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. The current port tries to modify the live DTB 137to add a memreserve region that reserves the previously mentioned area. 138 139If this is not possible, the user may manually add ``memmap=16M$256M`` to the 140command line passed to the kernel in ``cmdline.txt``. See the `Setup SD card`_ 141instructions to see how to do it. This system is strongly discouraged. 142 143The last 16 MiB of DRAM can only be accessed by the VideoCore, that has 144different mappings than the Arm cores in which the I/O addresses don't overlap 145the DRAM. The memory reserved to be used by the VideoCore is always placed at 146the end of the DRAM, so this space isn't wasted. 147 148Considering the 128 MiB allocated to the GPU and the 16 MiB allocated for 149TF-A, there are 880 MiB available for Linux. 150 151Boot sequence 152~~~~~~~~~~~~~ 153 154The boot sequence of TF-A is the usual one except when booting an AArch32 155kernel. In that case, BL33 is booted in AArch32 Hypervisor mode so that it 156can jump to the kernel in the same mode and let it take over that privilege 157level. If BL33 was running in EL2 in AArch64 (as in the default bootflow of 158TF-A) it could only jump to the kernel in AArch32 in Supervisor mode. 159 160The `Linux kernel tree`_ has instructions on how to jump to the Linux kernel 161in ``Documentation/arm/Booting`` and ``Documentation/arm64/booting.txt``. The 162bootstrap should take care of this. 163 164This port support a direct boot of the Linux kernel from the firmware (as a BL33 165image). Alternatively, U-Boot or other bootloaders may be used. 166 167Secondary cores 168~~~~~~~~~~~~~~~ 169 170This port of the Trusted Firmware-A supports ``PSCI_CPU_ON``, 171`PSCI_SYSTEM_RESET`` and ``PSCI_SYSTEM_OFF``. The last one doesn't really turn 172the system off, it simply reboots it and asks the VideoCore firmware to keep it 173in a low power mode permanently. 174 175The kernel used by `Raspbian`_ doesn't have support for PSCI, so it is needed to 176use mailboxes to trap the secondary cores until they are ready to jump to the 177kernel. This mailbox is located at a different address in the AArch32 default 178kernel than in the AArch64 kernel. 179 180Kernels with PSCI support can use the PSCI calls instead for a cleaner boot. 181 182Also, this port of TF-A has another Trusted Mailbox in Shared BL RAM. During 183cold boot, all secondary cores wait in a loop until they are given given an 184address to jump to in this Mailbox (``bl31_warm_entrypoint``). 185 186Once BL31 has finished and the primary core has jumped to the BL33 payload, it 187has to call ``PSCI_CPU_ON`` to release the secondary CPUs from the wait loop. 188The payload then makes them wait in another waitloop listening from messages 189from the kernel. When the primary CPU jumps into the kernel, it will send an 190address to the mailbox so that the secondary CPUs jump to it and are recognised 191by the kernel. 192 193Build Instructions 194------------------ 195 196To boot a AArch64 kernel, only the AArch64 toolchain is required. 197 198To boot a AArch32 kernel, both AArch64 and AArch32 toolchains are required. The 199AArch32 toolchain is needed for the AArch32 bootstrap needed to load a 32-bit 200kernel. 201 202The build system concatenates BL1 and the FIP so that the addresses match the 203ones in the memory map. The resulting file is ``armstub8.bin``, located in the 204build folder (e.g. ``build/rpi3/debug/armstub8.bin``). To know how to use this 205file, follow the instructions in `Setup SD card`_. 206 207The following build options are supported: 208 209- ``RPI3_BL33_IN_AARCH32``: This port can load a AArch64 or AArch32 BL33 image. 210 By default this option is 0, which means that TF-A will jump to BL33 in EL2 211 in AArch64 mode. If set to 1, it will jump to BL33 in Hypervisor in AArch32 212 mode. 213 214- ``PRELOADED_BL33_BASE``: Used to specify the address of a BL33 binary that has 215 been preloaded by any other system than using the firmware. ``BL33`` isn't 216 needed in the build command line if this option is used. Specially useful 217 because the file ``kernel8.img`` can be loaded anywhere by modifying the file 218 ``config.txt``. It doesn't have to contain a kernel, it could have any 219 arbitrary payload. 220 221- ``RPI3_DIRECT_LINUX_BOOT``: Disabled by default. Set to 1 to enable the direct 222 boot of the Linux kernel from the firmware. Option ``RPI3_PRELOADED_DTB_BASE`` 223 is mandatory when the direct Linux kernel boot is used. Options 224 ``PRELOADED_BL33_BASE`` will most likely be needed as well because it is 225 unlikely that the kernel image will fit in the space reserved for BL33 images. 