u-boot/doc/README.u-boot_on_efi

1 # SPDX-License-Identifier: GPL-2.0+
5 U-Boot on EFI
7 This document provides information about U-Boot running on top of EFI, either
8 as an application or just as a means of getting U-Boot onto a new platform.
22 32/64-bit
24 Where is the code?
28 ----------
29 Running U-Boot on EFI is useful in several situations:
31 - You have EFI running on a board but U-Boot does not natively support it
32 fully yet. You can boot into U-Boot from EFI and use that until U-Boot is
35 - You need to use an EFI implementation (e.g. UEFI) because your vendor
38 - You plan to use coreboot to boot into U-Boot but coreboot support does
39 not currently exist for your platform. In the meantime you can use U-Boot
40 on EFI and then move to U-Boot on coreboot when ready
42 - You use EFI but want to experiment with a simpler alternative like U-Boot
46 ------
47 Only x86 is supported at present. If you are using EFI on another architecture
48 you may want to reconsider. However, much of the code is generic so could be
51 U-Boot supports running as an EFI application for 32-bit EFI only. This is
52 not very useful since only a serial port is provided. You can look around at
53 memory and type 'help' but that is about it.
55 More usefully, U-Boot supports building itself as a payload for either 32-bit
56 or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once
57 started, U-Boot changes to 32-bit mode (currently) and takes over the
62 ------------------
64 for that board. It will be either 32-bit or 64-bit. Alternatively, you can
65 opt for using QEMU [1] and the OVMF [2], as detailed below.
67 To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI
68 and CONFIG_EFI_APP. The efi-x86_app config (efi-x86_app_defconfig) is set up
69 for this. Just build U-Boot as normal, e.g.
71    make efi-x86_app_defconfig
74 To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), enable
76 CONFIG_EFI_STUB_64BIT. The efi-x86_payload configs (efi-x86_payload32_defconfig
77 and efi-x86_payload32_defconfig) are set up for this. Then build U-Boot as
80    make efi-x86_payload32_defconfig (or efi-x86_payload64_defconfig)
85    u-boot-app.efi      - U-Boot EFI application
86    u-boot-payload.efi  - U-Boot EFI payload application
90 -------------
91 QEMU is an emulator and it can emulate an x86 machine. Please make sure your
92 QEMU version is 2.3.0 or above to test this. You can run the payload with
96    cp /path/to/u-boot*.efi /tmp/efi
97    qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/
99 Add -nographic if you want to use the terminal for output. Once it starts
100 type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to
101 run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a
102 prebuilt EFI BIOS for QEMU or you can build one from source as well.
104 To try it on real hardware, put u-boot-app.efi on a suitable boot medium,
105 such as a USB stick. Then you can type something like this to start it:
107    fs0:u-boot-payload.efi
109 (or fs0:u-boot-app.efi for the application)
111 This will start the payload, copy U-Boot into RAM and start U-Boot. Note
112 that EFI does not support booting a 64-bit application from a 32-bit
126 ---------------
127 For the application the whole of U-Boot is built as a shared library. The
128 efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI
129 functions with efi_init(), sets up U-Boot global_data, allocates memory for
130 U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f()
133 Since U-Boot limits its memory access to the allocated regions very little
134 special code is needed. The CONFIG_EFI_APP option controls a few things
138 The only available driver is the serial driver. This calls back into EFI
139 'boot services' to send and receive characters. Although it is implemented
140 as a serial driver the console device is not necessarilly serial. If you
144 consoles will be active. Even though U-Boot does the same thing normally,
145 These are features of EFI, not U-Boot.
147 Very little code is involved in implementing the EFI application feature.
148 U-Boot is highly portable. Most of the difficulty is in modifying the
149 Makefile settings to pass the right build flags. In particular there is very
150 little x86-specific code involved - you can find most of it in
157 -----------
158 The payload approach is a different kettle of fish. It works by building
159 U-Boot exactly as normal for your target board, then adding the entire
161 for booting it. The stub application is built as a normal EFI application
164 The stub application is implemented in lib/efi/efi_stub.c. The efi_main()
165 function is called by EFI. It is responsible for copying U-Boot from its
167 U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc.
169 The stub application is architecture-dependent. At present it has some
170 x86-specific code and a comment at the top of efi_stub.c describes this.
172 While the stub application does allocate some memory from EFI this is not
173 used by U-Boot (the payload). In fact when U-Boot starts it has all of the
174 memory available to it and can operate as it pleases (but see the next
178 ------
179 The payload can pass information to U-Boot in the form of EFI tables. At
180 present this feature is used to pass the EFI memory map, an inordinately
182 display this list. U-Boot uses the list to work out where to relocate
185 Although U-Boot can use any memory it likes, EFI marks some memory as used
186 by 'run-time services', code that hangs around while U-Boot is running and
187 is even present when Linux is running. This is common on x86 and provides
189 fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It
194 ----------
195 U-Boot drivers typically don't use interrupts. Since EFI enables interrupts
196 it is possible that an interrupt will fire that U-Boot cannot handle. This
197 seems to cause problems. For this reason the U-Boot payload runs with
200 32/64-bit
201 ---------
202 While the EFI application can in principle be built as either 32- or 64-bit,
203 only 32-bit is currently supported. This means that the application can only
204 be used with 32-bit EFI.
206 The payload stub can be build as either 32- or 64-bits. Only a small amount
207 of code is built this way (see the extra- line in lib/efi/Makefile).
208 Everything else is built as a normal U-Boot, so is always 32-bit on x86 at
212 -----------
215 - Add ARM support
217 - Add 64-bit application support
219 - Figure out how to solve the interrupt problem
221 - Add more drivers to the application side (e.g. video, block devices, USB,
222 environment access). This would mostly be an academic exercise as a strong
223 use case is not readily apparent, but it might be fun.
225 - Avoid turning off boot services in the stub. Instead allow U-Boot to make
226 use of boot services in case it wants to. It is unclear what it might want
229 Where is the code?
230 ------------------
232 	payload stub, application, support code. Mostly arch-neutral
235 	x86 support code for running as an EFI application and payload
237 board/efi/efi-x86_app/efi.c
238 	x86 board code for running as an EFI application
240 board/efi/efi-x86_payload
246 --