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U-Boot, UEFI, and GRUB can form a boot chain, but they are not three mandatory stages. U-Boot can provide UEFI services and launch GRUB as an EFI application; GRUB can then load an operating system or chain-load another bootloader. Alternatively, U-Boot can boot a Linux kernel directly, without GRUB or its UEFI subsystem. Which path works depends on the board and its firmware build.
What each component does
- U-Boot is bootloader software commonly used on embedded platforms. It can start a kernel with its own commands, or, if built with the relevant support, provide a UEFI environment.
- UEFI is a firmware interface and boot policy mechanism, not another name for GRUB. Its boot manager selects UEFI drivers and applications, including operating-system loaders, according to boot variables. The UEFI Specification 2.11 describes the boot manager as a firmware policy engine configured through global NVRAM variables.
- GRUB is a bootloader that can run as a UEFI application. Depending on the operating system, GRUB build, and platform, it can load an OS directly or pass control to another loader. The GNU GRUB Manual 2.14 describes direct loading, kexec from userspace, and chain-loading as its three boot methods.
Can U-Boot start GRUB as a UEFI application?
Yes, when the U-Boot build includes the required UEFI support and the board can supply the information GRUB and the operating system need. U-Boot’s UEFI documentation says its implementation can execute Linux and bootloaders such as GRUB, while noting that U-Boot does not aim to implement unrestricted, full UEFI functionality.
A documented U-Boot example loads a device tree and a GRUB EFI binary from storage, then calls bootefi with the GRUB image and device-tree addresses. The example’s files, partitions, and commands describe that environment; they are not a universal recipe for other boards.
Three valid boot paths
U-Boot provides UEFI and launches GRUB
- The board starts U-Boot.
- A U-Boot build with
CONFIG_EFI_LOADERand thebooteficommand enabled provides the UEFI services needed to run an EFI image. - U-Boot loads the required files, such as a device tree and GRUB’s EFI executable.
- U-Boot starts GRUB with
bootefi, or asks its UEFI boot manager to follow configured boot options withbootefi bootmgr. - GRUB loads a supported operating system directly or chain-loads another loader if needed.
For boot-manager selection, BootNext can specify an option for the next boot, while BootOrder gives the sequence the manager should try. See the U-Boot bootefi command documentation for command behavior and the UEFI boot-manager specification for the general policy model.
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U-Boot boots Linux directly
GRUB and U-Boot’s UEFI subsystem are optional in this path. U-Boot documents booti, bootm, and bootz as ways to start Linux without using its UEFI subsystem. The appropriate command and required kernel, initrd, and hardware-description files depend on the board and image format.
Platform UEFI firmware launches GRUB
U-Boot is not required if other platform firmware implements UEFI. In that arrangement, the firmware boot manager selects GRUB or another EFI loader using its boot policy and variables.
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GRUB chain-loads another loader
GRUB can hand execution to another bootloader when direct OS loading is unavailable or unsuitable. That adds a compatibility and maintenance link; the GRUB manual generally favors direct loading or kexec when those methods are available, but the right choice depends on the OS and device design.
Compare the paths before choosing one
| Path | What happens | What to verify |
|---|---|---|
| U-Boot native kernel boot | U-Boot loads Linux using its own boot commands, without UEFI. | Board support, kernel/initrd/device-tree handling, and whether the platform needs EFI services. |
| U-Boot UEFI → GRUB | U-Boot provides UEFI services and starts GRUB as an EFI application. | U-Boot build options, EFI application compatibility, device-tree or ACPI handoff, and boot-variable support and persistence. |
| UEFI firmware → GRUB | Platform UEFI firmware selects and loads GRUB. | Firmware boot entries and order, available drivers and filesystems, and Secure Boot configuration where relevant. |
| GRUB → another loader | GRUB chain-loads a second bootloader. | Whether GRUB can load the OS directly, compatibility of the next loader, and the additional maintenance and failure points. |
Configuration details that affect the handoff
Build options and commands
U-Boot documents CONFIG_CMD_BOOTEFI=y and CONFIG_EFI_LOADER=y for enabling its UEFI support. The boot-manager subcommand and other features can be configured separately, so check the target build rather than assuming that a command shown in an example exists on your board. The U-Boot UEFI documentation and bootefi command reference describe these options.
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Hardware description and loaded-file order
For an operating-system handoff, the platform must supply an appropriate hardware description through ACPI or a device tree. U-Boot documents how bootefi receives an FDT address or falls back to environment variables; the exact behavior should be checked against the target build’s command documentation.
In U-Boot’s documented manual-loading case, the last PE/COFF file loaded supplies the file path used by the loaded-image protocol. Its example loads GRUB after the device tree. A partial command sequence copied without that context may therefore behave differently from the example.
Boot-variable storage and Secure Boot
U-Boot’s eficonfig documentation describes managing UEFI variables and using bootefi bootmgr to attempt options in BootOrder. It describes tamper-resistant variable storage using OP-TEE support and RPMB-backed eMMC for a particular configuration; that storage method is not established for all boards.
Secure Boot is a separate concern from selecting GRUB. U-Boot documents Secure Boot settings and enrolled signature variables, but a working signed loader chain depends on the platform’s trust configuration. The component names alone do not establish whether Secure Boot is enabled or how keys are managed.
Quick Recap
How to diagnose a failing chain
bootefiis missing: check whether the target build enablesCONFIG_CMD_BOOTEFIandCONFIG_EFI_LOADER, and whether the needed subcommand is configured.- GRUB starts but cannot continue: verify that the correct EFI binary was loaded, that the storage paths match the board, and that an appropriate ACPI or device-tree description is available for the OS handoff.
- The boot manager ignores an expected option: inspect the UEFI boot variables, including
BootNextandBootOrder, and confirm that the board’s variable storage persists as expected. - A signed loader is rejected: check the platform’s Secure Boot policy, enrolled keys, and signatures across the complete loader chain.
- The sequence works only on one board or build: treat it as platform-specific. U-Boot’s implementation, device-tree or ACPI handling, storage access, and variable persistence vary by configuration.
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