1 =================
2 The EFI Boot Stub
3 =================
4
5 On the x86 and ARM platforms, a kernel zImage/
6 as a PE/COFF image, thereby convincing EFI fir
7 it as an EFI executable. The code that modifie
8 along with the EFI-specific entry point that t
9 jumps to are collectively known as the "EFI bo
10 arch/x86/boot/header.S and drivers/firmware/ef
11 respectively. For ARM the EFI stub is implemen
12 arch/arm/boot/compressed/efi-header.S and
13 drivers/firmware/efi/libstub/arm32-stub.c. EFI
14 between architectures is in drivers/firmware/e
15
16 For arm64, there is no compressed kernel suppo
17 masquerades as a PE/COFF image and the EFI stu
18 kernel. The arm64 EFI stub lives in drivers/fi
19 and drivers/firmware/efi/libstub/arm64-stub.c.
20
21 By using the EFI boot stub it's possible to bo
22 without the use of a conventional EFI boot loa
23 elilo. Since the EFI boot stub performs the jo
24 a certain sense it *IS* the boot loader.
25
26 The EFI boot stub is enabled with the CONFIG_E
27
28
29 How to install bzImage.efi
30 --------------------------
31
32 The bzImage located in arch/x86/boot/bzImage m
33 System Partition (ESP) and renamed with the ex
34 the extension the EFI firmware loader will ref
35 not possible to execute bzImage.efi from the u
36 because EFI firmware doesn't have support for
37 arch/arm/boot/zImage should be copied to the s
38 may not need to be renamed. Similarly for arm6
39 should be copied but not necessarily renamed.
40
41
42 Passing kernel parameters from the EFI shell
43 --------------------------------------------
44
45 Arguments to the kernel can be passed after bz
46
47 fs0:> bzImage.efi console=ttyS0 root=/
48
49
50 The "initrd=" option
51 --------------------
52
53 Like most boot loaders, the EFI stub allows th
54 multiple initrd files using the "initrd=" opti
55 stub-specific command line parameter, everythi
56 kernel when it boots.
57
58 The path to the initrd file must be an absolut
59 beginning of the ESP, relative path names do n
60 is an EFI-style path and directory elements mu
61 backslashes (\). For example, given the follow
62
63 fs0:>
64 Kernels\
65 bzImage.efi
66 initrd-large.img
67
68 Ramdisks\
69 initrd-small.img
70 initrd-medium.img
71
72 to boot with the initrd-large.img file if the
73 directory is fs0:\Kernels, the following comma
74
75 fs0:\Kernels> bzImage.efi initrd=\Kern
76
77 Notice how bzImage.efi can be specified with a
78 because the image we're executing is interpret
79 which understands relative paths, whereas the
80 is passed to bzImage.efi.
81
82
83 The "dtb=" option
84 -----------------
85
86 For the ARM and arm64 architectures, a device
87 the kernel. Normally firmware shall supply the
88 EFI CONFIGURATION TABLE. However, the "dtb=" c
89 be used to override the firmware supplied devi
90 one when firmware is unable to.
91
92 Please note: Firmware adds runtime configurati
93 device tree before booting the kernel. If dtb=
94 the device tree, then any runtime data provide
95 lost. The dtb= option should only be used eith
96 as a last resort when a device tree is not pro
97 CONFIGURATION TABLE.
98
99 "dtb=" is processed in the same manner as the
100 described above.
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