Streamlining the Final Leap: Unifying U-Boot’s Pre-OS Cleanup

written by Teemikemedia | 18 July 2025

What happens in the final moments before U-Boot hands control over to the operating system? Until recently, the answer was, “it’s complicated.” Each architecture like ARM, x86, and RISC-V had its own way of handling the final pre-boot cleanup, leading to a maze of slightly different functions and duplicated code. It was difficult to know what was really happening just before the kernel started.

Thanks to a recent series of commits in Concept, this critical part of the boot process has been significantly cleaned up and unified.

A Simpler, Centralized Approach

The core of this effort is the introduction of a new generic function: bootm_final(). This function’s purpose is to consolidate all the common steps that must happen right before booting an OS. By moving to this centralized model, we’ve replaced various architecture-specific functions, like bootm_announce_and_cleanup(), with a single, unified call.

This new approach has been adopted across the x86, RISC-V, and ARM architectures, as well as for the EFI loader.

Key Improvements in This Series

  • Unified Cleanup: Common tasks like disabling interrupts, quiescing board devices, and calling cleanup_before_linux() are now handled in one place, reducing code duplication and increasing consistency.
  • Better Bootstage Reporting: The EFI boot path now benefits from bootstage processing. If enabled, U-Boot will produce a bootstage report, offering better insights into boot-time performance when launching an EFI application. This report is emitted when exit-boot-services is called, thus allowing timing of GRUB and the kernel EFI stuff, if present.
  • Code Simplification: With the new generic function in place, redundant architecture-specific functions have been removed. We also took the opportunity to drop an outdated workaround for an old version of GRUB (EFI_GRUB_ARM32_WORKAROUND).

This cleanup makes the boot process more robust, easier to understand, and simpler to maintain. While there is still future work to be done in this area, this is a major step forward in standardizing the final hand-off from U-Boot to the OS.