Flattened uImage Tree documentation

uImage.FIT is the new format used for uImage payloads developed by U-boot.

Supported architectures

  • aarch64

Supported FIT sections

The FIT can contain multiple sections, each holding a unique kernel, initrd or config. Out of the sections one kernel and one initrd is chosen, depending on the matching config.

The config is selected depending on the compat string.

The section must be named in order to be found by the FIT parser:

  • kernel
  • fdt
  • ramdisk

Architecture specifics

The FIT parser needs architecure support.

aarch64

The source code can be found in src/arch/arm64/fit.c.

On aarch64 the kernel (a section named ‘kernel’) must be in Image format and it needs a devicetree (a section named ‘fdt’) to boot. The kernel will be placed close to “DRAMSTART”.

Other

Other architectures aren’t supported.

Supported compression

The FIT image has to be included uncompressed into the CBFS. The sections inside the FIT image can use different compression schemes.

Supported compression algorithms:

  • LZMA
  • LZ4
  • none

Compat string

The config entries contain a compatible string, that is used to find a matching config.

The following mainboard specific funtions provide the BOARDID and SKUID:

uint32_t board_id(void);
uint32_t sku_id(void);

By default the following compat strings are added:

  • CONFIG_MAINBOARD_VENDOR,CONFIG_MAINBOARD_PART_NUMBER
  • CONFIG_MAINBOARD_VENDOR,CONFIG_MAINBOARD_PART_NUMBER-revBOARDID
  • CONFIG_MAINBOARD_VENDOR,CONFIG_MAINBOARD_PART_NUMBER-revBOARDID-skuSKUID

Example:

cavium,cn8100_sff_evb

If board_id() or sku_id() return invalid, the single comapt string isn’t added.

You can add custom compat strings by calling:

fit_add_compat_string(const char *str);

If no matching compat string is found, the default config is chosen.

Building FIT image

The FIT image has to be built by calling mkimage. You can use the following example configuration:

/*
 * Simple U-Boot uImage source file containing a single kernel and FDT blob
 */

/dts-v1/;

/ {
    description = "Simple image with single Linux kernel and FDT blob";
    #address-cells = <1>;

    images {
        kernel {
            description = "Vanilla Linux kernel";
            data = /incbin/("Image.lzma");
            type = "kernel";
            arch = "arm64";
            os = "linux";
            compression = "lzma";
            load = <0x80000>;
            entry = <0x80000>;
            hash-1 {
                algo = "crc32";
            };
        };
        fdt-1 {
            description = "Flattened Device Tree blob";
            data = /incbin/("target.dtb");
            type = "flat_dt";
            arch = "arm64";
            compression = "none";
            hash-1 {
                algo = "crc32";
            };
        };
        ramdisk-1 {
            description = "Compressed Initramfs";
            data = /incbin/("initramfs.cpio.xz");
            type = "ramdisk";
            arch = "arm64";
            os = "linux";
            compression = "none";
            load = <00000000>;
            entry = <00000000>;
            hash-1 {
                algo = "sha1";
            };
        };
    };

    configurations {
        default = "conf-1";
        conf-1 {
            description = "Boot Linux kernel with FDT blob";
            kernel = "kernel";
            fdt = "fdt-1";
            ramdisk = "ramdisk-1";
        };
    };
};

Save it as ITS file config.its along with the other files defined here:

  • target.dtb
  • initramfs.cpio.xz
  • Image.lzma

Generate the uImage that will be included into the CBFS by calling

mkimage -f config.its uImage

The generated file includes a compressed initrd initramfs.cpio.xz, which will be decompressed by the Linux kernel, a compressed kernel Image.lzma, which will be decompressed by the FIT loader and an uncompressed devicetree blob.