5.9 KiB
Compiling wolfBoot
WolfBoot is portable across different types of embedded systems. The platform-specific code
is contained in a single file under the hal directory, and implements the hardware-specific functions.
To enable specific compile options, use environment variables while calling make, e.g.
make CORTEX_M0=1
Platform selection
If supported natively, the target platform can be specified using the TARGET variable.
Make will automatically select the correct compile option, and include the corresponding HAL for
the selected target.
For a list of the platforms currently supported, see the HAL documentation.
To add a new platform, simply create the corresponding HAL driver and linker script file in the hal directory.
Default option if none specified: TARGET=stm32f4
Some platform will require extra options, specific for the architecture. By default, wolfBoot is compiled for ARM Cortex-M3/4/7. To compile for Cortex-M0, use:
CORTEX_M0=1
Flash partitions
The file include/target.h must be modified according to flash geometry, partitions size and offset of the target system. The following values must be set to provide the desired flash configuration:
WOLFBOOT_SECTOR_SIZE
This variable determines the size of the physical sector on the flash memory. If areas with different block sizes are used for the two partitions (e.g. update partition on an external flash), this variable should indicate the size of the biggest sector shared between the two partitions.
WolfBoot uses this value as minimum unit when swapping the firmware images in place. For this reason, this value is also used to set the size of the SWAP partition.
WOLFBOOT_PARTITION_BOOT_ADDRESS
This is the start address of the boot partition, aligned to the beginning of a new flash sector. The application code starts after a further offset, equal to the partition header size (256B for Ed25519 and ECC signature headers).
WOLFBOOT_PARTITION_UPDATE_ADDRESS
This is the start address of the update partition. If an external memory is used via the
EXT_FLASH option, this variable contains the offset of the update partition from the
beginning of the external memory addressable space.
WOLFBOOT_PARTITION_SWAP_ADDRESS
The address for the swap spaced used by wolfBoot to swap the two firmware images in place, in order to perform a reversable update. The size of the SWAP partition is exactly one sector on the flash. If an external memory is used, the variable contains the offset of the SWAP area from the beginning of its addressable space.
WOLFBOOT_PARTITION_SIZE
The size of the BOOT and UPDATE partition. The size is the same for both partitions.
Bootloader features
A number of characteristics can be turned on/off during wolfBoot compilation. Bootloader size, performance and activated features are affected by compile-time flags.
Change DSA algorithm
By default, wolfBoot is compiled to use Ed25519 DSA. The implementation of ed25519 is smaller, while giving a good compromise in terms of boot-up time.
Better performance can be achieved using ECDSA with curve p-256. To activate ECC256 support, use
SIGN=ECC256
when invoking make.
The default option, if no value is provided for the SIGN variable, is
SIGN=ED25519
Changing the DSA algorithm will also result in compiling a different set of tools for key generation and firmware signature.
Find the corresponding key generation and firmware signing tools in the tools directory.
Enable debug symbols
To debug the bootloader, simply compile with DEBUG=1. The size of the bootloade will increase
consistently, so ensure that you have enough space at the beginning of the flash before
WOLFBOOT_PARTITION_BOOT_ADDRESS.
Disable interrupt vector relocation
On some platforms, it might be convenient to avoid the interrupt vector relocation before boot-up. This is required when a component on the system already manages the interrupt relocation at a different stage, or on these platform that do not support interrupt vector relocation.
To disable interrupt vector table relocation, compile with VTOR=0. By default, wolfBoot will relocate the
interrupt vector by setting the offset in the vector relocation offset register (VTOR).
Enable workaround for 'write once' flash memories
On some microcontrollers, the internal flash memory does not allow subsequent writes (adding zeroes) to a sector, after the entire sector has been erased. WolfBoot relies on the mechanism of adding zeroes to the 'flags' fields at the end of both partitions to provide a fail-safe swap mechanism.
To enable the workaround for 'write once' internal flash, compile with
NVM_FLASH_WRITEONCE=1
warning When this option is enabled, the fail-safe swap is not guaranteed, i.e. the microcontroller cannot be safely powered down or restarted during a swap operation.
Allow version roll-back
WolfBoot will not allow updates to a firmware with a version number smaller than the current one. To allow
downgrades, compile with ALLOW_DOWNGRADE=1.
Warning: this option will disable version checking before the updates, thus exposing the system to potential forced downgrade attacks.
Enable optional support for external flash memory
WolfBoot can be compiled with the makefile option EXT_FLASH=1. When the external flash support is
enabled, update and swap partitions can be associated to an external memory, and will use alternative
HAL function for read/write/erase access.
To associate the update or the swap partition to an external memory, define PART_UPDATE_EXT and/or
PART_SWAP_EXT, respectively. By default, the makefile assumes that if an external memory is present,
both PART_UPDATE_EXT and PART_SWAP_EXT are defined.
When external memory is used, the HAL API must be extended to define methods to access the custom memory.
Refer to the HAL page for the description of the ext_flash_* API.