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@Dan Handley Dan Handley on 17 Jan 2014 48 KB Update year in copyright text to 2014
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ARM Trusted Firmware User Guide

Contents :

  1. Introduction
  2. Using the Software
  3. Firmware Design
  4. References

1. Introduction

The ARM Trusted Firmware implements a subset of the Trusted Board Boot Requirements (TBBR) Platform Design Document (PDD) [1] for ARM reference platforms. The TBB sequence starts when the platform is powered on and runs up to the stage where it hands-off control to firmware running in the normal world in DRAM. This is the cold boot path.

The ARM Trusted Firmware also implements the Power State Coordination Interface (PSCI) PDD [2] as a runtime service. PSCI is the interface from normal world software to firmware implementing power management use-cases (for example, secondary CPU boot, hotplug and idle). Normal world software can access ARM Trusted Firmware runtime services via the ARM SMC (Secure Monitor Call) instruction. The SMC instruction must be used as mandated by the SMC Calling Convention PDD [3].

2. Using the Software

Host machine requirements

The minimum recommended machine specification for building the software and running the FVP (Fixed Virtual Platform) model is a dual-core processor running at 2GHz with 12GB of RAM. For best performance, use a machine with a quad-core processor running at 2.6GHz with 16GB of RAM.

The software has been tested on Ubuntu 12.04.02 (64-bit). Packages used for building the software were installed from that distribution unless otherwise specified.

Tools

The following tools are required to use the ARM Trusted Firmware:

  • git package to obtain source code

  • ia32-libs package

  • build-essential and uuid-dev packages for building UEFI

  • bc and ncurses-dev packages for building Linux

  • Baremetal GNU GCC tools. Verified packages can be downloaded from [Linaro] [Linaro Toolchain]. The rest of this document assumes that the gcc-linaro-aarch64-none-elf-4.8-2013.09-01_linux.tar.xz tools are used.

    wget http://releases.linaro.org/13.09/components/toolchain/binaries/gcc-linaro-aarch64-none-elf-4.8-2013.09-01_linux.tar.xz
tar -xf gcc-linaro-aarch64-none-elf-4.8-2013.09-01_linux.tar.xz

  • The Device Tree Compiler (DTC) included with Linux kernel 3.12-rc4 is used to build the Flattened Device Tree (FDT) source files (.dts files) provided with this software.

  • (Optional) For debugging, ARM Development Studio 5 (DS-5) v5.16.

Building the Trusted Firmware

To build the software for the FVPs, follow these steps:

  1. Clone the ARM Trusted Firmware repository from Github:

    git clone https://github.com/ARM-software/arm-trusted-firmware.git

  2. Change to the trusted firmware directory:

    cd arm-trusted-firmware

  3. Set the compiler path and build:

    CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf- make

    By default this produces a release version of the build. To produce a debug version instead, refer to the "Debugging options" section below.

    The build process creates products in a build directory tree, building the objects and binaries for each boot loader stage in separate sub-directories. The following boot loader binary files are created from the corresponding ELF files:

    • build/<platform>/<build-type>/bl1.bin
    • build/<platform>/<build-type>/bl2.bin
    • build/<platform>/<build-type>/bl31.bin

    ... where <platform> currently defaults to fvp and <build-type> is either debug or release.

  4. Copy the three boot loader binary files to the directory from which the FVP will be launched. Symbolic links of the same names may be created instead.

  5. (Optional) Build products for a specific build variant can be removed using:

    make DEBUG=<D> clean

    ... where <D> is 0 or 1, as specified when building.

    The build tree can be removed completely using:

    make realclean

Debugging options

To compile a debug version and make the build more verbose use

CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf- make DEBUG=1 V=1

AArch64 GCC uses DWARF version 4 debugging symbols by default. Some tools (for example DS-5) might not support this and may need an older version of DWARF symbols to be emitted by GCC. This can be achieved by using the -gdwarf-<version> flag, with the version being set to 2 or 3. Setting the version to 2 is recommended for DS-5 versions older than 5.16.

When debugging logic problems it might also be useful to disable all compiler optimizations by using -O0.

NOTE: Using -O0 could cause output images to be larger and base addresses might need to be recalculated (see the later memory layout section).

Extra debug options can be passed to the build system by setting CFLAGS:

CFLAGS='-O0 -gdwarf-2'                                    \
CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf- \
make DEBUG=1 V=1

NOTE: The Foundation FVP does not provide a debugger interface.

Obtaining the normal world software

Obtaining UEFI

Clone the EDK2 (EFI Development Kit 2) source code from Github. This version supports the Base and Foundation FVPs. EDK2 is an open source implementation of the UEFI specification:

git clone -n https://github.com/tianocore/edk2.git
cd edk2
git checkout 75f630347cace34e2d3abed2a5556ba71cfc50a9

To build the software to be compatible with Foundation and Base FVPs, follow these steps:

  1. Change into the EDK2 source directory

    cd edk2

  2. Copy build config templates to local workspace

    export EDK_TOOLS_PATH=$(pwd)/BaseTools
. edksetup.sh $(pwd)/BaseTools/

  3. Rebuild EDK2 host tools

    make -C "$EDK_TOOLS_PATH" clean
make -C "$EDK_TOOLS_PATH"

  4. Build the software

    CROSS_COMPILE=<absolute-path-to-aarch64-gcc>/bin/aarch64-none-elf- \
build -v -d3 -a AARCH64 -t ARMGCC                                  \
-p ArmPlatformPkg/ArmVExpressPkg/ArmVExpress-FVP-AArch64.dsc       \
-D ARM_FOUNDATION_FVP=1

    The EDK2 binary for use with the ARM Trusted Firmware can then be found here:

    Build/ArmVExpress-FVP-AArch64/DEBUG_ARMGCC/FV/FVP_AARCH64_EFI.fd

This will build EDK2 for the default settings as used by the FVPs.

