1# SPDX-License-Identifier: GPL-2.0+ 2# 3# Copyright (C) 2015 Google, Inc 4 5U-Boot on EFI 6============= 7This document provides information about U-Boot running on top of EFI, either 8as an application or just as a means of getting U-Boot onto a new platform. 9 10 11=========== Table of Contents =========== 12 13Motivation 14Status 15Build Instructions 16Trying it out 17Inner workings 18EFI Application 19EFI Payload 20Tables 21Interrupts 2232/64-bit 23Future work 24Where is the code? 25 26 27Motivation 28---------- 29Running U-Boot on EFI is useful in several situations: 30 31- You have EFI running on a board but U-Boot does not natively support it 32fully yet. You can boot into U-Boot from EFI and use that until U-Boot is 33fully ported 34 35- You need to use an EFI implementation (e.g. UEFI) because your vendor 36requires it in order to provide support 37 38- You plan to use coreboot to boot into U-Boot but coreboot support does 39not currently exist for your platform. In the meantime you can use U-Boot 40on EFI and then move to U-Boot on coreboot when ready 41 42- You use EFI but want to experiment with a simpler alternative like U-Boot 43 44 45Status 46------ 47Only x86 is supported at present. If you are using EFI on another architecture 48you may want to reconsider. However, much of the code is generic so could be 49ported. 50 51U-Boot supports running as an EFI application for 32-bit EFI only. This is 52not very useful since only a serial port is provided. You can look around at 53memory and type 'help' but that is about it. 54 55More usefully, U-Boot supports building itself as a payload for either 32-bit 56or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once 57started, U-Boot changes to 32-bit mode (currently) and takes over the 58machine. You can use devices, boot a kernel, etc. 59 60 61Build Instructions 62------------------ 63First choose a board that has EFI support and obtain an EFI implementation 64for that board. It will be either 32-bit or 64-bit. Alternatively, you can 65opt for using QEMU [1] and the OVMF [2], as detailed below. 66 67To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI 68and CONFIG_EFI_APP. The efi-x86 config (efi-x86_defconfig) is set up for this. 69Just build U-Boot as normal, e.g. 70 71 make efi-x86_defconfig 72 make 73 74To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), adjust an 75existing config (like qemu-x86_defconfig) to enable CONFIG_EFI, CONFIG_EFI_STUB 76and either CONFIG_EFI_STUB_32BIT or CONFIG_EFI_STUB_64BIT. All of these are 77boolean Kconfig options. Then build U-Boot as normal, e.g. 78 79 make qemu-x86_defconfig 80 make 81 82You will end up with one of these files depending on what you build for: 83 84 u-boot-app.efi - U-Boot EFI application 85 u-boot-payload.efi - U-Boot EFI payload application 86 87 88Trying it out 89------------- 90QEMU is an emulator and it can emulate an x86 machine. Please make sure your 91QEMU version is 2.3.0 or above to test this. You can run the payload with 92something like this: 93 94 mkdir /tmp/efi 95 cp /path/to/u-boot*.efi /tmp/efi 96 qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/ 97 98Add -nographic if you want to use the terminal for output. Once it starts 99type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to 100run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a 101prebuilt EFI BIOS for QEMU or you can build one from source as well. 102 103To try it on real hardware, put u-boot-app.efi on a suitable boot medium, 104such as a USB stick. Then you can type something like this to start it: 105 106 fs0:u-boot-payload.efi 107 108(or fs0:u-boot-app.efi for the application) 109 110This will start the payload, copy U-Boot into RAM and start U-Boot. Note 111that EFI does not support booting a 64-bit application from a 32-bit 112EFI (or vice versa). Also it will often fail to print an error message if 113you get this wrong. 114 115 116Inner workings 117============== 118Here follow a few implementation notes for those who want to fiddle with 119this and perhaps contribute patches. 120 121The application and payload approaches sound similar but are in fact 122implemented completely differently. 123 124EFI Application 125--------------- 126For the application the whole of U-Boot is built as a shared library. The 127efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI 128functions with efi_init(), sets up U-Boot global_data, allocates memory for 129U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f() 130and board_init_r()). 131 132Since U-Boot limits its memory access to the allocated regions very little 133special code is needed. The CONFIG_EFI_APP option controls a few things 134that need to change so 'git grep CONFIG_EFI_APP' may be instructive. 135The CONFIG_EFI option controls more general EFI adjustments. 