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Bootloader Information

The bootloader information (abbreviated to blinfo) subsystem is an extension of the Retention System which allows for reading shared data from a bootloader and allowing applications to query it. It has an optional feature of organising the information retrieved from the bootloader and storing it in the Settings with the blinfo/ prefix.

Devicetree setup

To use the bootloader information subsystem, a retention area needs to be created which has a retained data section as its parent, generally non-init RAM is used for this purpose. See the following example (examples in this guide are based on the nRF52840 DK board and memory layout):

/ {
        sram@2003F000 {
                compatible = "zephyr,memory-region", "mmio-sram";
                reg = <0x2003F000 DT_SIZE_K(1)>;
                zephyr,memory-region = "RetainedMem";
                status = "okay";

                retainedmem {
                        compatible = "zephyr,retained-ram";
                        status = "okay";
                        #address-cells = <1>;
                        #size-cells = <1>;

                        boot_info0: boot_info@0 {
                                compatible = "zephyr,retention";
                                status = "okay";
                                reg = <0x0 0x100>;

        chosen {
                zephyr,bootloader-info = &boot_info0;

/* Reduce SRAM0 usage by 1KB to account for non-init area */
&sram0 {
        reg = <0x20000000 DT_SIZE_K(255)>;

Note that this configuration needs to be applied on both the bootloader (MCUboot) and application to be usable. It can be combined with other retention system APIs such as the Boot mode

MCUboot setup

Once the above devicetree configuration is applied, MCUboot needs to be configured to store the shared data in this area, the following Kconfigs need to be set for this:

  • CONFIG_RETAINED_MEM - Enables retained memory driver

  • CONFIG_RETENTION - Enables retention system

  • CONFIG_BOOT_SHARE_DATA - Enables shared data

  • CONFIG_BOOT_SHARE_DATA_BOOTINFO - Enables boot information shared data type

  • CONFIG_BOOT_SHARE_BACKEND_RETENTION - Stores shared data using retention/blinfo subsystem

Application setup

The application must enable the following base Kconfig options for the bootloader information subsystem to function:

The following include is needed to use the bootloader information subsystem:

#include <zephyr/retention/blinfo.h>

By default, only the lookup function is provided: blinfo_lookup(), the application can call this to query the information from the bootloader. This function is enabled by default with CONFIG_RETENTION_BOOTLOADER_INFO_OUTPUT_FUNCTION, however, applications can optionally choose to use the settings storage feature instead. In this mode, the bootloader information can be queries by using settings keys, the following Kconfig options need to be enabled for this mode:

This allows the information to be queried via the settings_runtime_get() function with the following keys:

  • blinfo/mode The mode that MCUboot is configured for (enum mcuboot_mode value)

  • blinfo/signature_type The signature type MCUboot is configured for (enum mcuboot_signature_type value)

  • blinfo/recovery The recovery type enabled in MCUboot (enum mcuboot_recovery_mode value)

  • blinfo/running_slot The running slot, useful for direct-XIP mode to know which slot to use for an update

  • blinfo/bootloader_version Version of the bootloader (struct image_version object)

  • blinfo/max_application_size Maximum size of an application (in bytes) that can be loaded

In addition to the previous include, the following includes are required for this mode:

#include <bootutil/boot_status.h>
#include <bootutil/image.h>
#include <mcuboot_version.h>
#include <zephyr/settings/settings.h>

API Reference

Bootloader information API

group bootloader_info_interface

Bootloader info interface.


int blinfo_lookup(uint16_t key, char *val, int val_len_max)

Returns bootinfo information.

  • key – The information to return (for MCUboot: minor TLV).

  • val – Where the return information will be placed.

  • val_len_max – The maximum size of the provided buffer.

Return values:
  • 0 – If successful.

  • -EOVERFLOW – If the data is too large to fit the supplied buffer.

  • -EIO – If the requested key was not found.

  • -errno – Error code.