To boot Linux using a VirtioBlock file-system, the command line passed from EDK2 to the Linux kernel must be modified as described in the "Obtaining a root file-system" section below.

If legacy GICv2 locations are used, the EDK2 platform description must be updated. This is required as EDK2 does not support probing for the GIC location. To do this, build the software as described above with the ARM_FVP_LEGACY_GICV2_LOCATION flag:

-D ARM_FVP_LEGACY_GICV2_LOCATION=1

Then make clean before rebuilding EDK2.

The EDK2 binary FVP_AARCH64_EFI.fd should be loaded into FVP FLASH0 via model parameters as described in the "Running the Software" section below.

Obtaining a Linux kernel

The software has been verified using Linux kernel version 3.12-rc4. Patches have been applied to the kernel in order to enable CPU hotplug.

Preparing a Linux kernel for use on the FVPs with hotplug support can be done as follows (GICv2 support only):

  1. Clone Linux:

    git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

    The CPU hotplug features are not yet included in the mainline kernel. To use these, add the patches from Mark Rutland's kernel, based on Linux 3.12-rc4:

    cd linux
git remote add -f --tags markr git://linux-arm.org/linux-mr.git
git checkout -b hotplug arm64-cpu-hotplug-20131023

  2. Build with the Linaro GCC tools.

    # in linux/
make mrproper
make ARCH=arm64 defconfig

# Enable Hotplug
make ARCH=arm64 menuconfig
#   Kernel Features ---> [*] Support for hot-pluggable CPUs

CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf- \
make -j6 ARCH=arm64

  3. Copy the Linux image arch/arm64/boot/Image to the working directory from where the FVP is launched. Alternatively a symbolic link may be used.

Obtaining the Flattened Device Trees

Depending on the FVP configuration and Linux configuration used, different FDT files are required. FDTs for the Foundation and Base FVPs can be found in the Trusted Firmware source directory under fdts/. The Foundation FVP has a subset of the Base FVP components. For example, the Foundation FVP lacks CLCD and MMC support, and has only one CPU cluster.

  • fvp-base-gicv2-psci.dtb

    (Default) For use with both AEMv8 and Cortex-A57-A53 Base FVPs with Base memory map configuration.

  • fvp-base-gicv2legacy-psci.dtb

    For use with both AEMv8 and Cortex-A57-A53 Base FVPs with legacy VE GIC memory map configuration.

  • fvp-base-gicv3-psci.dtb

    For use with AEMv8 Base FVP with Base memory map configuration and Linux GICv3 support.

  • fvp-foundation-gicv2-psci.dtb

    (Default) For use with Foundation FVP with Base memory map configuration.

  • fvp-foundation-gicv2legacy-psci.dtb

    For use with Foundation FVP with legacy VE GIC memory map configuration.

  • fvp-foundation-gicv3-psci.dtb

    For use with Foundation FVP with Base memory map configuration and Linux GICv3 support.

Copy the chosen FDT blob as fdt.dtb to the directory from which the FVP is launched. Alternatively a symbolic link may be used.

Obtaining a root file-system

To prepare a Linaro LAMP based Open Embedded file-system, the following instructions can be used as a guide. The file-system can be provided to Linux via VirtioBlock or as a RAM-disk. Both methods are described below.

Prepare VirtioBlock

To prepare a VirtioBlock file-system, do the following:

  1. Download and unpack the disk image.

    NOTE: The unpacked disk image grows to 2 GiB in size.

    wget http://releases.linaro.org/13.09/openembedded/aarch64/vexpress64-openembedded_lamp-armv8_20130927-7.img.gz
gunzip vexpress64-openembedded_lamp-armv8_20130927-7.img.gz

  2. Make sure the Linux kernel has Virtio support enabled using make ARCH=arm64 menuconfig.

    Device Drivers  ---> Virtio drivers  ---> <*> Platform bus driver for memory mapped virtio devices
Device Drivers  ---> [*] Block devices  --->  <*> Virtio block driver
File systems    ---> <*> The Extended 4 (ext4) filesystem

    If some of these configurations are missing, enable them, save the kernel configuration, then rebuild the kernel image using the instructions provided in the section "Obtaining a Linux kernel".

  3. Change the Kernel command line to include root=/dev/vda2. This can either be done in the EDK2 boot menu or in the platform file. Editing the platform file and rebuilding EDK2 will make the change persist. To do this:

    1. In EDK, edit the following file:

      ArmPlatformPkg/ArmVExpressPkg/ArmVExpress-FVP-AArch64.dsc

    2. Add root=/dev/vda2 to:

      gArmPlatformTokenSpaceGuid.PcdDefaultBootArgument|"<Other default options>"

    3. Remove the entry:

      gArmPlatformTokenSpaceGuid.PcdDefaultBootInitrdPath|""

    4. Rebuild EDK2 (see "Obtaining UEFI" section above).

  4. The file-system image file should be provided to the model environment by passing it the correct command line option. In the FVPs the following option should be provided in addition to the ones described in the "Running the software" section below.

    NOTE: A symbolic link to this file cannot be used with the FVP; the path to the real file must be provided.

