Device Tree Flash Partitions

Device tree can be used to describe a partition layout for any flash device in the system.

Two important uses for this mechanism are:

  1. To force the Zephyr image to be linked into a specific area on Flash.

    This is useful, for example, if the Zephyr image must be linked at some offset from the flash device’s start, to be loaded by a bootloader at runtime.

  2. To generate compile-time definitions for the partition layout, which can be shared by Zephyr subsystems and applications to operate on specific areas in flash.

    This is useful, for example, to create areas for storing file systems or other persistent state. These defines only describe the boundaries of each partition. They don’t, for example, initialize a partition’s flash contents with a file system.

Partitions are generally managed using device tree overlays. Refer to Device Tree Overlays for details on using overlay files.

Defining Partitions

The partition layout for a flash device is described inside the partitions child node of the flash device’s node in the device tree.

You can define partitions for any flash device on the system.

Most Zephyr-supported SoCs with flash support in device tree will define a label flash0. This label refers to the primary on-die flash programmed to run Zephyr. To generate partitions for this device, add the following snippet to a device tree overlay file:

&flash0 {
        partitions {
                compatible = "fixed-partitions";
                #address-cells = <1>;
                #size-cells = <1>;

                /* Define your partitions here; see below */
        };
};

To define partitions for another flash device, modify the above to either use its label or provide a complete path to the flash device node in the device tree.

The content of the partitions node looks like this:

partitions {
        compatible = "fixed-partitions";
        #address-cells = <1>;
        #size-cells = <1>;

        partition1_label: partition@START_OFFSET_1 {
                label = "partition1_name";
                reg = <0xSTART_OFFSET_1 0xSIZE_1>;
        };

        /* ... */

        partitionN_label: partition@START_OFFSET_N {
                label = "partitionN_name";
                reg = <0xSTART_OFFSET_N 0xSIZE_N>;
        };
};

Where:

  • partitionX_label are device tree labels that can be used elsewhere in the device tree to refer to the partition
  • partitionX_name controls how defines generated by the Zephyr build system for this partition will be named
  • START_OFFSET_x is the start offset in hexadecimal notation of the partition from the beginning of the flash device
  • SIZE_x is the hexadecimal size, in bytes, of the flash partition

The partitions do not have to cover the entire flash device. The device tree compiler currently does not check if partitions overlap; you must ensure they do not when defining them.

Example Primary Flash Partition Layout

Here is a complete (but hypothetical) example device tree overlay snippet illustrating these ideas. Notice how the partitions do not overlap, but also do not cover the entire device.

&flash0 {
        partitions {
                compatible = "fixed-partitions";
                #address-cells = <1>;
                #size-cells = <1>;

                code_dts_label: partition@8000 {
                        label = "zephyr-code";
                        reg = <0x00008000 0x34000>;
                };

                data_dts_label: partition@70000 {
                        label = "application-data";
                        reg = <0x00070000 0xD000>;
                };
        };
};

Linking Zephyr Within a Partition

To force the linker to output a Zephyr image within a given flash partition, add this to a device tree overlay:

/ {
        chosen {
                zephyr,code-partition = &slot0_partition;
        };
};

If the chosen node has no zephyr,code-partition property, the application image link uses the entire flash device. If a zephyr,code-partition property is defined, the application link will be restricted to that partition.

Flash Partition Macros

The Zephyr build system generates definitions for each flash device partition. These definitions are available to any files which include <zephyr.h>.

Consider this flash partition:

dts_label: partition@START_OFFSET {
        label = "def-name";
        reg = <0xSTART_OFFSET 0xSIZE>;
};

The build system will generate the following corresponding defines:

#define FLASH_AREA_DEF_NAME_LABEL        "def-name"
#define FLASH_AREA_DEF_NAME_OFFSET_0     0xSTART_OFFSET
#define FLASH_AREA_DEF_NAME_SIZE_0       0xSIZE
#define FLASH_AREA_DEF_NAME_OFFSET       FLASH_AREA_MCUBOOT_OFFSET_0
#define FLASH_AREA_DEF_NAME_SIZE         FLASH_AREA_MCUBOOT_SIZE_0

As you can see, the label property is capitalized when forming the macro names. Other simple conversions to ensure it is a valid C identifier, such as converting “-” to “_”, are also performed. The offsets and sizes are available as well.

MCUboot Partitions

MCUboot is a secure bootloader for 32-bit microcontrollers.

Some Zephyr boards provide definitions for the flash partitions which are required to build MCUboot itself, as well as any applications which must be chain-loaded by MCUboot.

The device tree labels for these partitions are:

boot_partition
This is the partition where the bootloader is expected to be placed. MCUboot’s build system will attempt to link the MCUboot image into this partition.
slot0_partition
MCUboot loads the executable application image from this partition. Any application bootable by MCUboot must be linked to run from this partition.
slot1_partition
This is the partition which stores firmware upgrade images. Zephyr applications which receive firmware updates must ensure the upgrade images are placed in this partition (the Zephyr DFU subsystem can be used for this purpose). MCUboot checks for upgrade images in this partition, and can move them to slot0_partition for execution. The slot0_partition and slot1_partition must be the same size.
scratch_partition
This partition is used as temporary storage while swapping the contents of slot0_partition and slot1_partition.

Important

Upgrade images are only temporarily stored in slot1_partition. They must be linked to execute of out of slot0_partition.

See the MCUboot documentation for more details on these partitions.

File System Partitions

storage_partition
This is the area where e.g. NFFS expects its partition.