What is U-Boot?

Das U-Boot (Universal Bootloader) is the most widely deployed open-source bootloader for embedded systems. Originally developed by Wolfgang Denk at DENX Software Engineering, it was initially called PPCBoot (for PowerPC targets), then MBXBoot, and eventually renamed U-Boot as it expanded support to dozens of architectures. The project is hosted at https://source.denx.de/u-boot/u-boot and mirrored on GitHub at https://github.com/u-boot/u-boot.

U-Boot sits between the hardware initialization firmware (ROM code, TF-A, etc.) and the operating system kernel. Its primary job is to:

1. Initialize hardware (DDR, clocks, PLLs, storage interfaces, etc.)
2. Locate a kernel image (from flash, eMMC, SD card, network, USB, etc.)
3. Set up the boot environment (device tree, kernel command line, etc.)
4. Transfer control to the kernel

The 2026.01 release (tag v2026.01) is the January 2026 quarterly stable release and includes improvements in RISC-V support, EFI capsule updates, driver model maturity, and enhanced security features including verified boot improvements.

---

Historical Timeline

Year	Milestone
1999	PPCBoot 0.0.1 released by Wolfgang Denk for PowerPC
2000	MBXBoot created for i.MX
2002	Project renamed to U-Boot (Universal Bootloader)
2008	Kconfig-like configuration introduced
2014	Driver Model (DM) introduced
2015	Verified Boot / FIT image signing
2016	UEFI support introduced
2018	ARM64 support matured
2020	RISC-V full support
2022	VPL (Verifying Program Loader) introduced
2024	EFI Capsule Update, DM full migration
2026	v2026.01 — latest stable release

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Where U-Boot Fits: The Boot Chain

A typical ARM64 SoC boot chain looks like:

+---------------------------+
|    ROM Boot Code (BL1)    |  ← On-chip ROM, hardwired
+---------------------------+
           |
           v
+---------------------------+
|  TF-A BL2 / SPL (BL2)   |  ← Loaded from storage by ROM
+---------------------------+
           |
           v
+---------------------------+
|   TF-A BL31 (EL3 runtime)|  ← Secure monitor
+---------------------------+
           |
           v
+---------------------------+
|   U-Boot (BL33 / BL2)    |  ← Non-secure world bootloader
+---------------------------+
           |
           v
+---------------------------+
|     Linux Kernel          |
+---------------------------+
           |
           v
+---------------------------+
|     Root Filesystem       |
+---------------------------+

On simpler MCU-class systems (like Cortex-M or RISC-V without TF-A):

ROM → SPL → U-Boot → Application/RTOS/Linux

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U-Boot vs Other Bootloaders

Bootloader	Target	License	Notes
U-Boot	Everything (MCU to server)	GPL-2.0+	Most feature-rich, huge hardware support
GRUB2	x86/ARM PC	GPL-3.0	Desktop/server focus
coreboot	x86	GPL-2.0	Open BIOS replacement
Barebox	Embedded Linux	GPL-2.0	U-Boot alternative, cleaner codebase
LILO	x86 only	BSD	Legacy, deprecated
EDK2 (UEFI)	x86/ARM	BSD-2	UEFI reference impl
Zephyr Bootloader	MCU	Apache-2.0	RTOS ecosystem

U-Boot is the dominant choice in:

• Automotive (AUTOSAR, ADAS platforms)
• Industrial IoT
• Networking equipment
• Consumer electronics (TV SoCs, set-top boxes)
• Single-board computers (Raspberry Pi, BeagleBone, etc.)
• Yocto/OpenEmbedded-built systems

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Key Features of U-Boot 2026.01

Core Features

• Multi-architecture support: ARM, ARM64 (AArch64), x86, x86_64, RISC-V (32/64), MIPS, PowerPC, m68k, sh, nios2, xtensa, arc, microblaze, openrisc
• Multi-storage support: NOR flash, NAND flash, eMMC, SD/MMC, USB Mass Storage, SATA, NVMe, SPI-NOR, SPI-NAND
• Network boot: TFTP, NFS, BOOTP, DHCP, PXE, HTTP
• Filesystems: FAT (12/16/32/exFAT), ext2/3/4, SquashFS, EROFS, BTRFS, JFFS2, UBIFS, EFI system partition
• Script engine: Hush shell, if/for/while, env variables
• Device Tree: Full FDT runtime manipulation
• Display: Framebuffer support, HDMI, MIPI DSI, logo splash screen
• USB: Host (EHCI/XHCI/OHCI), Gadget (DFU, fastboot, mass storage)
• DFU (Device Firmware Update): USB DFU 1.1 compliant
• Fastboot: Android fastboot protocol support

Security Features (2026.01)

• Verified Boot: FIT image signature verification using RSA/ECDSA
• UEFI Secure Boot: Microsoft-compatible UEFI secure boot
• UEFI Capsule Update: Signed firmware updates via UEFI capsule mechanism
• Measured Boot: TPM 1.2/2.0 measured boot with PCR extension
• OP-TEE integration: Secure enclave communication
• ARM TrustZone: Runtime separation of secure/non-secure world
• i.MX HAB/AHAB: NXP High Assurance Boot support
• Rockchip AVB: Android Verified Boot support

New in 2026.01

• Improved EFI capsule update with rollback protection
• Enhanced RISC-V SBI multi-hart support
• DM (Driver Model) migration fully complete — legacy board APIs removed
• New bootflow command with improved boot ordering
• Improved Sandbox testing coverage
• Better ACPI table generation for ARM64
• Rust bindings experimental preview
• SCMI (System Control and Management Interface) driver improvements

---

Licensing

U-Boot is licensed under the GNU General Public License v2.0 or later (GPL-2.0+).

Important licensing nuances:

• Files in include/linux/ are dual-licensed (GPL-2.0 + Linux exceptions)
• Some drivers may have additional BSD/MIT licensed components
• The GPL requires redistribution of source when modifying and distributing binaries
• The "system library exception" does NOT apply — linking your code with U-Boot statically makes it GPL
• Standalone programs (booted by U-Boot, not linked with it) are not subject to GPL viral effect

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Release Cycle

U-Boot follows a quarterly release cycle:

• Releases in January, April, July, October
• Tags follow vYYYY.MM format (e.g., v2026.01, v2025.10)
• Long-Term Support (LTS) releases are maintained for ~2 years
• Release candidates: v2026.01-rc1, v2026.01-rc2, etc.
• Patches are submitted to the mailing list: u-boot@lists.denx.de

---

Community and Resources

• Mailing list: u-boot@lists.denx.de
• Git repository: https://source.denx.de/u-boot/u-boot
• Patchwork: https://patchwork.ozlabs.org/project/uboot/
• Documentation: https://docs.u-boot.org
• IRC: #u-boot on Libera.Chat
• CI: https://source.denx.de/u-boot/u-boot/-/pipelines (GitLab CI)
• Custodians: Each architecture/subsystem has a designated custodian responsible for reviewing patches

Contributors: Vasu Grover

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