^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) =================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) The EFI Boot Stub
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3) =================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5) On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6) as a PE/COFF image, thereby convincing EFI firmware loaders to load
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7) it as an EFI executable. The code that modifies the bzImage header,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) along with the EFI-specific entry point that the firmware loader
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9) jumps to are collectively known as the "EFI boot stub", and live in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11) respectively. For ARM the EFI stub is implemented in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12) arch/arm/boot/compressed/efi-header.S and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14) between architectures is in drivers/firmware/efi/libstub.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) For arm64, there is no compressed kernel support, so the Image itself
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) masquerades as a PE/COFF image and the EFI stub is linked into the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18) kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) and drivers/firmware/efi/libstub/arm64-stub.c.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) By using the EFI boot stub it's possible to boot a Linux kernel
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) without the use of a conventional EFI boot loader, such as grub or
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) elilo. Since the EFI boot stub performs the jobs of a boot loader, in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) a certain sense it *IS* the boot loader.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) How to install bzImage.efi
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30) --------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) System Partition (ESP) and renamed with the extension ".efi". Without
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34) the extension the EFI firmware loader will refuse to execute it. It's
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) not possible to execute bzImage.efi from the usual Linux file systems
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36) because EFI firmware doesn't have support for them. For ARM the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37) arch/arm/boot/zImage should be copied to the system partition, and it
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39) should be copied but not necessarily renamed.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) Passing kernel parameters from the EFI shell
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43) --------------------------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45) Arguments to the kernel can be passed after bzImage.efi, e.g.::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47) fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50) The "initrd=" option
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51) --------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53) Like most boot loaders, the EFI stub allows the user to specify
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54) multiple initrd files using the "initrd=" option. This is the only EFI
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55) stub-specific command line parameter, everything else is passed to the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) kernel when it boots.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58) The path to the initrd file must be an absolute path from the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59) beginning of the ESP, relative path names do not work. Also, the path
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60) is an EFI-style path and directory elements must be separated with
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61) backslashes (\). For example, given the following directory layout::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63) fs0:>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64) Kernels\
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65) bzImage.efi
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66) initrd-large.img
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68) Ramdisks\
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69) initrd-small.img
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70) initrd-medium.img
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) to boot with the initrd-large.img file if the current working
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73) directory is fs0:\Kernels, the following command must be used::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75) fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) Notice how bzImage.efi can be specified with a relative path. That's
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78) because the image we're executing is interpreted by the EFI shell,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79) which understands relative paths, whereas the rest of the command line
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80) is passed to bzImage.efi.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83) The "dtb=" option
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84) -----------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86) For the ARM and arm64 architectures, a device tree must be provided to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87) the kernel. Normally firmware shall supply the device tree via the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) EFI CONFIGURATION TABLE. However, the "dtb=" command line option can
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89) be used to override the firmware supplied device tree, or to supply
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90) one when firmware is unable to.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92) Please note: Firmware adds runtime configuration information to the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93) device tree before booting the kernel. If dtb= is used to override
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94) the device tree, then any runtime data provided by firmware will be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95) lost. The dtb= option should only be used either as a debug tool, or
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) as a last resort when a device tree is not provided in the EFI
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97) CONFIGURATION TABLE.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99) "dtb=" is processed in the same manner as the "initrd=" option that is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100) described above.
Orange Pi5 kernel
Deprecated Linux kernel 5.10.110 for OrangePi 5/5B/5+ boards
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