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1 | # |
2 | # Copyright (C) 2015 Google, Inc | |
3 | # | |
4 | # SPDX-License-Identifier: GPL-2.0+ | |
5 | # | |
6 | ||
7 | U-Boot on EFI | |
8 | ============= | |
9 | This document provides information about U-Boot running on top of EFI, either | |
10 | as an application or just as a means of getting U-Boot onto a new platform. | |
11 | ||
12 | ||
13 | =========== Table of Contents =========== | |
14 | ||
15 | Motivation | |
16 | Status | |
17 | Build Instructions | |
18 | Trying it out | |
19 | Inner workings | |
20 | EFI Application | |
21 | EFI Payload | |
22 | Tables | |
23 | Interrupts | |
24 | 32/64-bit | |
25 | Future work | |
26 | Where is the code? | |
27 | ||
28 | ||
29 | Motivation | |
30 | ---------- | |
31 | Running U-Boot on EFI is useful in several situations: | |
32 | ||
33 | - You have EFI running on a board but U-Boot does not natively support it | |
34 | fully yet. You can boot into U-Boot from EFI and use that until U-Boot is | |
35 | fully ported | |
36 | ||
37 | - You need to use an EFI implementation (e.g. UEFI) because your vendor | |
38 | requires it in order to provide support | |
39 | ||
40 | - You plan to use coreboot to boot into U-Boot but coreboot support does | |
41 | not currently exist for your platform. In the meantime you can use U-Boot | |
42 | on EFI and then move to U-Boot on coreboot when ready | |
43 | ||
44 | - You use EFI but want to experiment with a simpler alternative like U-Boot | |
45 | ||
46 | ||
47 | Status | |
48 | ------ | |
49 | Only x86 is supported at present. If you are using EFI on another architecture | |
50 | you may want to reconsider. However, much of the code is generic so could be | |
51 | ported. | |
52 | ||
53 | U-Boot supports running as an EFI application for 32-bit EFI only. This is | |
54 | not very useful since only a serial port is provided. You can look around at | |
55 | memory and type 'help' but that is about it. | |
56 | ||
57 | More usefully, U-Boot supports building itself as a payload for either 32-bit | |
58 | or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once | |
59 | started, U-Boot changes to 32-bit mode (currently) and takes over the | |
60 | machine. You can use devices, boot a kernel, etc. | |
61 | ||
62 | ||
63 | Build Instructions | |
64 | ------------------ | |
65 | First choose a board that has EFI support and obtain an EFI implementation | |
66 | for that board. It will be either 32-bit or 64-bit. Alternatively, you can | |
67 | opt for using QEMU [1] and the OVMF [2], as detailed below. | |
68 | ||
69 | To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI | |
70 | and CONFIG_EFI_APP. The efi-x86 config (efi-x86_defconfig) is set up for this. | |
71 | Just build U-Boot as normal, e.g. | |
72 | ||
73 | make efi-x86_defconfig | |
74 | make | |
75 | ||
76 | To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), adjust an | |
77 | existing config (like qemu-x86_defconfig) to enable CONFIG_EFI, CONFIG_EFI_STUB | |
78 | and either CONFIG_EFI_STUB_32BIT or CONFIG_EFI_STUB_64BIT. All of these are | |
79 | boolean Kconfig options. Then build U-Boot as normal, e.g. | |
80 | ||
81 | make qemu-x86_defconfig | |
82 | make | |
83 | ||
84 | You will end up with one of these files depending on what you build for: | |
85 | ||
86 | u-boot-app.efi - U-Boot EFI application | |
87 | u-boot-payload.efi - U-Boot EFI payload application | |
88 | ||
89 | ||
90 | Trying it out | |
91 | ------------- | |
92 | QEMU is an emulator and it can emulate an x86 machine. Please make sure your | |
93 | QEMU version is 2.3.0 or above to test this. You can run the payload with | |
94 | something like this: | |
95 | ||
96 | mkdir /tmp/efi | |
97 | cp /path/to/u-boot*.efi /tmp/efi | |
98 | qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/ | |
99 | ||
100 | Add -nographic if you want to use the terminal for output. Once it starts | |
101 | type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to | |
102 | run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a | |
103 | prebuilt EFI BIOS for QEMU or you can build one from source as well. | |
104 | ||
105 | To try it on real hardware, put u-boot-app.efi on a suitable boot medium, | |
106 | such as a USB stick. Then you can type something like this to start it: | |
107 | ||
108 | fs0:u-boot-payload.efi | |
109 | ||
110 | (or fs0:u-boot-app.efi for the application) | |
111 | ||
112 | This will start the payload, copy U-Boot into RAM and start U-Boot. Note | |
113 | that EFI does not support booting a 64-bit application from a 32-bit | |
114 | EFI (or vice versa). Also it will often fail to print an error message if | |
115 | you get this wrong. | |
116 | ||
117 | ||
118 | Inner workings | |
119 | ============== | |
120 | Here follow a few implementation notes for those who want to fiddle with | |
121 | this and perhaps contribute patches. | |
122 | ||
123 | The application and payload approaches sound similar but are in fact | |
124 | implemented completely differently. | |
125 | ||
126 | EFI Application | |
127 | --------------- | |
128 | For the application the whole of U-Boot is built as a shared library. The | |
129 | efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI | |
130 | functions with efi_init(), sets up U-Boot global_data, allocates memory for | |
131 | U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f() | |
132 | and board_init_r()). | |
133 | ||
134 | Since U-Boot limits its memory access to the allocated regions very little | |
135 | special code is needed. The CONFIG_EFI_APP option controls a few things | |
136 | that need to change so 'git grep CONFIG_EFI_APP' may be instructive. | |
137 | The CONFIG_EFI option controls more general EFI adjustments. | |
138 | ||
139 | The only available driver is the serial driver. This calls back into EFI | |
