Porting How-To

This document describes the requirements and necessary steps required to port mcuboot to a new target OS.

Requirements

  • mcuboot requires a configuration file, which can be included as mcuboot_config/mcuboot_config.h, which configures various options (that begin with MCUBOOT_).
  • mcuboot requires that the target provides a flash API with ability to get the flash’s minimum write size, and read/write/erase individual sectors.
  • mcuboot doesn’t bundle a cryptographic library, which means the target OS must already have it bundled. The supported libraries at the moment are either mbed TLS or the set tinycrypt + mbed TLS (where mbed TLS is used to provide functionality not existing in tinycrypt).

Steps to port

Main app and calling the bootloader

From the perspective of the target OS, the bootloader can be seen as a library, so an entry point must be provided. This is likely a typical app for the target OS, and it must call the following function to run the bootloader:

int boot_go(struct boot_rsp *rsp);

This function is located at boot/bootutil/loader.c and receives a struct boot_rsp pointer. The struct boot_rsp is defined as:

struct boot_rsp {
    /** A pointer to the header of the image to be executed. */
    const struct image_header *br_hdr;

    /**
     * The flash offset of the image to execute.  Indicates the position of
     * the image header.
     */
    uint8_t br_flash_id;
    uint32_t br_image_addr;
};

After running the management functions of the bootloader, boot_go returns an initialized boot_rsp which has pointers to the location of the image where the target firmware is located which can be used to jump to.

Configuration file

You must provide a file, mcuboot_config/mcuboot_config.h. This is included by several files in the “library” portion of MCUboot; it provides preprocessor definitions that configure the library’s build.

See the file samples/mcuboot_config/mcuboot_config.template.h for a starting point and more information. This is a good place to convert settings in your environment’s configuration system to those required by MCUboot. For example, Mynewt uses MYNEWT_VAL() and Zephyr uses Kconfig; these configuration systems are converted to MCUBOOT_ options in the following files:

  • boot/zephyr/include/mcuboot_config/mcuboot_config.h
  • boot/mynewt/mcuboot_config/include/mcuboot_config/mcuboot_config.h

Flash Map

The bootloader requires a flash_map to be able to know how the flash is partitioned. A flash_map consists of struct flash_area entries specifying the partitions, where a flash_area defined as follows:

struct flash_area {
    uint8_t  fa_id;         /** The slot/scratch identification */
    uint8_t  fa_device_id;  /** The device id (usually there's only one) */
    uint16_t pad16;
    uint32_t fa_off;        /** The flash offset from the beginning */
    uint32_t fa_size;       /** The size of this sector */
};

fa_id is can be one of the following options:

#define FLASH_AREA_IMAGE_0 1
#define FLASH_AREA_IMAGE_1 2
#define FLASH_AREA_IMAGE_SCRATCH 3

The functions that must be defined for working with the flash_areas are:

/*< Opens the area for use. id is one of the `fa_id`s */
int     flash_area_open(uint8_t id, const struct flash_area **);
void    flash_area_close(const struct flash_area *);
/*< Reads `len` bytes of flash memory at `off` to the buffer at `dst` */
int     flash_area_read(const struct flash_area *, uint32_t off, void *dst,
                     uint32_t len);
/*< Writes `len` bytes of flash memory at `off` from the buffer at `src` */
int     flash_area_write(const struct flash_area *, uint32_t off,
                     const void *src, uint32_t len);
/*< Erases `len` bytes of flash memory at `off` */
int     flash_area_erase(const struct flash_area *, uint32_t off, uint32_t len);
/*< Returns this `flash_area`s alignment */
uint8_t flash_area_align(const struct flash_area *);
/*< Initializes an array of flash_area elements for the slot's sectors */
int     flash_area_to_sectors(int idx, int *cnt, struct flash_area *ret);
/*< Returns the `fa_id` for slot, where slot is 0 or 1 */
int     flash_area_id_from_image_slot(int slot);
/*< Returns the slot, for the `fa_id` supplied */
int     flash_area_id_to_image_slot(int area_id);

Memory management for mbed TLS

mbed TLS employs dynamic allocation of memory, making use of the pair calloc/free. If mbed TLS is to be used for crypto, your target RTOS needs to provide this pair of function.

To configure the what functions are called when allocating/deallocating memory mbed TLS uses the following call [^cite1]:

int mbedtls_platform_set_calloc_free (void *(*calloc_func)(size_t, size_t),
                                      void (*free_func)(void *));

If your system already provides functions with compatible signatures, those can be used directly here, otherwise create new functions that glue to your calloc/free implementations.

[^cite1]https://tls.mbed.org/api/platform_8h.html