1 .. SPDX-License-Identifier: GPL-2.0+
2 .. Copyright (C) 2015 Google, Inc
6 This document provides information about U-Boot running on top of EFI, either
7 as an application or just as a means of getting U-Boot onto a new platform.
12 Running U-Boot on EFI is useful in several situations:
14 - You have EFI running on a board but U-Boot does not natively support it
15 fully yet. You can boot into U-Boot from EFI and use that until U-Boot is
18 - You need to use an EFI implementation (e.g. UEFI) because your vendor
19 requires it in order to provide support
21 - You plan to use coreboot to boot into U-Boot but coreboot support does
22 not currently exist for your platform. In the meantime you can use U-Boot
23 on EFI and then move to U-Boot on coreboot when ready
25 - You use EFI but want to experiment with a simpler alternative like U-Boot
30 Only x86 is supported at present. If you are using EFI on another architecture
31 you may want to reconsider. However, much of the code is generic so could be
34 U-Boot supports running as an EFI application for 32-bit EFI only. This is
35 not very useful since only a serial port is provided. You can look around at
36 memory and type 'help' but that is about it.
38 More usefully, U-Boot supports building itself as a payload for either 32-bit
39 or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once
40 started, U-Boot changes to 32-bit mode (currently) and takes over the
41 machine. You can use devices, boot a kernel, etc.
46 First choose a board that has EFI support and obtain an EFI implementation
47 for that board. It will be either 32-bit or 64-bit. Alternatively, you can
48 opt for using QEMU [1] and the OVMF [2], as detailed below.
50 To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI
51 and CONFIG_EFI_APP. The efi-x86_app config (efi-x86_app_defconfig) is set up
52 for this. Just build U-Boot as normal, e.g.::
54 make efi-x86_app_defconfig
57 To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), enable
58 CONFIG_EFI, CONFIG_EFI_STUB, and select either CONFIG_EFI_STUB_32BIT or
59 CONFIG_EFI_STUB_64BIT. The efi-x86_payload configs (efi-x86_payload32_defconfig
60 and efi-x86_payload32_defconfig) are set up for this. Then build U-Boot as
63 make efi-x86_payload32_defconfig (or efi-x86_payload64_defconfig)
66 You will end up with one of these files depending on what you build for:
68 * u-boot-app.efi - U-Boot EFI application
69 * u-boot-payload.efi - U-Boot EFI payload application
74 QEMU is an emulator and it can emulate an x86 machine. Please make sure your
75 QEMU version is 2.3.0 or above to test this. You can run the payload with
79 cp /path/to/u-boot*.efi /tmp/efi
80 qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/
82 Add -nographic if you want to use the terminal for output. Once it starts
83 type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to
84 run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a
85 prebuilt EFI BIOS for QEMU or you can build one from source as well.
87 To try it on real hardware, put u-boot-app.efi on a suitable boot medium,
88 such as a USB stick. Then you can type something like this to start it::
90 fs0:u-boot-payload.efi
92 (or fs0:u-boot-app.efi for the application)
94 This will start the payload, copy U-Boot into RAM and start U-Boot. Note
95 that EFI does not support booting a 64-bit application from a 32-bit
96 EFI (or vice versa). Also it will often fail to print an error message if
102 Here follow a few implementation notes for those who want to fiddle with
103 this and perhaps contribute patches.
105 The application and payload approaches sound similar but are in fact
106 implemented completely differently.
110 For the application the whole of U-Boot is built as a shared library. The
111 efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI
112 functions with efi_init(), sets up U-Boot global_data, allocates memory for
113 U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f()
116 Since U-Boot limits its memory access to the allocated regions very little
117 special code is needed. The CONFIG_EFI_APP option controls a few things
118 that need to change so 'git grep CONFIG_EFI_APP' may be instructive.
119 The CONFIG_EFI option controls more general EFI adjustments.
121 The only available driver is the serial driver. This calls back into EFI
122 'boot services' to send and receive characters. Although it is implemented
123 as a serial driver the console device is not necessarilly serial. If you
124 boot EFI with video output then the 'serial' device will operate on your
125 target devices's display instead and the device's USB keyboard will also
126 work if connected. If you have both serial and video output, then both
127 consoles will be active. Even though U-Boot does the same thing normally,
128 These are features of EFI, not U-Boot.
130 Very little code is involved in implementing the EFI application feature.
131 U-Boot is highly portable. Most of the difficulty is in modifying the
132 Makefile settings to pass the right build flags. In particular there is very
133 little x86-specific code involved - you can find most of it in
134 arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave
135 enough) should be straightforward.
137 Use the 'reset' command to get back to EFI.
141 The payload approach is a different kettle of fish. It works by building
142 U-Boot exactly as normal for your target board, then adding the entire
143 image (including device tree) into a small EFI stub application responsible
144 for booting it. The stub application is built as a normal EFI application
145 except that it has a lot of data attached to it.
147 The stub application is implemented in lib/efi/efi_stub.c. The efi_main()
148 function is called by EFI. It is responsible for copying U-Boot from its
149 original location into memory, disabling EFI boot services and starting
150 U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc.
152 The stub application is architecture-dependent. At present it has some
153 x86-specific code and a comment at the top of efi_stub.c describes this.
155 While the stub application does allocate some memory from EFI this is not
156 used by U-Boot (the payload). In fact when U-Boot starts it has all of the
157 memory available to it and can operate as it pleases (but see the next
162 The payload can pass information to U-Boot in the form of EFI tables. At
163 present this feature is used to pass the EFI memory map, an inordinately
164 large list of memory regions. You can use the 'efi mem all' command to
165 display this list. U-Boot uses the list to work out where to relocate
168 Although U-Boot can use any memory it likes, EFI marks some memory as used
169 by 'run-time services', code that hangs around while U-Boot is running and
170 is even present when Linux is running. This is common on x86 and provides
171 a way for Linux to call back into the firmware to control things like CPU
172 fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It
173 will relocate itself to the top of the largest block of memory it can find
178 U-Boot drivers typically don't use interrupts. Since EFI enables interrupts
179 it is possible that an interrupt will fire that U-Boot cannot handle. This
180 seems to cause problems. For this reason the U-Boot payload runs with
181 interrupts disabled at present.
185 While the EFI application can in principle be built as either 32- or 64-bit,
186 only 32-bit is currently supported. This means that the application can only
187 be used with 32-bit EFI.
189 The payload stub can be build as either 32- or 64-bits. Only a small amount
190 of code is built this way (see the extra- line in lib/efi/Makefile).
191 Everything else is built as a normal U-Boot, so is always 32-bit on x86 at
196 This work could be extended in a number of ways:
200 - Add 64-bit application support
202 - Figure out how to solve the interrupt problem
204 - Add more drivers to the application side (e.g. video, block devices, USB,
205 environment access). This would mostly be an academic exercise as a strong
206 use case is not readily apparent, but it might be fun.
208 - Avoid turning off boot services in the stub. Instead allow U-Boot to make
209 use of boot services in case it wants to. It is unclear what it might want
215 payload stub, application, support code. Mostly arch-neutral
218 x86 support code for running as an EFI application and payload
220 board/efi/efi-x86_app/efi.c
221 x86 board code for running as an EFI application
223 board/efi/efi-x86_payload
224 generic x86 EFI payload board support code
230 Ben Stoltz, Simon Glass
234 * [1] http://www.qemu.org
235 * [2] http://www.tianocore.org/ovmf/