226 This option can be combined with ``RPI3_BL33_IN_AARCH32`` in order to boot a 227 32-bit kernel. The only thing this option does is to set the arguments in 228 registers x0-x3 or r0-r2 as expected by the kernel. 229 230- ``RPI3_PRELOADED_DTB_BASE``: Auxiliary build option needed when using 231 ``RPI3_DIRECT_LINUX_BOOT=1``. This option allows to specify the location of a 232 DTB in memory. 233 234- ``RPI3_RUNTIME_UART``: Indicates whether the UART should be used at runtime 235 or disabled. ``-1`` (default) disables the runtime UART. Any other value 236 enables the default UART (currently UART1) for runtime messages. 237 238- ``BL32``: This port can load and run OP-TEE. The OP-TEE image is optional. 239 Please use the code from `here <https://github.com/OP-TEE/optee_os>`__. 240 Build the Trusted Firmware with option ``BL32=tee-header_v2.bin 241 BL32_EXTRA1=tee-pager_v2.bin BL32_EXTRA2=tee-pageable_v2.bin`` 242 to put the binaries into the FIP. 243 244 Note: If OP-TEE is used it may be needed to add the following options to the 245 Linux command line so that the USB driver doesn't use FIQs: 246 ``dwc_otg.fiq_enable=0 dwc_otg.fiq_fsm_enable=0 dwc_otg.nak_holdoff=0``. 247 This will unfortunately reduce the performance of the USB driver. It is needed 248 when using Raspbian, for example. 249 250- ``TRUSTED_BOARD_BOOT``: This port supports TBB. Set this option to 1 to enable 251 it. In order to use TBB, you might want to set ``GENERATE_COT=1`` to let the 252 contents of the FIP automatically signed by the build process. The ROT key 253 will be generated and output to ``rot_key.pem`` in the build directory. It is 254 able to set ROT_KEY to your own key in PEM format. Also in order to build, 255 you need to clone mbed TLS from `here <https://github.com/ARMmbed/mbedtls>`__. 256 ``MBEDTLS_DIR`` must point at the mbed TLS source directory. 257 258- ``ENABLE_STACK_PROTECTOR``: Disabled by default. It uses the hardware RNG of 259 the board. 260 261The following is not currently supported: 262 263- AArch32 for TF-A itself. 264 265- ``EL3_PAYLOAD_BASE``: The reason is that you can already load anything to any 266 address by changing the file ``armstub8.bin``, so there's no point in using 267 TF-A in this case. 268 269- ``MULTI_CONSOLE_API=0``: The multi console API must be enabled. Note that the 270 crash console uses the internal 16550 driver functions directly in order to be 271 able to print error messages during early crashes before setting up the 272 multi console API. 273 274Building the firmware for kernels that don't support PSCI 275~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 276 277This is the case for the 32-bit image of Raspbian, for example. 64-bit kernels 278always support PSCI, but they may not know that the system understands PSCI due 279to an incorrect DTB file. 280 281First, clone and compile the 32-bit version of the `Raspberry Pi 3 TF-A 282bootstrap`_. Choose the one needed for the architecture of your kernel. 283 284Then compile TF-A. For a 32-bit kernel, use the following command line: 285 286.. code:: shell 287 288 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 289 RPI3_BL33_IN_AARCH32=1 \ 290 BL33=../rpi3-arm-tf-bootstrap/aarch32/el2-bootstrap.bin 291 292For a 64-bit kernel, use this other command line: 293 294.. code:: shell 295 296 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 297 BL33=../rpi3-arm-tf-bootstrap/aarch64/el2-bootstrap.bin 298 299However, enabling PSCI support in a 64-bit kernel is really easy. In the 300repository `Raspberry Pi 3 TF-A bootstrap`_ there is a patch that can be applied 301to the Linux kernel tree maintained by the Raspberry Pi foundation. It modifes 302the DTS to tell the kernel to use PSCI. Once this patch is applied, follow the 303instructions in `AArch64 kernel build instructions`_ to get a working 64-bit 304kernel image and supporting files. 305 306Building the firmware for kernels that support PSCI 307~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 308 309For a 64-bit kernel: 310 311.. code:: shell 312 313 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 314 PRELOADED_BL33_BASE=0x02000000 \ 315 RPI3_PRELOADED_DTB_BASE=0x01000000 \ 316 RPI3_DIRECT_LINUX_BOOT=1 317 318For a 32-bit kernel: 319 320.. code:: shell 321 322 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 323 PRELOADED_BL33_BASE=0x02000000 \ 324 RPI3_PRELOADED_DTB_BASE=0x01000000 \ 325 RPI3_DIRECT_LINUX_BOOT=1 \ 326 RPI3_BL33_IN_AARCH32=1 327 328AArch64 kernel build instructions 329--------------------------------- 330 331The following instructions show how to install and run a AArch64 kernel by 332using a SD card with the default `Raspbian`_ install as base. Skip them if you 333want to use the default 32-bit kernel. 