1Arm Trusted Firmware 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 10Cortex-A53 cores, which makes it possible to have a port of the Arm Trusted 11Firmware. 12 13The following instructions explain how to use this port of the Trusted Firmware 14with the default distribution of `Raspbian`_ because that's the distribution 15officially supported by the Raspberry Pi Foundation. At the moment of writing 16this, the officially supported kernel is a AArch32 kernel. This doesn't mean 17that this port of the Trusted Firmware can't boot a AArch64 kernel. The `Linux 18tree fork`_ maintained by the Foundation can be compiled for AArch64 by 19following the steps in `AArch64 kernel build instructions`_. 20 21**IMPORTANT NOTE**: This port isn't secure. All of the memory used is DRAM, 22which is available from both the Non-secure and Secure worlds. This port 23shouldn't be considered more than a prototype to play with and implement 24elements like PSCI to support the Linux kernel. 25 26Design 27------ 28 29The SoC used by the Raspberry Pi 3 is the Broadcom BCM2837. It is a SoC with a 30VideoCore IV that acts as primary processor (and loads everything from the SD 31card) and is located between all Arm cores and the DRAM. Check the `Raspberry Pi 323 documentation`_ for more information. 33 34This explains why it is possible to change the execution state (AArch64/AArch32) 35depending on a few files on the SD card. We only care about the cases in which 36the cores boot in AArch64 mode. 37 38The rules are simple: 39 40- If a file called ``kernel8.img`` is located on the ``boot`` partition of the 41 SD card, it will load it and execute in EL2 in AArch64. Basically, it executes 42 a `default AArch64 stub`_ at address **0x0** that jumps to the kernel. 43 44- If there is also a file called ``armstub8.bin``, it will load it at address 45 **0x0** (instead of the default stub) and execute it in EL3 in AArch64. All 46 the cores are powered on at the same time and start at address **0x0**. 47 48This means that we can use the default AArch32 kernel provided in the official 49`Raspbian`_ distribution by renaming it to ``kernel8.img``, while the Trusted 50Firmware and anything else we need is in ``armstub8.bin``. This way we can 51forget about the default bootstrap code. When using a AArch64 kernel, it is only 52needed to make sure that the name on the SD card is ``kernel8.img``. 53 54Ideally, we want to load the kernel and have all cores available, which means 55that we need to make the secondary cores work in the way the kernel expects, as 56explained in `Secondary cores`_. In practice, a small bootstrap is needed 57between the Trusted Firmware and the kernel. 58 59To get the most out of a AArch32 kernel, we want to boot it in Hypervisor mode 60in AArch32. This means that BL33 can't be in EL2 in AArch64 mode. The 61architecture specifies that AArch32 Hypervisor mode isn't present when AArch64 62is used for EL2. When using a AArch64 kernel, it should simply start in EL2. 63 64Placement of images 65~~~~~~~~~~~~~~~~~~~ 66 67The file ``armstub8.bin`` contains BL1 and the FIP. It is needed to add padding 68between them so that the addresses they are loaded to match the ones specified 69when compiling the Trusted Firmware. 70 71The device tree block is loaded by the VideoCore loader from an appropriate 72file, but we can specify the address it is loaded to in ``config.txt``. 73 74The file ``kernel8.img`` contains a kernel image that is loaded to the address 75specified in ``config.txt``. The `Linux kernel tree`_ has information about how 76a AArch32 Linux kernel image is loaded in ``Documentation/arm/Booting``: 77 78:: 79 80 The zImage may also be placed in system RAM and called there. The 81 kernel should be placed in the first 128MiB of RAM. It is recommended 82 that it is loaded above 32MiB in order to avoid the need to relocate 83 prior to decompression, which will make the boot process slightly 84 faster. 