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Qualcomm® Linux uses systemd-boot as the UEFI boot manager. The Linux kernel is compiled with EFI stub support so that UEFI firmware and systemd-boot can load it directly as an EFI executable. There is no GRUB or other second-stage boot loader in the chain.

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UEFI boot manager overview

A UEFI boot manager is a program that runs after firmware platform initialization and is responsible for selecting and launching an OS image. On Qualcomm Linux:
  1. The XBL firmware initialises hardware (DDR, clocks, security) and hands off to a UEFI image stored in the abl_a / abl_b partitions.
  2. UEFI firmware performs UEFI platform initialization, discovers the EFI System Partition (ESP), and launches the default EFI application (systemd-boot).
  3. systemd-boot reads boot entries from the ESP, displays a menu if needed, and executes the selected kernel EFI binary.
For more information on the UEFI specification and boot manager behavior, see UEFI Boot Manager.

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systemd-boot behavior

systemd-boot is a minimal, spec-compliant UEFI boot manager from the systemd project. It implements the Boot Loader Specification (BLS) and supports both Type 1 and Type 2 boot entries.

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Boot entry discovery

On startup, systemd-boot scans the ESP for valid entries in the following locations: On Qualcomm Linux, kernels are packaged exclusively as Type 2 UKI images placed in EFI/Linux/.

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loader.conf

Global defaults for systemd-boot are stored in loader/loader.conf on the ESP. The Qualcomm Linux Yocto build writes a minimal loader.conf:

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View Boot entry formats

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Type 1 drop-in .conf files

Type 1 entries are plain-text .conf files placed in loader/entries/ on the ESP. Each file describes one boot entry:
Qualcomm Linux does not use Type 1 entries in production. The layout is shown here for reference when adding custom entries.

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Type 2 Unified Kernel Images (UKI)

A UKI is a single EFI binary that embeds the Linux kernel image, initramfs, kernel command line, and optionally a device tree. UEFI firmware (or systemd-boot) can execute a UKI directly without any supporting .conf file. UKI files are placed in EFI/Linux/ on the ESP and named with an .efi extension (for example linux.efi, linux-rt.efi). systemd-boot identifies them automatically by scanning that directory. For the UKI image format specification, see Type 2 EFI Unified Kernel Images.

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Generate Yocto ESP image

The Yocto meta-qcom layer provides the recipe recipes-kernel/images/esp-qcom-image.bb that assembles the ESP image (efi.bin). The recipe:
  1. Builds systemd-boot from the systemd package and copies the systemd-boot.efi binary to EFI/BOOT/BOOTAA64.EFI which is the default UEFI removable media boot path.
  2. Packages the kernel, initramfs, and command line into a UKI using ukify and writes it to EFI/Linux/linux.efi (or EFI/Linux/linux-rt.efi for RT builds).
  3. Writes loader/loader.conf with the default timeout and entry policy.
  4. Formats everything into a FAT32 image and stores it as efi.bin in the build deploy directory.
The resulting efi.bin is flashed to the efi partition on the device. For flashing instructions, see Install & boot the kernel. For more information about the secure boot integration, see Configure and secure boot with systemd-boot and UKI.