    On the Base FVPs:

    -C bp.virtioblockdevice.image_path="<path-to>/vexpress64-openembedded_lamp-armv8_20130927-7.img"

    On the Foundation FVP:

    --block-device="<path-to>/vexpress64-openembedded_lamp-armv8_20130927-7.img"
  1. Ensure that the FVP doesn't output any error messages. If the following error message is displayed: 
    ERROR: BlockDevice: Failed to open "vexpress64-openembedded_lamp-armv8_20130927-7.img"!
    then make sure the path to the file-system image in the model parameter is correct and that read permission is correctly set on the file-system image file.
Prepare RAM-disk

To prepare a RAM-disk root file-system, do the following:

  1. Download the file-system image:

    wget http://releases.linaro.org/13.09/openembedded/aarch64/linaro-image-lamp-genericarmv8-20130912-487.rootfs.tar.gz

  2. Modify the Linaro image:

    # Prepare for use as RAM-disk. Normally use MMC, NFS or VirtioBlock.
# Be careful, otherwise you could damage your host file-system.
mkdir tmp; cd tmp
sudo sh -c "zcat ../linaro-image-lamp-genericarmv8-20130912-487.rootfs.tar.gz | cpio -id"
sudo ln -s sbin/init .
sudo ln -s S35mountall.sh etc/rcS.d/S03mountall.sh
sudo sh -c "echo 'devtmpfs /dev devtmpfs mode=0755,nosuid 0 0' >> etc/fstab"
sudo sh -c "find . | cpio --quiet -H newc -o | gzip -3 -n > ../filesystem.cpio.gz"
cd ..

  3. Copy the resultant filesystem.cpio.gz to the directory where the FVP is launched from. Alternatively a symbolic link may be used.

Running the software

This version of the ARM Trusted Firmware has been tested on the following ARM FVPs (64-bit versions only).

  • Foundation_v8 (Version 2.0, Build 0.8.5206)
  • FVP_Base_AEMv8A-AEMv8A (Version 5.2, Build 0.8.5202)
  • FVP_Base_Cortex-A57x4-A53x4 (Version 5.2, Build 0.8.5202)

NOTE: The software will not work on Version 1.0 of the Foundation FVP. The commands below would report an unhandled argument error in this case.

Please refer to the FVP documentation for a detailed description of the model parameter options. A brief description of the important ones that affect the ARM Trusted Firmware and normal world software behavior is provided below.

The Foundation FVP is a cut down version of the AArch64 Base FVP. It can be downloaded for free from [ARM's website][ARM FVP website].

Running on the Foundation FVP

The following Foundation_v8 parameters should be used to boot Linux with 4 CPUs using the ARM Trusted Firmware.

NOTE: Using the --block-device parameter is not necessary if a Linux RAM-disk file-system is used (see the "Obtaining a File-system" section above).

<path-to>/Foundation_v8                   \
--cores=4                                 \
--no-secure-memory                        \
--visualization                           \
--gicv3                                   \
--data="<path to bl1.bin>"@0x0            \
--data="<path to UEFI binary>"@0x8000000  \
--block-device="<path-to>/vexpress64-openembedded_lamp-armv8_20130927-7.img"

The default use-case for the Foundation FVP is to enable the GICv3 device in the model but use the GICv2 FDT, in order for Linux to drive the GIC in GICv2 emulation mode.

The memory mapped addresses 0x0 and 0x8000000 correspond to the start of trusted ROM and NOR FLASH0 respectively.

Running on the AEMv8 Base FVP

The following FVP_Base_AEMv8A-AEMv8A parameters should be used to boot Linux with 8 CPUs using the ARM Trusted Firmware.

NOTE: Using cache_state_modelled=1 makes booting very slow. The software will still work (and run much faster) without this option but this will hide any cache maintenance defects in the software.

NOTE: Using the -C bp.virtioblockdevice.image_path parameter is not necessary if a Linux RAM-disk file-system is used (see the "Obtaining a root file-system" section above).

<path-to>/FVP_Base_AEMv8A-AEMv8A                    \
-C pctl.startup=0.0.0.0                             \
-C bp.secure_memory=0                               \
-C cluster0.NUM_CORES=4                             \
-C cluster1.NUM_CORES=4                             \
-C cache_state_modelled=1                           \
-C bp.pl011_uart0.untimed_fifos=1                   \
-C bp.secureflashloader.fname=<path to bl1.bin>     \
-C bp.flashloader0.fname=<path to UEFI binary>      \
-C bp.virtioblockdevice.image_path="<path-to>/vexpress64-openembedded_lamp-armv8_20130927-7.img"

Running on the Cortex-A57-A53 Base FVP

The following FVP_Base_Cortex-A57x4-A53x4 model parameters should be used to boot Linux with 8 CPUs using the ARM Trusted Firmware.

NOTE: Using cache_state_modelled=1 makes booting very slow. The software will still work (and run much faster) without this option but this will hide any cache maintenance defects in the software.

NOTE: Using the -C bp.virtioblockdevice.image_path parameter is not necessary if a Linux RAM-disk file-system is used (see the "Obtaining a root file-system" section above).

<path-to>/FVP_Base_Cortex-A57x4-A53x4               \
-C pctl.startup=0.0.0.0                             \
-C bp.secure_memory=0                               \
-C cache_state_modelled=1                           \
-C bp.pl011_uart0.untimed_fifos=1                   \
-C bp.secureflashloader.fname=<path to bl1.bin>     \
-C bp.flashloader0.fname=<path to UEFI binary>      \
-C bp.virtioblockdevice.image_path="<path-to>/vexpress64-openembedded_lamp-armv8_20130927-7.img"

Configuring the GICv2 memory map

The Base FVP models support GICv2 with the default model parameters at the following addresses. The Foundation FVP also supports these addresses when configured for GICv3 in GICv2 emulation mode.