334 335Note that this system won't be fully 64-bit because all the tools in the 336filesystem are 32-bit binaries, but it's a quick way to get it working, and it 337allows the user to run 64-bit binaries in addition to 32-bit binaries. 338 3391. Clone the `Linux tree fork`_ maintained by the Raspberry Pi Foundation. To 340 speed things up, do a shallow clone of the desired branch. 341 342.. code:: shell 343 344 git clone --depth=1 -b rpi-4.18.y https://github.com/raspberrypi/linux 345 cd linux 346 3472. Configure and compile the kernel. Adapt the number after ``-j`` so that it is 348 1.5 times the number of CPUs in your computer. This may take some time to 349 finish. 350 351.. code:: shell 352 353 make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcmrpi3_defconfig 354 make -j 6 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- 355 3563. Copy the kernel image and the device tree to the SD card. Replace the path 357 by the corresponding path in your computers to the ``boot`` partition of the 358 SD card. 359 360.. code:: shell 361 362 cp arch/arm64/boot/Image /path/to/boot/kernel8.img 363 cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b.dtb /path/to/boot/ 364 cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b-plus.dtb /path/to/boot/ 365 3664. Install the kernel modules. Replace the path by the corresponding path to the 367 filesystem partition of the SD card on your computer. 368 369.. code:: shell 370 371 make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \ 372 INSTALL_MOD_PATH=/path/to/filesystem modules_install 373 3745. Follow the instructions in `Setup SD card`_ except for the step of renaming 375 the existing ``kernel7.img`` (we have already copied a AArch64 kernel). 376 377Setup SD card 378------------- 379 380The instructions assume that you have an SD card with a fresh install of 381`Raspbian`_ (or that, at least, the ``boot`` partition is untouched, or nearly 382untouched). They have been tested with the image available in 2018-03-13. 383 3841. Insert the SD card and open the ``boot`` partition. 385 3862. Rename ``kernel7.img`` to ``kernel8.img``. This tricks the VideoCore 387 bootloader into booting the Arm cores in AArch64 mode, like TF-A needs, 388 even though the kernel is not compiled for AArch64. 389 3903. Copy ``armstub8.bin`` here. When ``kernel8.img`` is available, The VideoCore 391 bootloader will look for a file called ``armstub8.bin`` and load it at 392 address **0x0** instead of a predefined one. 393 3944. To enable the serial port "Mini UART" in Linux, open ``cmdline.txt`` and add 395 ``console=serial0,115200 console=tty1``. 396 3975. Open ``config.txt`` and add the following lines at the end (``enable_uart=1`` 398 is only needed to enable debugging through the Mini UART): 399 400:: 401 402 enable_uart=1 403 kernel_address=0x02000000 404 device_tree_address=0x01000000 405 406If you connect a serial cable to the Mini UART and your computer, and connect 407to it (for example, with ``screen /dev/ttyUSB0 115200``) you should see some 408text. In the case of an AArch32 kernel, you should see something like this: 409 410:: 411 412 NOTICE: Booting Trusted Firmware 413 NOTICE: BL1: v1.4(release):v1.4-329-g61e94684-dirty 414 NOTICE: BL1: Built : 00:09:25, Nov 6 2017 415 NOTICE: BL1: Booting BL2 416 NOTICE: BL2: v1.4(release):v1.4-329-g61e94684-dirty 417 NOTICE: BL2: Built : 00:09:25, Nov 6 2017 418 NOTICE: BL1: Booting BL31 419 NOTICE: BL31: v1.4(release):v1.4-329-g61e94684-dirty 420 NOTICE: BL31: Built : 00:09:25, Nov 6 2017 421 [ 0.266484] bcm2835-aux-uart 3f215040.serial: could not get clk: -517 422 423 Raspbian GNU/Linux 9 raspberrypi ttyS0 424 raspberrypi login: 425 426Just enter your credentials, everything should work as expected. Note that the 427HDMI output won't show any text during boot. 428 429.. _default Arm stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub7.S 430.. _default AArch64 stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub8.S 431.. _Linux kernel tree: https://github.com/torvalds/linux 432.. _Linux tree fork: https://github.com/raspberrypi/linux 433.. _Raspberry Pi 3: https://www.raspberrypi.org/products/raspberry-pi-3-model-b/ 434.. _Raspberry Pi 3 TF-A bootstrap: https://github.com/AntonioND/rpi3-arm-tf-bootstrap 435.. _Raspberry Pi 3 documentation: https://www.raspberrypi.org/documentation/ 436.. _Raspbian: https://www.raspberrypi.org/downloads/raspbian/ 437