85 86There are no similar restrictions for AArch64 kernels, as specified in the file 87``Documentation/arm64/booting.txt``. 88 89This means that we need to avoid the first 128 MiB of RAM when placing the 90Trusted Firmware images (and specially the first 32 MiB, as they are directly 91used to place the uncompressed AArch32 kernel image. This way, both AArch32 and 92AArch64 kernels can be placed at the same address. 93 94In the end, the images look like the following diagram when placed in memory. 95All addresses are Physical Addresses from the point of view of the Arm cores. 96Again, note that this is all just part of the same DRAM that goes from 97**0x00000000** to **0x3F000000**, it just has different names to simulate a real 98secure platform! 99 100:: 101 102 0x00000000 +-----------------+ 103 | ROM | BL1 104 0x00010000 +-----------------+ 105 | FIP | 106 0x00200000 +-----------------+ 107 | | 108 | ... | 109 | | 110 0x01000000 +-----------------+ 111 | Kernel | 112 +-----------------+ 113 | | 114 | ... | 115 | | 116 0x02000000 +-----------------+ 117 | DTB | 118 +-----------------+ 119 | | 120 | ... | 121 | | 122 0x10000000 +-----------------+ 123 | Secure SRAM | BL2, BL31 124 0x10100000 +-----------------+ 125 | Secure DRAM | 126 0x10300000 +-----------------+ 127 | Non-secure DRAM | BL33 128 0x11000000 +-----------------+ 129 | | 130 | ... | 131 | | 132 0x3F000000 +-----------------+ 133 | I/O | 134 0x40000000 +-----------------+ 135 136The area between **0x10000000** and **0x11000000** has to be protected so that 137the kernel doesn't use it. That is done by adding ``memmap=256M$16M`` to the 138command line passed to the kernel. See the `Setup SD card`_ instructions to see 139how to do it. 140 141The last 16 MiB of DRAM can only be accessed by the VideoCore, that has 142different mappings than the Arm cores in which the I/O addresses don't overlap 143the DRAM. The memory reserved to be used by the VideoCore is always placed at 144the end of the DRAM, so this space isn't wasted. 145 146Considering the 128 MiB allocated to the GPU and the 16 MiB allocated for the 147Trusted Firmware, there are 880 MiB available for Linux. 148 149Boot sequence 150~~~~~~~~~~~~~ 151 152The boot sequence of the Trusted Firmware is the usual one except when booting 153a AArch32 kernel. In that case, BL33 is booted in AArch32 Hypervisor mode so 154that it can jump to the kernel in the same mode and let it take over that 155privilege level. If BL33 was running in EL2 in AArch64 (as in the default 156bootflow of the Trusted Firmware) it could only jump to the kernel in AArch32 in 157Supervisor mode. 158 159The `Linux kernel tree`_ has instructions on how to jump to the Linux kernel 160in ``Documentation/arm/Booting`` and ``Documentation/arm64/booting.txt``. The 161bootstrap should take care of this. 162 163Secondary cores 164~~~~~~~~~~~~~~~ 165 166The kernel used by `Raspbian`_ doesn't have support for PSCI, so it is needed to 167use mailboxes to trap the secondary cores until they are ready to jump to the 168kernel. This mailbox is located at a different address in the AArch32 default 169kernel than in the AArch64 kernel. 170 171Also, this port of the Trusted Firmware has another Trusted Mailbox in Shared BL 172RAM. During cold boot, all secondary cores wait in a loop until they are given 173given an address to jump to in this Mailbox (``bl31_warm_entrypoint``). 174 175Once BL31 has finished and the primary core has jumped to the BL33 payload, it 176has to call ``PSCI_CPU_ON`` to release the secondary CPUs from the wait loop. 177The payload then makes them wait in another waitloop listening from messages 178from the kernel. When the primary CPU jumps into the kernel, it will send an 179address to the mailbox so that the secondary CPUs jump to it and are recognised 180by the kernel. 