GICv2 Distributor Interface     0x2f000000
GICv2 CPU Interface             0x2c000000
GICv2 Virtual CPU Interface     0x2c010000
GICv2 Hypervisor Interface      0x2c02f000

The Base FVP models can be configured to support GICv2 at addresses corresponding to the legacy (Versatile Express) memory map as follows. These are the default addresses when using the Foundation FVP in GICv2 mode.

GICv2 Distributor Interface     0x2c001000
GICv2 CPU Interface             0x2c002000
GICv2 Virtual CPU Interface     0x2c004000
GICv2 Hypervisor Interface      0x2c006000

The choice of memory map is reflected in the build variant field (bits[15:12]) in the SYS_ID register (Offset 0x0) in the Versatile Express System registers memory map (0x1c010000).

  • SYS_ID.Build[15:12]

    0x1 corresponds to the presence of the Base GIC memory map. This is the default value on the Base FVPs.

  • SYS_ID.Build[15:12]

    0x0 corresponds to the presence of the Legacy VE GIC memory map. This is the default value on the Foundation FVP.

This register can be configured as described in the following sections.

NOTE: If the legacy VE GIC memory map is used, then the corresponding FDT and UEFI images should be used.

Configuring AEMv8 Foundation FVP GIC for legacy VE memory map

The following parameters configure the Foundation FVP to use GICv2 with the legacy VE memory map:

<path-to>/Foundation_v8                  \
--cores=4                                \
--no-secure-memory                       \
--visualization                          \
--no-gicv3                               \
--data="<path to bl1.bin>"@0x0           \
--data="<path to UEFI binary>"@0x8000000 \
--block-device="<path-to>/vexpress64-openembedded_lamp-armv8_20130927-7.img"

Explicit configuration of the SYS_ID register is not required.

Configuring AEMv8 Base FVP GIC for legacy VE memory map

The following parameters configure the AEMv8 Base FVP to use GICv2 with the legacy VE memory map:

NOTE: Using the -C bp.virtioblockdevice.image_path parameter is not necessary if a Linux RAM-disk file-system is used (see the "Obtaining a root file-system" section above).

<path-to>/FVP_Base_AEMv8A-AEMv8A                    \
-C cluster0.gic.GICD-offset=0x1000                  \
-C cluster0.gic.GICC-offset=0x2000                  \
-C cluster0.gic.GICH-offset=0x4000                  \
-C cluster0.gic.GICH-other-CPU-offset=0x5000        \
-C cluster0.gic.GICV-offset=0x6000                  \
-C cluster0.gic.PERIPH-size=0x8000                  \
-C cluster1.gic.GICD-offset=0x1000                  \
-C cluster1.gic.GICC-offset=0x2000                  \
-C cluster1.gic.GICH-offset=0x4000                  \
-C cluster1.gic.GICH-other-CPU-offset=0x5000        \
-C cluster1.gic.GICV-offset=0x6000                  \
-C cluster1.gic.PERIPH-size=0x8000                  \
-C gic_distributor.GICD-alias=0x2c001000            \
-C bp.variant=0x0                                   \
-C bp.virtioblockdevice.image_path="<path-to>/vexpress64-openembedded_lamp-armv8_20130927-7.img"

The bp.variant parameter corresponds to the build variant field of the SYS_ID register. Setting this to 0x0 allows the ARM Trusted Firmware to detect the legacy VE memory map while configuring the GIC.

Configuring Cortex-A57-A53 Base FVP GIC for legacy VE memory map

The following parameters configure the Cortex-A57-A53 Base FVP to use GICv2 with the legacy VE memory map:

NOTE: Using the -C bp.virtioblockdevice.image_path parameter is not necessary if a Linux RAM-disk file-system is used (see the "Obtaining a root file-system" section above).

<path-to>/FVP_Base_Cortex-A57x4-A53x4               \
-C legacy_gicv2_map=1                               \
-C bp.virtioblockdevice.image_path="<path-to>/vexpress64-openembedded_lamp-armv8_20130927-7.img"

Explicit configuration of the SYS_ID register is not required.

3. Firmware Design

The cold boot path starts when the platform is physically turned on. One of the CPUs released from reset is chosen as the primary CPU, and the remaining CPUs are considered secondary CPUs. The primary CPU is chosen through platform-specific means. The cold boot path is mainly executed by the primary CPU, other than essential CPU initialization executed by all CPUs. The secondary CPUs are kept in a safe platform-specific state until the primary CPU has performed enough initialization to boot them.

The cold boot path in this implementation of the ARM Trusted Firmware is divided into three stages (in order of execution):

  • Boot Loader stage 1 (BL1)
  • Boot Loader stage 2 (BL2)
  • Boot Loader stage 3 (BL3-1). The '1' distinguishes this from other 3rd level boot loader stages.

The ARM Fixed Virtual Platforms (FVPs) provide trusted ROM, trusted SRAM and trusted DRAM regions. Each boot loader stage uses one or more of these memories for its code and data.

BL1

This stage begins execution from the platform's reset vector in trusted ROM at EL3. BL1 code starts at 0x00000000 (trusted ROM) in the FVP memory map. The BL1 data section is placed at the start of trusted SRAM, 0x04000000. The functionality implemented by this stage is as follows.

Determination of boot path

Whenever a CPU is released from reset, BL1 needs to distinguish between a warm boot and a cold boot. This is done using a platform-specific mechanism. The ARM FVPs implement a simple power controller at 0x1c100000. The PSYS register (0x10) is used to distinguish between a cold and warm boot. This information is contained in the PSYS.WK[25:24] field. Additionally, a per-CPU mailbox is maintained in trusted DRAM (0x00600000), to which BL1 writes an entrypoint. Each CPU jumps to this entrypoint upon warm boot. During cold boot, BL1 places the secondary CPUs in a safe platform-specific state while the primary CPU executes the remaining cold boot path as described in the following sections.

