable=0 dwc_otg.fiq_fsm_enable=0 dwc_otg.nak_holdoff=0``. 240 This will unfortunately reduce the performance of the USB driver. It is needed 241 when using Raspbian, for example. 242 243- ``TRUSTED_BOARD_BOOT``: This port supports TBB. Set this option to 1 to enable 244 it. In order to use TBB, you might want to set ``GENERATE_COT=1`` to let the 245 contents of the FIP automatically signed by the build process. The ROT key 246 will be generated and output to ``rot_key.pem`` in the build directory. It is 247 able to set ROT_KEY to your own key in PEM format. Also in order to build, 248 you need to clone mbed TLS from `here <https://github.com/ARMmbed/mbedtls>`__. 249 ``MBEDTLS_DIR`` must point at the mbed TLS source directory. 250 251- ``ENABLE_STACK_PROTECTOR``: Disabled by default. It uses the hardware RNG of 252 the board. 253 254The following is not currently supported: 255 256- AArch32 for TF-A itself. 257 258- ``EL3_PAYLOAD_BASE``: The reason is that you can already load anything to any 259 address by changing the file ``armstub8.bin``, so there's no point in using 260 TF-A in this case. 261 262- ``MULTI_CONSOLE_API=0``: The multi console API must be enabled. Note that the 263 crash console uses the internal 16550 driver functions directly in order to be 264 able to print error messages during early crashes before setting up the 265 multi console API. 266 267Building the firmware for kernels that don't support PSCI 268~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 269 270This is the case for the 32-bit image of Raspbian, for example. 64-bit kernels 271always support PSCI, but they may not know that the system understands PSCI due 272to an incorrect DTB file. 273 274First, clone and compile the 32-bit version of the `Raspberry Pi 3 TF-A 275bootstrap`_. Choose the one needed for the architecture of your kernel. 276 277Then compile TF-A. For a 32-bit kernel, use the following command line: 278 279.. code:: shell 280 281 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 282 RPI3_BL33_IN_AARCH32=1 \ 283 BL33=../rpi3-arm-tf-bootstrap/aarch32/el2-bootstrap.bin 284 285For a 64-bit kernel, use this other command line: 286 287.. code:: shell 288 289 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 290 BL33=../rpi3-arm-tf-bootstrap/aarch64/el2-bootstrap.bin 291 292However, enabling PSCI support in a 64-bit kernel is really easy. In the 293repository `Raspberry Pi 3 TF-A bootstrap`_ there is a patch that can be applied 294to the Linux kernel tree maintained by the Raspberry Pi foundation. It modifes 295the DTS to tell the kernel to use PSCI. Once this patch is applied, follow the 296instructions in `AArch64 kernel build instructions`_ to get a working 64-bit 297kernel image and supporting files. 298 299Building the firmware for kernels that support PSCI 300~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 301 302For a 64-bit kernel: 303 304.. code:: shell 305 306 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 307 PRELOADED_BL33_BASE=0x02000000 \ 308 RPI3_PRELOADED_DTB_BASE=0x01000000 \ 309 RPI3_DIRECT_LINUX_BOOT=1 310 311For a 32-bit kernel: 312 313.. code:: shell 314 315 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 316 PRELOADED_BL33_BASE=0x02000000 \ 317 RPI3_PRELOADED_DTB_BASE=0x01000000 \ 318 RPI3_DIRECT_LINUX_BOOT=1 \ 319 RPI3_BL33_IN_AARCH32=1 320 321AArch64 kernel build instructions 322--------------------------------- 323 324The following instructions show how to install and run a AArch64 kernel by 325using a SD card with the default `Raspbian`_ install as base. Skip them if you 326want to use the default 32-bit kernel. 327 328Note that this system won't be fully 64-bit because all the tools in the 329filesystem are 32-bit binaries, but it's a quick way to get it working, and it 330allows the user to run 64-bit binaries in addition to 32-bit binaries. 331 3321. Clone the `Linux tree fork`_ maintained by the Raspberry Pi Foundation. To 333 speed things up, do a shallow clone of the desired branch. 334 335.. code:: shell 336 337 git clone --depth=1 -b rpi-4.18.y https://github.com/raspberrypi/linux 338 cd linux 339 3402. Configure and compile the kernel. Adapt the number after ``-j`` so that it is 341 1.5 times the number of CPUs in your computer. This may take some time to 342 finish. 