181 182Build Instructions 183------------------ 184 185To boot a AArch64 kernel, only the AArch64 toolchain is required. 186 187To boot a AArch32 kernel, both AArch64 and AArch32 toolchains are required. The 188AArch32 toolchain is needed for the AArch32 bootstrap needed to load a 32-bit 189kernel. 190 191First, clone and compile `Raspberry Pi 3 Arm Trusted Firmware bootstrap`_. 192Choose the one needed for the architecture of your kernel. 193 194Then compile the Arm Trusted Firmware. For a AArch32 kernel, use the following 195command line: 196 197.. code:: shell 198 199 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 200 RPI3_BL33_IN_AARCH32=1 \ 201 BL33=../rpi3-arm-tf-bootstrap/aarch32/el2-bootstrap.bin \ 202 all fip 203 204For a AArch64 kernel, use this other command line: 205 206.. code:: shell 207 208 CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi3 \ 209 BL33=../rpi3-arm-tf-bootstrap/aarch64/el2-bootstrap.bin \ 210 all fip 211 212Then, join BL1 and the FIP with the following instructions (replace ``release`` 213by ``debug`` if you set the build option ``DEBUG=1``): 214 215.. code:: shell 216 217 cp build/rpi3/release/bl1.bin bl1.pad.bin 218 truncate --size=65536 bl1.pad.bin 219 cat bl1.pad.bin build/rpi3/release/fip.bin > armstub8.bin 220 221The resulting file, ``armstub8.bin``, contains BL1 and the FIP in the place they 222need to be for the Trusted Firmware to boot correctly. Now, follow the 223instructions in `Setup SD card`_. 224 225The following build options are supported: 226 227- ``PRELOADED_BL33_BASE``: Specially useful because the file ``kernel8.img`` can 228 be loaded anywhere by modifying the file ``config.txt``. It doesn't have to 229 contain a kernel, it could have any arbitrary payload. 230 231- ``RESET_TO_BL31``: Set to 1 by default. If using a 32-bit kernel like 232 `Raspbian`_, the space used by BL1 can overwritten by the kernel when it is 233 being loaded. Even when using a AArch64 kernel the region used by 234 BL1 isn't protected and the kernel could overwrite it. The space used by BL31 235 is reserved by the command line passed to the kernel. 236 237- ``RPI3_BL33_IN_AARCH32``: This port can load a AArch64 or AArch32 BL33 image. 238 By default this option is 0, which means that the Trusted Firmware will jump 239 to BL33 in EL2 in AArch64 mode. If set to 1, it will jump to BL33 in 240 Hypervisor in AArch32 mode. 241 242The following is not currently supported: 243 244- AArch32 for the Trusted Firmware itself. 245 246- ``EL3_PAYLOAD_BASE``: The reason is that you can already load anything to any 247 address by changing the file ``armstub8.bin``, so there's no point in using 248 the Trusted Firmware in this case. 249 250- ``LOAD_IMAGE_V2=0``: Only version 2 is supported. 251 252AArch64 kernel build instructions 253--------------------------------- 254 255The following instructions show how to install and run a AArch64 kernel by 256using a SD card with the default `Raspbian`_ install as base. Skip them if you 257want to use the default 32-bit kernel. 258 259Note that this system won't be fully 64-bit because all the tools in the 260filesystem are 32-bit binaries, but it's a quick way to get it working, and it 261allows the user to run 64-bit binaries in addition to 32-bit binaries. 262 2631. Clone the `Linux tree fork`_ maintained by the Raspberry Pi Foundation. To 264 speed things up, do a shallow clone of the desired branch. 265 266.. code:: shell 267 268 git clone --depth=1 -b rpi-4.14.y https://github.com/raspberrypi/linux 269 cd linux 270 2712. Configure and compile the kernel. Adapt the number after ``-j`` so that it is 272 1.5 times the number of CPUs in your computer. This may take some time to 273 finish. 