Architectural initialization

BL1 performs minimal architectural initialization as follows.

  • Exception vectors

    BL1 sets up simple exception vectors for both synchronous and asynchronous exceptions. The default behavior upon receiving an exception is to set a status code. In the case of the FVP this code is written to the Versatile Express System LED register in the following format:

    SYS_LED[0]   - Security state (Secure=0/Non-Secure=1)
SYS_LED[2:1] - Exception Level (EL3=0x3, EL2=0x2, EL1=0x1, EL0=0x0)
SYS_LED[7:3] - Exception Class (Sync/Async & origin). The values for
               each exception class are:

0x0 : Synchronous exception from Current EL with SP_EL0
0x1 : IRQ exception from Current EL with SP_EL0
0x2 : FIQ exception from Current EL with SP_EL0
0x3 : System Error exception from Current EL with SP_EL0
0x4 : Synchronous exception from Current EL with SP_ELx
0x5 : IRQ exception from Current EL with SP_ELx
0x6 : FIQ exception from Current EL with SP_ELx
0x7 : System Error exception from Current EL with SP_ELx
0x8 : Synchronous exception from Lower EL using aarch64
0x9 : IRQ exception from Lower EL using aarch64
0xa : FIQ exception from Lower EL using aarch64
0xb : System Error exception from Lower EL using aarch64
0xc : Synchronous exception from Lower EL using aarch32
0xd : IRQ exception from Lower EL using aarch32
0xe : FIQ exception from Lower EL using aarch32
0xf : System Error exception from Lower EL using aarch32

    A write to the LED register reflects in the System LEDs (S6LED0..7) in the CLCD window of the FVP. This behavior is because this boot loader stage does not expect to receive any exceptions other than the SMC exception. For the latter, BL1 installs a simple stub. The stub expects to receive only a single type of SMC (determined by its function ID in the general purpose register X0). This SMC is raised by BL2 to make BL1 pass control to BL3-1 (loaded by BL2) at EL3. Any other SMC leads to an assertion failure.

  • MMU setup

    BL1 sets up EL3 memory translation by creating page tables to cover the first 4GB of physical address space. This covers all the memories and peripherals needed by BL1.

  • Control register setup

    • SCTLR_EL3. Instruction cache is enabled by setting the SCTLR_EL3.I bit. Alignment and stack alignment checking is enabled by setting the SCTLR_EL3.A and SCTLR_EL3.SA bits. Exception endianness is set to little-endian by clearing the SCTLR_EL3.EE bit.

    • CPUECTLR. When the FVP includes a model of a specific ARM processor implementation (for example A57 or A53), then intra-cluster coherency is enabled by setting the CPUECTLR.SMPEN bit. The AEMv8 Base FVP is inherently coherent so does not implement CPUECTLR.

    • SCR. Use of the HVC instruction from EL1 is enabled by setting the SCR.HCE bit. FIQ exceptions are configured to be taken in EL3 by setting the SCR.FIQ bit. The register width of the next lower exception level is set to AArch64 by setting the SCR.RW bit. External Aborts and SError Interrupts are configured to be taken in EL3 by setting the SCR.EA bit.

    • CPTR_EL3. Accesses to the CPACR from EL1 or EL2, or the CPTR_EL2 from EL2 are configured to not trap to EL3 by clearing the CPTR_EL3.TCPAC bit. Instructions that access the registers associated with Floating Point and Advanced SIMD execution are configured to not trap to EL3 by clearing the CPTR_EL3.TFP bit.

    • CNTFRQ_EL0. The CNTFRQ_EL0 register is programmed with the base frequency of the system counter, which is retrieved from the first entry in the frequency modes table.

    • Generic Timer. The system level implementation of the generic timer is enabled through the memory mapped interface.

Platform initialization

BL1 enables issuing of snoop and DVM (Distributed Virtual Memory) requests from the CCI-400 slave interface corresponding to the cluster that includes the primary CPU. BL1 also initializes UART0 (PL011 console), which enables access to the printf family of functions.

BL2 image load and execution

BL1 execution continues as follows:

  1. BL1 determines the amount of free trusted SRAM memory available by calculating the extent of its own data section, which also resides in trusted SRAM. BL1 loads a BL2 raw binary image through semi-hosting, at a platform-specific base address. The filename of the BL2 raw binary image on the host file system must be bl2.bin. If the BL2 image file is not present or if there is not enough free trusted SRAM the following error message is printed:

    "Failed to load boot loader stage 2 (BL2) firmware."

    If the load is successful, BL1 updates the limits of the remaining free trusted SRAM. It also populates information about the amount of trusted SRAM used by the BL2 image. The exact load location of the image is provided as a base address in the platform header. Further description of the memory layout can be found later in this document.

  2. BL1 prints the following string from the primary CPU to indicate successful execution of the BL1 stage:

    "Booting trusted firmware boot loader stage 1"

  3. BL1 passes control to the BL2 image at Secure EL1, starting from its load address.

  4. BL1 also passes information about the amount of trusted SRAM used and available for use. This information is populated at a platform-specific memory address.

BL2

BL1 loads and passes control to BL2 at Secure EL1. BL2 is linked against and loaded at a platform-specific base address (more information can be found later in this document). The functionality implemented by BL2 is as follows.