140 | 'boot services' to send and receive characters. Although it is implemented | |
141 | as a serial driver the console device is not necessarilly serial. If you | |
142 | boot EFI with video output then the 'serial' device will operate on your | |
143 | target devices's display instead and the device's USB keyboard will also | |
144 | work if connected. If you have both serial and video output, then both | |
145 | consoles will be active. Even though U-Boot does the same thing normally, | |
146 | These are features of EFI, not U-Boot. | |
147 | ||
148 | Very little code is involved in implementing the EFI application feature. | |
149 | U-Boot is highly portable. Most of the difficulty is in modifying the | |
150 | Makefile settings to pass the right build flags. In particular there is very | |
151 | little x86-specific code involved - you can find most of it in | |
152 | arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave | |
153 | enough) should be straightforward. | |
154 | ||
155 | Use the 'reset' command to get back to EFI. | |
156 | ||
157 | EFI Payload | |
158 | ----------- | |
159 | The payload approach is a different kettle of fish. It works by building | |
160 | U-Boot exactly as normal for your target board, then adding the entire | |
161 | image (including device tree) into a small EFI stub application responsible | |
162 | for booting it. The stub application is built as a normal EFI application | |
163 | except that it has a lot of data attached to it. | |
164 | ||
165 | The stub application is implemented in lib/efi/efi_stub.c. The efi_main() | |
166 | function is called by EFI. It is responsible for copying U-Boot from its | |
167 | original location into memory, disabling EFI boot services and starting | |
168 | U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc. | |
169 | ||
170 | The stub application is architecture-dependent. At present it has some | |
171 | x86-specific code and a comment at the top of efi_stub.c describes this. | |
172 | ||
173 | While the stub application does allocate some memory from EFI this is not | |
174 | used by U-Boot (the payload). In fact when U-Boot starts it has all of the | |
175 | memory available to it and can operate as it pleases (but see the next | |
176 | section). | |
177 | ||
178 | Tables | |
179 | ------ | |
180 | The payload can pass information to U-Boot in the form of EFI tables. At | |
181 | present this feature is used to pass the EFI memory map, an inordinately | |
182 | large list of memory regions. You can use the 'efi mem all' command to | |
183 | display this list. U-Boot uses the list to work out where to relocate | |
184 | itself. | |
185 | ||
186 | Although U-Boot can use any memory it likes, EFI marks some memory as used | |
187 | by 'run-time services', code that hangs around while U-Boot is running and | |
188 | is even present when Linux is running. This is common on x86 and provides | |
189 | a way for Linux to call back into the firmware to control things like CPU | |
190 | fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It | |
191 | will relocate itself to the top of the largest block of memory it can find | |
192 | below 4GB. | |
193 | ||
194 | Interrupts | |
195 | ---------- | |
196 | U-Boot drivers typically don't use interrupts. Since EFI enables interrupts | |
197 | it is possible that an interrupt will fire that U-Boot cannot handle. This | |
198 | seems to cause problems. For this reason the U-Boot payload runs with | |
199 | interrupts disabled at present. | |
200 | ||
201 | 32/64-bit | |
202 | --------- | |
203 | While the EFI application can in principle be built as either 32- or 64-bit, | |
204 | only 32-bit is currently supported. This means that the application can only | |
205 | be used with 32-bit EFI. | |
206 | ||
207 | The payload stub can be build as either 32- or 64-bits. Only a small amount | |
208 | of code is built this way (see the extra- line in lib/efi/Makefile). | |
209 | Everything else is built as a normal U-Boot, so is always 32-bit on x86 at | |
210 | present. | |
211 | ||
212 | Future work | |
213 | ----------- | |
214 | This work could be extended in a number of ways: | |
215 | ||
216 | - Add a generic x86 EFI payload configuration. At present you need to modify | |
217 | an existing one, but mostly the low-level x86 code is disabled when booting | |
218 | on EFI anyway, so a generic 'EFI' board could be created with a suitable set | |
219 | of drivers enabled. | |
220 | ||
221 | - Add ARM support | |
222 | ||
223 | - Add 64-bit application support | |
224 | ||
225 | - Figure out how to solve the interrupt problem | |
226 | ||
227 | - Add more drivers to the application side (e.g. video, block devices, USB, | |
228 | environment access). This would mostly be an academic exercise as a strong | |
229 | use case is not readily apparent, but it might be fun. | |
230 | ||
231 | - Avoid turning off boot services in the stub. Instead allow U-Boot to make | |
232 | use of boot services in case it wants to. It is unclear what it might want | |
233 | though. | |
234 | ||
235 | Where is the code? | |
236 | ------------------ | |
237 | lib/efi | |
238 | payload stub, application, support code. Mostly arch-neutral | |
239 | ||
240 | arch/x86/lib/efi | |
241 | helper functions for the fake DRAM init, etc. These can be used by | |
242 | any board that runs as a payload. | |
243 | ||
244 | arch/x86/cpu/efi | |
245 | x86 support code for running as an EFI application | |
246 | ||
247 | board/efi/efi-x86/efi.c | |
248 | x86 board code for running as an EFI application | |
249 | ||
250 | common/cmd_efi.c | |
251 | the 'efi' command | |
252 | ||
253 | -- | |
254 | Ben Stoltz, Simon Glass | |
255 | Google, Inc | |
256 | July 2015 | |
257 | ||
258 | [1] http://www.qemu.org | |
259 | [2] http://www.tianocore.org/ovmf/ |