343 344.. code:: shell 345 346 make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcmrpi3_defconfig 347 make -j 6 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- 348 3493. Copy the kernel image and the device tree to the SD card. Replace the path 350 by the corresponding path in your computers to the ``boot`` partition of the 351 SD card. 352 353.. code:: shell 354 355 cp arch/arm64/boot/Image /path/to/boot/kernel8.img 356 cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b.dtb /path/to/boot/ 357 cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b-plus.dtb /path/to/boot/ 358 3594. Install the kernel modules. Replace the path by the corresponding path to the 360 filesystem partition of the SD card on your computer. 361 362.. code:: shell 363 364 make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \ 365 INSTALL_MOD_PATH=/path/to/filesystem modules_install 366 3675. Follow the instructions in `Setup SD card`_ except for the step of renaming 368 the existing ``kernel7.img`` (we have already copied a AArch64 kernel). 369 370Setup SD card 371------------- 372 373The instructions assume that you have an SD card with a fresh install of 374`Raspbian`_ (or that, at least, the ``boot`` partition is untouched, or nearly 375untouched). They have been tested with the image available in 2018-03-13. 376 3771. Insert the SD card and open the ``boot`` partition. 378 3792. Rename ``kernel7.img`` to ``kernel8.img``. This tricks the VideoCore 380 bootloader into booting the Arm cores in AArch64 mode, like TF-A needs, 381 even though the kernel is not compiled for AArch64. 382 3833. Copy ``armstub8.bin`` here. When ``kernel8.img`` is available, The VideoCore 384 bootloader will look for a file called ``armstub8.bin`` and load it at 385 address **0x0** instead of a predefined one. 386 3874. Open ``cmdline.txt`` and add ``memmap=16M$256M`` to prevent the kernel from 388 using the memory needed by TF-A. If you want to enable the serial port 389 "Mini UART", make sure that this file also contains 390 ``console=serial0,115200 console=tty1``. 391 392 Note that the 16 MiB reserved this way won't be available for Linux, the same 393 way as the memory reserved in DRAM for the GPU isn't available. 394 3955. Open ``config.txt`` and add the following lines at the end (``enable_uart=1`` 396 is only needed to enable debugging through the Mini UART): 397 398:: 399 400 enable_uart=1 401 kernel_address=0x02000000 402 device_tree_address=0x01000000 403 404If you connect a serial cable to the Mini UART and your computer, and connect 405to it (for example, with ``screen /dev/ttyUSB0 115200``) you should see some 406text. In the case of an AArch32 kernel, you should see something like this: 407 408:: 409 410 NOTICE: Booting Trusted Firmware 411 NOTICE: BL1: v1.4(release):v1.4-329-g61e94684-dirty 412 NOTICE: BL1: Built : 00:09:25, Nov 6 2017 413 NOTICE: BL1: Booting BL2 414 NOTICE: BL2: v1.4(release):v1.4-329-g61e94684-dirty 415 NOTICE: BL2: Built : 00:09:25, Nov 6 2017 416 NOTICE: BL1: Booting BL31 417 NOTICE: BL31: v1.4(release):v1.4-329-g61e94684-dirty 418 NOTICE: BL31: Built : 00:09:25, Nov 6 2017 419 [ 0.266484] bcm2835-aux-uart 3f215040.serial: could not get clk: -517 420 421 Raspbian GNU/Linux 9 raspberrypi ttyS0 422 raspberrypi login: 423 424Just enter your credentials, everything should work as expected. Note that the 425HDMI output won't show any text during boot. 426 427.. _default Arm stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub7.S 428.. _default AArch64 stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub8.S 429.. _Linux kernel tree: https://github.com/torvalds/linux 430.. _Linux tree fork: https://github.com/raspberrypi/linux 431.. _Raspberry Pi 3: https://www.raspberrypi.org/products/raspberry-pi-3-model-b/ 432.. _Raspberry Pi 3 TF-A bootstrap: https://github.com/AntonioND/rpi3-arm-tf-bootstrap 433.. _Raspberry Pi 3 documentation: https://www.raspberrypi.org/documentation/ 434.. _Raspbian: https://www.raspberrypi.org/downloads/raspbian/ 435