Architectural initialization

BL2 performs minimal architectural initialization required for subsequent stages of the ARM Trusted Firmware and normal world software. It sets up Secure EL1 memory translation by creating page tables to address the first 4GB of the physical address space in a similar way to BL1. EL1 and EL0 are given access to Floating Point & Advanced SIMD registers by clearing the CPACR.FPEN bits.

Platform initialization

BL2 does not perform any platform initialization that affects subsequent stages of the ARM Trusted Firmware or normal world software. It copies the information regarding the trusted SRAM populated by BL1 using a platform-specific mechanism. It also calculates the limits of DRAM (main memory) to determine whether there is enough space to load the normal world software images. A platform defined base address is used to specify the load address for the BL3-1 image.

Normal world image load

BL2 loads a rich boot firmware image (UEFI). The image executes in the normal world. BL2 relies on BL3-1 to pass control to the normal world software image it loads. Hence, BL2 populates a platform-specific area of memory with the entrypoint and Current Program Status Register (CPSR) of the normal world software image. The entrypoint is the load address of the normal world software image. The CPSR is determined as specified in Section 5.13 of the PSCI PDD. This information is passed to BL3-1.

UEFI firmware load

By default, BL2 assumes the UEFI image is present at the base of NOR flash0 (0x08000000), and arranges for BL3-1 to pass control to that location. As mentioned earlier, BL2 populates platform-specific memory with the entrypoint and CPSR of the UEFI image.

BL3-1 image load and execution

BL2 execution continues as follows:

  1. BL2 loads the BL3-1 image into a platform-specific address in trusted SRAM. This is done using semi-hosting. The image is identified by the file bl31.bin on the host file-system. If there is not enough memory to load the image or the image is missing it leads to an assertion failure. If the BL3-1 image loads successfully, BL1 updates the amount of trusted SRAM used and available for use by BL3-1. This information is populated at a platform-specific memory address.

  2. BL2 passes control back to BL1 by raising an SMC, providing BL1 with the BL3-1 entrypoint. The exception is handled by the SMC exception handler installed by BL1.

  3. BL1 turns off the MMU and flushes the caches. It clears the SCTLR_EL3.M/I/C bits, flushes the data cache to the point of coherency and invalidates the TLBs.

  4. BL1 passes control to BL3-1 at the specified entrypoint at EL3.

BL3-1

The image for this stage is loaded by BL2 and BL1 passes control to BL3-1 at EL3. BL3-1 executes solely in trusted SRAM. BL3-1 is linked against and loaded at a platform-specific base address (more information can be found later in this document). The functionality implemented by BL3-1 is as follows.

Architectural initialization

Currently, BL3-1 performs a similar architectural initialization to BL1 as far as system register settings are concerned. Since BL1 code resides in ROM, architectural initialization in BL3-1 allows override of any previous initialization done by BL1. BL3-1 creates page tables to address the first 4GB of physical address space and initializes the MMU accordingly. It replaces the exception vectors populated by BL1 with its own. BL3-1 exception vectors signal error conditions in the same way as BL1 does if an unexpected exception is raised. They implement more elaborate support for handling SMCs since this is the only mechanism to access the runtime services implemented by BL3-1 (PSCI for example). BL3-1 checks each SMC for validity as specified by the SMC calling convention PDD before passing control to the required SMC handler routine.

Platform initialization

BL3-1 performs detailed platform initialization, which enables normal world software to function correctly. It also retrieves entrypoint information for the normal world software image loaded by BL2 from the platform defined memory address populated by BL2.

  • GICv2 initialization:

    • Enable group0 interrupts in the GIC CPU interface.
    • Configure group0 interrupts to be asserted as FIQs.
    • Disable the legacy interrupt bypass mechanism.
    • Configure the priority mask register to allow interrupts of all priorities to be signaled to the CPU interface.
    • Mark SGIs 8-15, the secure physical timer interrupt (#29) and the trusted watchdog interrupt (#56) as group0 (secure).
    • Target the trusted watchdog interrupt to CPU0.
    • Enable these group0 interrupts in the GIC distributor.
    • Configure all other interrupts as group1 (non-secure).
    • Enable signaling of group0 interrupts in the GIC distributor.

  • GICv3 initialization:

    If a GICv3 implementation is available in the platform, BL3-1 initializes the GICv3 in GICv2 emulation mode with settings as described for GICv2 above.

  • Power management initialization:

    BL3-1 implements a state machine to track CPU and cluster state. The state can be one of OFF, ON_PENDING, SUSPEND or ON. All secondary CPUs are initially in the OFF state. The cluster that the primary CPU belongs to is ON; any other cluster is OFF. BL3-1 initializes the data structures that implement the state machine, including the locks that protect them. BL3-1 accesses the state of a CPU or cluster immediately after reset and before the MMU is enabled in the warm boot path. It is not currently possible to use 'exclusive' based spinlocks, therefore BL3-1 uses locks based on Lamport's Bakery algorithm instead. BL3-1 allocates these locks in device memory. They are accessible irrespective of MMU state.

  • Runtime services initialization:

    The only runtime service implemented by BL3-1 is PSCI. The complete PSCI API is not yet implemented. The following functions are currently implemented:

    • PSCI_VERSION
    • CPU_OFF
    • CPU_ON
    • CPU_SUSPEND
    • AFFINITY_INFO

    The CPU_ON, CPU_OFF and CPU_SUSPEND functions implement the warm boot path in ARM Trusted Firmware. CPU_ON and CPU_OFF have undergone testing on all the supported FVPs. CPU_SUSPEND & AFFINITY_INFO have undergone testing only on the AEM v8 Base FVP. Support for AFFINITY_INFO is still experimental. Support for CPU_SUSPEND is stable for entry into power down states. Standby states are currently not supported. PSCI_VERSION is present but completely untested in this version of the software.