136 137The only available driver is the serial driver. This calls back into EFI 138'boot services' to send and receive characters. Although it is implemented 139as a serial driver the console device is not necessarilly serial. If you 140boot EFI with video output then the 'serial' device will operate on your 141target devices's display instead and the device's USB keyboard will also 142work if connected. If you have both serial and video output, then both 143consoles will be active. Even though U-Boot does the same thing normally, 144These are features of EFI, not U-Boot. 145 146Very little code is involved in implementing the EFI application feature. 147U-Boot is highly portable. Most of the difficulty is in modifying the 148Makefile settings to pass the right build flags. In particular there is very 149little x86-specific code involved - you can find most of it in 150arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave 151enough) should be straightforward. 152 153Use the 'reset' command to get back to EFI. 154 155EFI Payload 156----------- 157The payload approach is a different kettle of fish. It works by building 158U-Boot exactly as normal for your target board, then adding the entire 159image (including device tree) into a small EFI stub application responsible 160for booting it. The stub application is built as a normal EFI application 161except that it has a lot of data attached to it. 162 163The stub application is implemented in lib/efi/efi_stub.c. The efi_main() 164function is called by EFI. It is responsible for copying U-Boot from its 165original location into memory, disabling EFI boot services and starting 166U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc. 167 168The stub application is architecture-dependent. At present it has some 169x86-specific code and a comment at the top of efi_stub.c describes this. 170 171While the stub application does allocate some memory from EFI this is not 172used by U-Boot (the payload). In fact when U-Boot starts it has all of the 173memory available to it and can operate as it pleases (but see the next 174section). 175 176Tables 177------ 178The payload can pass information to U-Boot in the form of EFI tables. At 179present this feature is used to pass the EFI memory map, an inordinately 180large list of memory regions. You can use the 'efi mem all' command to 181display this list. U-Boot uses the list to work out where to relocate 182itself. 183 184Although U-Boot can use any memory it likes, EFI marks some memory as used 185by 'run-time services', code that hangs around while U-Boot is running and 186is even present when Linux is running. This is common on x86 and provides 187a way for Linux to call back into the firmware to control things like CPU 188fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It 189will relocate itself to the top of the largest block of memory it can find 190below 4GB. 191 192Interrupts 193---------- 194U-Boot drivers typically don't use interrupts. Since EFI enables interrupts 195it is possible that an interrupt will fire that U-Boot cannot handle. This 196seems to cause problems. For this reason the U-Boot payload runs with 197interrupts disabled at present. 198 19932/64-bit 200--------- 201While the EFI application can in principle be built as either 32- or 64-bit, 202only 32-bit is currently supported. This means that the application can only 203be used with 32-bit EFI. 204 205The payload stub can be build as either 32- or 64-bits. Only a small amount 206of code is built this way (see the extra- line in lib/efi/Makefile). 207Everything else is built as a normal U-Boot, so is always 32-bit on x86 at 208present. 209 210Future work 211----------- 212This work could be extended in a number of ways: 213 214- Add a generic x86 EFI payload configuration. At present you need to modify 215an existing one, but mostly the low-level x86 code is disabled when booting 216on EFI anyway, so a generic 'EFI' board could be created with a suitable set 217of drivers enabled. 218 219- Add ARM support 220 221- Add 64-bit application support 222 223- Figure out how to solve the interrupt problem 224 225- Add more drivers to the application side (e.g. video, block devices, USB, 226environment access). This would mostly be an academic exercise as a strong 227use case is not readily apparent, but it might be fun. 228 229- Avoid turning off boot services in the stub. Instead allow U-Boot to make 230use of boot services in case it wants to. It is unclear what it might want 231though. 232 233Where is the code? 234------------------ 235lib/efi 236 payload stub, application, support code. Mostly arch-neutral 237 238arch/x86/lib/efi 239 helper functions for the fake DRAM init, etc. These can be used by 240 any board that runs as a payload. 241 242arch/x86/cpu/efi 243 x86 support code for running as an EFI application 244 245board/efi/efi-x86/efi.c 246 x86 board code for running as an EFI application 247 248common/cmd_efi.c 249 the 'efi' command 250 251-- 252Ben Stoltz, Simon Glass 253Google, Inc 254July 2015 255 256[1] http://www.qemu.org 257[2] http://www.tianocore.org/ovmf/ 258