274 275.. code:: shell 276 277 make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcmrpi3_defconfig 278 make -j 6 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- 279 2803. Copy the kernel image and the device tree to the SD card. Replace the path 281 by the corresponding path in your computers to the ``boot`` partition of the 282 SD card. 283 284.. code:: shell 285 286 cp arch/arm64/boot/Image /path/to/boot/kernel8.img 287 cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b.dtb /path/to/boot/ 288 2894. Install the kernel modules. Replace the path by the corresponding path to the 290 filesystem partition of the SD card on your computer. 291 292.. code:: shell 293 294 make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \ 295 INSTALL_MOD_PATH=/path/to/filesystem modules_install 296 2975. Follow the instructions in `Setup SD card`_ except for the step of renaming 298 the existing ``kernel7.img`` (we have already copied a AArch64 kernel). 299 300Setup SD card 301------------- 302 303The instructions assume that you have an SD card with a fresh install of 304`Raspbian`_ (or that, at least, the ``boot`` partition is untouched, or nearly 305untouched). They have been tested with the image available in 2017-09-07. 306 3071. Insert the SD card and open the ``boot`` partition. 308 3092. Rename ``kernel7.img`` to ``kernel8.img``. This tricks the VideoCore 310 bootloader into booting the Arm cores in AArch64 mode, like the Trusted 311 Firmware needs, even though the kernel is not compiled for AArch64. 312 3133. Copy ``armstub8.bin`` here. When ``kernel8.img`` is available, The VideoCore 314 bootloader will look for a file called ``armstub8.bin`` and load it at 315 address **0x0** instead of a predefined one. 316 3174. Open ``cmdline.txt`` and add ``memmap=256M$16M`` to prevent the kernel from 318 using the memory needed by the Trusted Firmware. If you want to enable the 319 serial port "Mini UART", make sure that this file also contains 320 ``console=serial0,115200 console=tty1``. 321 322 Note that the 16 MiB reserved this way won't be available for Linux, the same 323 way as the memory reserved in DRAM for the GPU isn't available. 324 3255. Open ``config.txt`` and add the following lines at the end (``enable_uart=1`` 326 is only needed to enable debugging through the Mini UART): 327 328:: 329 330 enable_uart=1 331 kernel_address=0x01000000 332 device_tree_address=0x02000000 333 334If you connect a serial cable to the Mini UART and your computer, and connect 335to it (for example, with ``screen /dev/ttyUSB0 115200``) you should see some 336text. In the case of an AArch32 kernel, you should see something like this: 337 338:: 339 340 NOTICE: Booting Trusted Firmware 341 NOTICE: BL1: v1.4(release):v1.4-329-g61e94684-dirty 342 NOTICE: BL1: Built : 00:09:25, Nov 6 2017 343 NOTICE: BL1: Booting BL2 344 NOTICE: BL2: v1.4(release):v1.4-329-g61e94684-dirty 345 NOTICE: BL2: Built : 00:09:25, Nov 6 2017 346 NOTICE: BL1: Booting BL31 347 NOTICE: BL31: v1.4(release):v1.4-329-g61e94684-dirty 348 NOTICE: BL31: Built : 00:09:25, Nov 6 2017 349 [ 0.266484] bcm2835-aux-uart 3f215040.serial: could not get clk: -517 350 351 Raspbian GNU/Linux 9 raspberrypi ttyS0 352 raspberrypi login: 353 354Just enter your credentials, everything should work as expected. Note that the 355HDMI output won't show any text during boot. 356 357.. _default Arm stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub7.S 358.. _default AArch64 stub: https://github.com/raspberrypi/tools/blob/master/armstubs/armstub8.S 359.. _Linux kernel tree: https://github.com/torvalds/linux 360.. _Linux tree fork: https://github.com/raspberrypi/linux 361.. _Raspberry Pi 3: https://www.raspberrypi.org/products/raspberry-pi-3-model-b/ 362.. _Raspberry Pi 3 Arm Trusted Firmware bootstrap: https://github.com/AntonioND/rpi3-arm-tf-bootstrap 363.. _Raspberry Pi 3 documentation: https://www.raspberrypi.org/documentation/ 364.. _Raspbian: https://www.raspberrypi.org/downloads/raspbian/ 365