u-boot/doc/README.efi

4 # SPDX-License-Identifier:	GPL-2.0+
9   1  U-Boot on EFI
19   1.10 32/64-bit
23   2  EFI on U-Boot
29 U-Boot on EFI
31 This document provides information about U-Boot running on top of EFI, either
32 as an application or just as a means of getting U-Boot onto a new platform.
36 -------------------
39 - You have EFI running on a board but U-Boot does not natively support it
40 fully yet. You can boot into U-Boot from EFI and use that until U-Boot is
43 - You need to use an EFI implementation (e.g. UEFI) because your vendor
46 - You plan to use coreboot to boot into U-Boot but coreboot support does
47 not currently exist for your platform. In the meantime you can use U-Boot
48 on EFI and then move to U-Boot on coreboot when ready
50 - You use EFI but want to experiment with a simpler alternative like U-Boot
54 ------
59 U-Boot supports running as an EFI application for 32-bit EFI only. This is
63 More usefully, U-Boot supports building itself as a payload for either 32-bit
64 or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once
65 started, U-Boot changes to 32-bit mode (currently) and takes over the
66 machine. You can use devices, boot a kernel, etc.
70 ------------------
72 for that board. It will be either 32-bit or 64-bit. Alternatively, you can
75 To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI
76 and CONFIG_EFI_APP. The efi-x86 config (efi-x86_defconfig) is set up for this.
77 Just build U-Boot as normal, e.g.
79    make efi-x86_defconfig
82 To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), adjust an
83 existing config (like qemu-x86_defconfig) to enable CONFIG_EFI, CONFIG_EFI_STUB
85 boolean Kconfig options. Then build U-Boot as normal, e.g.
87    make qemu-x86_defconfig
92    u-boot-app.efi      - U-Boot EFI application
93    u-boot-payload.efi  - U-Boot EFI payload application
97 -------------
103    cp /path/to/u-boot*.efi /tmp/efi
104    qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/
106 Add -nographic if you want to use the terminal for output. Once it starts
107 type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to
111 To try it on real hardware, put u-boot-app.efi on a suitable boot medium,
114    fs0:u-boot-payload.efi
116 (or fs0:u-boot-app.efi for the application)
118 This will start the payload, copy U-Boot into RAM and start U-Boot. Note
119 that EFI does not support booting a 64-bit application from a 32-bit
133 ---------------
134 For the application the whole of U-Boot is built as a shared library. The
136 functions with efi_init(), sets up U-Boot global_data, allocates memory for
137 U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f()
140 Since U-Boot limits its memory access to the allocated regions very little
146 'boot services' to send and receive characters. Although it is implemented
148 boot EFI with video output then the 'serial' device will operate on your
151 consoles will be active. Even though U-Boot does the same thing normally,
152 These are features of EFI, not U-Boot.
155 U-Boot is highly portable. Most of the difficulty is in modifying the
157 little x86-specific code involved - you can find most of it in
164 -----------
166 U-Boot exactly as normal for your target board, then adding the entire
172 function is called by EFI. It is responsible for copying U-Boot from its
173 original location into memory, disabling EFI boot services and starting
174 U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc.
176 The stub application is architecture-dependent. At present it has some
177 x86-specific code and a comment at the top of efi_stub.c describes this.
180 used by U-Boot (the payload). In fact when U-Boot starts it has all of the
185 ------
186 The payload can pass information to U-Boot in the form of EFI tables. At
189 display this list. U-Boot uses the list to work out where to relocate
192 Although U-Boot can use any memory it likes, EFI marks some memory as used
193 by 'run-time services', code that hangs around while U-Boot is running and
196 fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It
201 ----------
202 U-Boot drivers typically don't use interrupts. Since EFI enables interrupts
203 it is possible that an interrupt will fire that U-Boot cannot handle. This
204 seems to cause problems. For this reason the U-Boot payload runs with
207 32/64-bit
208 ---------
209 While the EFI application can in principle be built as either 32- or 64-bit,
210 only 32-bit is currently supported. This means that the application can only
211 be used with 32-bit EFI.
213 The payload stub can be build as either 32- or 64-bits. Only a small amount
214 of code is built this way (see the extra- line in lib/efi/Makefile).
215 Everything else is built as a normal U-Boot, so is always 32-bit on x86 at
219 -----------
222 - Add a generic x86 EFI payload configuration. At present you need to modify
223 an existing one, but mostly the low-level x86 code is disabled when booting
227 - Add ARM support
229 - Add 64-bit application support
231 - Figure out how to solve the interrupt problem
233 - Add more drivers to the application side (e.g. video, block devices, USB,
237 - Avoid turning off boot services in the stub. Instead allow U-Boot to make
238 use of boot services in case it wants to. It is unclear what it might want
242 ------------------
244 	payload stub, application, support code. Mostly arch-neutral
253 board/efi/efi-x86/efi.c
259 --
267 -------------------------------------------------------------------------------
269 EFI on U-Boot
272 In addition to support for running U-Boot as a UEFI application, U-Boot itself
277 -------------------
280 kernel, grub2 or gummiboot) on U-Boot. This dramatically simplifies boot loader
281 configuration, as U-Boot based systems now look and feel (almost) the same way
285 ----------------
287 EFI support for 32bit ARM and AArch64 is already included in U-Boot. All you
294 an efi application as well as snippet in the default distro boot script that
298 ------
303 When enabled, the resulting U-Boot binary only grows by ~10KB, so it's very
308 Removable media booting (search for /efi/boot/boota{a64,arm}.efi) is supported.
315 ---------------------
317 You can run a simple 'hello world' EFI program in U-Boot.
320 Then you can boot into U-Boot and type:
328 -----------
332    - Improve disk media detection (don't scan, use what information we
334    - Add EFI variable support using NVRAM
335    - Add GFX support
336    - Make EFI Shell work
337    - Network device support
338    - Support for payload exit
339    - Payload Watchdog support