    Unsupported PSCI functions can be divided into ones that can return execution to the caller and ones that cannot. The following functions return with a error code as documented in the Power State Coordination Interface PDD.

    • MIGRATE : -1 (NOT_SUPPORTED)
    • MIGRATE_INFO_TYPE : 2 (Trusted OS is either not present or does not require migration)
    • MIGRATE_INFO_UP_CPU : 0 (Return value is UNDEFINED)

    The following unsupported functions do not return and signal an assertion failure if invoked.

    • SYSTEM_OFF
    • SYSTEM_RESET

    BL3-1 returns the error code -1 if an SMC is raised for any other runtime service. This behavior is mandated by the SMC calling convention PDD.

Normal world software execution

BL3-1 uses the entrypoint information provided by BL2 to jump to the normal world software image at the highest available Exception Level (EL2 if available, otherwise EL1).

Memory layout on FVP platforms

On FVP platforms, we use the Trusted ROM and Trusted SRAM to store the trusted firmware binaries. BL1 is originally sitting in the Trusted ROM. Its read-write data are relocated at the base of the Trusted SRAM at runtime. BL1 loads BL2 image near the top of the the trusted SRAM. BL2 loads BL3-1 image between BL1 and BL2. This memory layout is illustrated by the following diagram.

Trusted SRAM
+----------+ 0x04040000
|          |
|----------|
|   BL2    |
|----------|
|          |
|----------|
|   BL31   |
|----------|
|          |
|----------|
| BL1 (rw) |
+----------+ 0x04000000

Trusted ROM
+----------+ 0x04000000
| BL1 (ro) |
+----------+ 0x00000000

Each bootloader stage image layout is described by its own linker script. The linker scripts export some symbols into the program symbol table. Their values correspond to particular addresses. The trusted firwmare code can refer to these symbols to figure out the image memory layout.

Linker symbols follow the following naming convention in the trusted firmware.

  • __<SECTION>_START__

    Start address of a given section named <SECTION>.

  • __<SECTION>_END__

    End address of a given section named <SECTION>. If there is an alignment constraint on the section's end address then __<SECTION>_END__ corresponds to the end address of the section's actual contents, rounded up to the right boundary. Refer to the value of __<SECTION>_UNALIGNED_END__ to know the actual end address of the section's contents.

  • __<SECTION>_UNALIGNED_END__

    End address of a given section named <SECTION> without any padding or rounding up due to some alignment constraint.

  • __<SECTION>_SIZE__

    Size (in bytes) of a given section named <SECTION>. If there is an alignment constraint on the section's end address then __<SECTION>_SIZE__ corresponds to the size of the section's actual contents, rounded up to the right boundary. In other words, __<SECTION>_SIZE__ = __<SECTION>_END__ - _<SECTION>_START__. Refer to the value of __<SECTION>_UNALIGNED_SIZE__ to know the actual size of the section's contents.

  • __<SECTION>_UNALIGNED_SIZE__

    Size (in bytes) of a given section named <SECTION> without any padding or rounding up due to some alignment constraint. In other words, __<SECTION>_UNALIGNED_SIZE__ = __<SECTION>_UNALIGNED_END__ - __<SECTION>_START__.

Some of the linker symbols are mandatory as the trusted firmware code relies on them to be defined. They are listed in the following subsections. Some of them must be provided for each bootloader stage and some are specific to a given bootloader stage.

The linker scripts define some extra, optional symbols. They are not actually used by any code but they help in undertanding the bootloader images' memory layout as they are easy to spot in the link map files.

Common linker symbols

Early setup code needs to know the extents of the BSS section to zero-initialise it before executing any C code. The following linker symbols are defined for this purpose:

  • __BSS_START__ This address must be aligned on a 16-byte boundary.
  • __BSS_SIZE__

Similarly, the coherent memory section must be zero-initialised. Also, the MMU setup code needs to know the extents of this section to set the right memory attributes for it. The following linker symbols are defined for this purpose:

  • __COHERENT_RAM_START__ This address must be aligned on a page-size boundary.
  • __COHERENT_RAM_END__ This address must be aligned on a page-size boundary.
  • __COHERENT_RAM_UNALIGNED_SIZE__

BL1's linker symbols

BL1's early setup code needs to know the extents of the .data section to relocate it from ROM to RAM before executing any C code. The following linker symbols are defined for this purpose:

  • __DATA_ROM_START__ This address must be aligned on a 16-byte boundary.
  • __DATA_RAM_START__ This address must be aligned on a 16-byte boundary.
  • __DATA_SIZE__

BL1's platform setup code needs to know the extents of its read-write data region to figure out its memory layout. The following linker symbols are defined for this purpose:

  • __BL1_RAM_START__ This is the start address of BL1 RW data.
  • __BL1_RAM_END__ This is the end address of BL1 RW data.

BL2's and BL3-1's linker symbols

Both BL2 and BL3-1 need to know the extents of their read-only section to set the right memory attributes for this memory region in their MMU setup code. The following linker symbols are defined for this purpose:

  • __RO_START__
  • __RO_END__

How to choose the right base address for each bootloader stage image

The current implementation of the image loader has some limitations. It is designed to load images dynamically, at a load address chosen to minimize memory fragmentation. The chosen image location can be either at the top or the bottom of free memory. However, until this feature is fully functional, the code also contains support for loading images at a link-time fixed address.

BL1 is always loaded at address 0x0. BL2 and BL3-1 are loaded at specified locations in Trusted SRAM. The lack of dynamic image loader support means these load addresses must currently be adjusted as the code grows. The individual images must be linked against their ultimate runtime locations.

