FIT Images in Embedded Linux

While continuing my exploration on the LuckFox Mini A Board, I stumbled upon something interesting — the board uses a FIT Image for its kernel.

At first glance, it looked like a single binary file. But as I dug deeper, I realized — it’s not just a kernel image. It’s a single container combining multiple images together — the kernel, device tree, ramdisk, and sometimes even configuration data — all neatly packaged as one.

Let’s understand what this means, why it matters, and how you can create one yourself.

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⚙️ What Is a FIT Image?

FIT (Flattened Image Tree) is an image format introduced by U-Boot, the popular open-source bootloader used in Embedded Linux systems.

It’s designed to bundle multiple boot components — like the kernel, device tree blobs (DTBs), and root filesystem (initramfs) — into one single structured file.

You can think of a FIT Image as a well-organized ZIP file for your embedded boot process — except that it’s parsed and verified by U-Boot at runtime.

A FIT image typically has:

• One kernel image (zImage or Image)
• One or more device tree blobs (DTBs)
• Optional ramdisk/initramfs image
• Optional configurations for different boards or revisions

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🧩 Why FIT Images? — Advantages Over the Traditional Approach

In the traditional boot process, U-Boot loads the kernel, device tree, and optionally initramfs separately. This works fine — but it comes with limitations, especially when you start scaling.

🔹 1. Simplified Boot Process

Instead of managing multiple files and load addresses, the bootloader just loads one unified image — reducing complexity and possible errors.

🔹 2. Version Integrity

FIT supports cryptographic verification using SHA-1/SHA-256 and RSA signatures, ensuring that what’s loaded is exactly what was intended. This is crucial for production-grade systems.

🔹 3. Multiple Configurations in One Image

You can store multiple kernel + DTB pairs (for different hardware variants or revisions) inside the same FIT.
At boot, U-Boot chooses the right configuration dynamically — based on environment variables or board detection.

🔹 4. Easier Updates

When performing OTA updates or SD card flashing, you just replace one FIT file instead of three or four different binaries — making system maintenance much simpler.

🔹 5. Portability & Clean Integration

FIT aligns perfectly with the Device Tree philosophy — separating hardware details from core kernel logic while keeping things portable.

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🧠 The Internal Structure — It’s All About the Device Tree

Internally, the FIT image is described using a Device Tree–like syntax (also known as .its — Image Tree Source).

Here’s a simplified example:

/dts-v1/;

/ {
    description = "My FIT Image";
    images {
        kernel {
            description = "Linux Kernel";
            data = /incbin/("zImage");
            type = "kernel";
            arch = "arm";
            os = "linux";
            compression = "none";
            load = <0x8000>;
            entry = <0x8000>;
        };

        fdt {
            description = "Device Tree Blob";
            data = /incbin/("myboard.dtb");
            type = "flat_dt";
            arch = "arm";
            compression = "none";
        };
    };

    configurations {
        default = "config1";
        config1 {
            kernel = "kernel";
            fdt = "fdt";
        };
    };
};

🛠️ How to Create a FIT Image

You can generate a FIT image using mkimage, a U-Boot utility tool.

Here’s the typical flow:

# Step 1: Create your ITS description file (fit.its)
# Step 2: Generate the FIT image
mkimage -f fit.its fitImage

This will create a fitImage — your single packaged binary ready to be flashed or booted by U-Boot.

Then, in U-Boot:

setenv bootfile fitImage
load mmc 0:1 ${loadaddr} ${bootfile}
bootm ${loadaddr}

And you’re done — U-Boot parses the image, picks the right configuration, and boots Linux seamlessly.

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⚡ FIT Image vs. Legacy Image — Quick Comparison

Feature	Legacy uImage	FIT Image
Single file support	No	Yes
Multiple configurations	No	Yes
Signature verification	No	Yes
DTB inclusion	External	Embedded
Boot simplicity	Manual setup	Auto selection
Update management	Multi-file	Single file

🧩 Where You’ll Encounter FIT Images

You’ll often find FIT images in:

• Modern development boards (like LuckFox Mini, BeaglePlay, and i.MX8 platforms)
• Yocto-based builds where FIT is integrated into kernel-fitimage.bbclass
• Production-grade embedded products that require secure or OTA updates

FIT images are becoming the default standard for modern boot setups — for their modularity, security, and convenience.

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🎓 Want to Go Beyond FIT Images?

If you’re fascinated by how FIT ties into the broader Embedded Linux boot flow — from bootloaders to kernel and root filesystem — then you’ll love the Embedded Linux Porting Hacks Course.

This hands-on program helps you build a solid foundation in Embedded Linux by covering:
✅ Booting up the board
✅ Recovering a bricked system
✅ Understanding U-Boot bootloader internals
✅ Configuring & building Linux images
✅ Using build systems like Buildroot & Yocto

Every concept comes with practical exercises to help you truly get how Linux boots and runs on embedded platforms.

👉 Explore the course here: https://embitude.in/elph/

💌 Stay Connected with Our Embedded Linux Community

We regularly share practical insights, blog updates, and hands-on challenges for embedded engineers who love getting closer to the kernel.

Join our growing community here 👇
🔗 https://embitudeinfotech.graphy.com/s/community

🧭 Wrapping Up

FIT images represent the evolution of embedded boot management — moving from scattered binaries to a structured, secure, and scalable approach.

Whether you’re exploring boards like the LuckFox Mini A or building your own embedded product, understanding FIT is a must-have skill for any Linux engineer.

In upcoming blogs, we’ll continue exploring more such porting hacks and kernel concepts that form the foundation of a robust embedded system. Stay tuned!