BL2 is loaded near the top of the Trusted SRAM. BL3-1 is loaded between BL1 and BL2. All three images are resident concurrently in Trusted RAM during boot so overlaps are not permitted.

The image end addresses can be determined from the link map files of the different images. These are the build/<platform>/<build-type>/bl<x>/bl<x>.map files, with <x> the stage bootloader.

  • bl1.map link map file provides __BL1_RAM_END__ address.
  • bl2.map link map file provides __BL2_END__ address.
  • bl31.map link map file provides __BL31_END__ address.

To prevent images from overlapping each other, the following constraints must be enforced:

  1. __BL1_RAM_END__ <= BL31_BASE
  2. __BL31_END__ <= BL2_BASE
  3. __BL2_END__ <= (<Top of Trusted SRAM>)

This is illustrated by the following memory layout diagram:

+----------+ 0x04040000
|          |
|----------| __BL2_END__
|   BL2    |
|----------| BL2_BASE
|          |
|----------| __BL31_END__
|   BL31   |
|----------| BL31_BASE
|          |
|----------| __BL1_RAM_END__
| BL1 (rw) |
+----------+ 0x04000000

Overlaps are detected during image linking as follows.

Constraint 1 is enforced by BL1's linker script. If it is violated then the linker will report an error while building BL1 to indicate that it doesn't fit:

aarch64-none-elf-ld: BL31 image overlaps BL1 image.

This error means that the BL3-1 base address needs to be incremented. Ensure that the new memory layout still obeys all constraints.

Constraint 2 is enforced by BL3-1's linker script. If it is violated then the linker will report an error while building BL3-1 to indicate that it doesn't fit:

aarch64-none-elf-ld: BL31 image overlaps BL2 image.

This error can either mean that the BL3-1 base address needs to be decremented or that BL2 base address needs to be incremented. Ensure that the new memory layout still obeys all constraints.

Constraint 3 is enforced by BL2's linker script. If it is violated then the linker will report an error while building BL2 to indicate that it doesn't fit. For example:

aarch64-none-elf-ld: address 0x40400c8 of bl2.elf section `.bss' is not
within region `RAM'

This error means that the BL2 base address needs to be decremented. Ensure that the new memory layout still obeys all constraints.

Since constraint checks are scattered across linker scripts, it is required to make clean prior to building to ensure that all possible overlapping scenarios are checked.

The current implementation of the image loader can result in wasted space because of the simplified data structure used to represent the extents of free memory. For example, to load BL2 at address 0x0402D000, the resulting memory layout should be as follows:

------------ 0x04040000
|          |  <- Free space (1)
|----------|
|   BL2    |
|----------| BL2_BASE (0x0402D000)
|          |  <- Free space (2)
|----------|
|   BL1    |
------------ 0x04000000

In the current implementation, we need to specify whether BL2 is loaded at the top or bottom of the free memory. BL2 is top-loaded so in the example above, the free space (1) above BL2 is hidden, resulting in the following view of memory:

------------ 0x04040000
|          |
|          |
|   BL2    |
|----------| BL2_BASE (0x0402D000)
|          |  <- Free space (2)
|----------|
|   BL1    |
------------ 0x04000000

BL3-1 is bottom-loaded above BL1. For example, if BL3-1 is bottom-loaded at 0x0400E000, the memory layout should look like this:

------------ 0x04040000
|          |
|          |
|   BL2    |
|----------| BL2_BASE (0x0402D000)
|          |  <- Free space (2)
|          |
|----------|
|          |
|   BL31   |
|----------|  BL31_BASE (0x0400E000)
|          |  <- Free space (3)
|----------|
|   BL1    |
------------ 0x04000000

But the free space (3) between BL1 and BL3-1 is wasted, resulting in the following view:

------------ 0x04040000
|          |
|          |
|   BL2    |
|----------| BL2_BASE (0x0402D000)
|          |  <- Free space (2)
|          |
|----------|
|          |
|          |
|   BL31   | BL31_BASE (0x0400E000)
|          |
|----------|
|   BL1    |
------------ 0x04000000

Code Structure

Trusted Firmware code is logically divided between the three boot loader stages mentioned in the previous sections. The code is also divided into the following categories (present as directories in the source code):

  • Architecture specific. This could be AArch32 or AArch64.
  • Platform specific. Choice of architecture specific code depends upon the platform.
  • Common code. This is platform and architecture agnostic code.
  • Library code. This code comprises of functionality commonly used by all other code.
  • Stage specific. Code specific to a boot stage.
  • Drivers.

Each boot loader stage uses code from one or more of the above mentioned categories. Based upon the above, the code layout looks like this:

Directory    Used by BL1?    Used by BL2?    Used by BL3?
bl1          Yes             No              No
bl2          No              Yes             No
bl31         No              No              Yes
arch         Yes             Yes             Yes
plat         Yes             Yes             Yes
drivers      Yes             No              Yes
common       Yes             Yes             Yes
lib          Yes             Yes             Yes

All assembler files have the .S extension. The linker source files for each boot stage have the extension .ld.S. These are processed by GCC to create the linker scripts which have the extension .ld.

FDTs provide a description of the hardware platform and are used by the Linux kernel at boot time. These can be found in the fdts directory.

4. References

  1. Trusted Board Boot Requirements CLIENT PDD (ARM DEN 0006B-5). Available under NDA through your ARM account representative.

  2. Power State Coordination Interface PDD (ARM DEN 0022B.b).

  3. SMC Calling Convention PDD (ARM DEN 0028A).

Copyright (c) 2013-2014, ARM Limited and Contributors. All rights reserved.