1.. SPDX-License-Identifier: GPL-2.0+ 2.. Copyright (C) 2015 Google, Inc 3 4U-Boot on EFI 5============= 6This document provides information about U-Boot running on top of EFI, either 7as an application or just as a means of getting U-Boot onto a new platform. 8 9 10Motivation 11---------- 12Running U-Boot on EFI is useful in several situations: 13 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 16 fully ported 17 18- You need to use an EFI implementation (e.g. UEFI) because your vendor 19 requires it in order to provide support 20 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 24 25- You use EFI but want to experiment with a simpler alternative like U-Boot 26 27 28Status 29------ 30Only x86 is supported at present. If you are using EFI on another architecture 31you may want to reconsider. However, much of the code is generic so could be 32ported. 33 34U-Boot supports running as an EFI application for 32-bit EFI only. This is 35not very useful since only a serial port is provided. You can look around at 36memory and type 'help' but that is about it. 37 38More usefully, U-Boot supports building itself as a payload for either 32-bit 39or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once 40started, U-Boot changes to 32-bit mode (currently) and takes over the 41machine. You can use devices, boot a kernel, etc. 42 43 44Build Instructions 45------------------ 46First choose a board that has EFI support and obtain an EFI implementation 47for that board. It will be either 32-bit or 64-bit. Alternatively, you can 48opt for using QEMU [1] and the OVMF [2], as detailed below. 49 50To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI 51and CONFIG_EFI_APP. The efi-x86_app config (efi-x86_app_defconfig) is set up 52for this. Just build U-Boot as normal, e.g.:: 53 54 make efi-x86_app_defconfig 55 make 56 57To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), enable 58CONFIG_EFI, CONFIG_EFI_STUB, and select either CONFIG_EFI_STUB_32BIT or 59CONFIG_EFI_STUB_64BIT. The efi-x86_payload configs (efi-x86_payload32_defconfig 60and efi-x86_payload32_defconfig) are set up for this. Then build U-Boot as 61normal, e.g.:: 62 63 make efi-x86_payload32_defconfig (or efi-x86_payload64_defconfig) 64 make 65 66You will end up with one of these files depending on what you build for: 67 68* u-boot-app.efi - U-Boot EFI application 69* u-boot-payload.efi - U-Boot EFI payload application 70 71 72Trying it out 73------------- 74QEMU is an emulator and it can emulate an x86 machine. Please make sure your 75QEMU version is 6.0.0 or above to test this. You can run the payload with 76something like this:: 77 78 mkdir /tmp/efi 79 cp /path/to/u-boot*.efi /tmp/efi 80 qemu-system-x86_64 -pflash edk2-x86_64-code.fd -hda fat:rw:/tmp/efi/ 81 82Add -nographic if you want to use the terminal for output. Once it starts 83type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to 84run the application. 'edk2-x86_64-code.fd' is the EFI 'BIOS'. QEMU already 85ships both 32-bit and 64-bit EFI BIOS images. For 32-bit EFI 'BIOS' image, 86use 'edk2-i386-code.fd'. 87 88 89To try it on real hardware, put u-boot-app.efi on a suitable boot medium, 90such as a USB stick. Then you can type something like this to start it:: 91 92 fs0:u-boot-payload.efi 93 94(or fs0:u-boot-app.efi for the application) 95 96This will start the payload, copy U-Boot into RAM and start U-Boot. Note 97that EFI does not support booting a 64-bit application from a 32-bit 98EFI (or vice versa). Also it will often fail to print an error message if 99you get this wrong. 100 101 102Inner workings 103-------------- 104Here follow a few implementation notes for those who want to fiddle with 105this and perhaps contribute patches. 106 107The application and payload approaches sound similar but are in fact 108implemented completely differently. 109 110EFI Application 111~~~~~~~~~~~~~~~ 112For the application the whole of U-Boot is built as a shared library. The 113efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI 114functions with efi_init(), sets up U-Boot global_data, allocates memory for 115U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f() 116and board_init_r()). 117 118Since U-Boot limits its memory access to the allocated regions very little 119special code is needed. The CONFIG_EFI_APP option controls a few things 120that need to change so 'git grep CONFIG_EFI_APP' may be instructive. 121The CONFIG_EFI option controls more general EFI adjustments. 122 123The only available driver is the serial driver. This calls back into EFI 124'boot services' to send and receive characters. Although it is implemented 125as a serial driver the console device is not necessarilly serial. If you 126boot EFI with video output then the 'serial' device will operate on your 127target devices's display instead and the device's USB keyboard will also 128work if connected. If you have both serial and video output, then both 129consoles will be active. Even though U-Boot does the same thing normally, 130These are features of EFI, not U-Boot. 131 132Very little code is involved in implementing the EFI application feature. 133U-Boot is highly portable. Most of the difficulty is in modifying the 134Makefile settings to pass the right build flags. In particular there is very 135little x86-specific code involved - you can find most of it in 136arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave 137enough) should be straightforward. 138 139Use the 'reset' command to get back to EFI. 140 141EFI Payload 142~~~~~~~~~~~ 143The payload approach is a different kettle of fish. It works by building 144U-Boot exactly as normal for your target board, then adding the entire 145image (including device tree) into a small EFI stub application responsible 146for booting it. The stub application is built as a normal EFI application 147except that it has a lot of data attached to it. 148 149The stub application is implemented in lib/efi/efi_stub.c. The efi_main() 150function is called by EFI. It is responsible for copying U-Boot from its 151original location into memory, disabling EFI boot services and starting 152U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc. 153 154The stub application is architecture-dependent. At present it has some 155x86-specific code and a comment at the top of efi_stub.c describes this. 156 157While the stub application does allocate some memory from EFI this is not 158used by U-Boot (the payload). In fact when U-Boot starts it has all of the 159memory available to it and can operate as it pleases (but see the next 160section). 161 162Tables 163~~~~~~ 164The payload can pass information to U-Boot in the form of EFI tables. At 165present this feature is used to pass the EFI memory map, an inordinately 166large list of memory regions. You can use the 'efi mem all' command to 167display this list. U-Boot uses the list to work out where to relocate 168itself. 169 170Although U-Boot can use any memory it likes, EFI marks some memory as used 171by 'run-time services', code that hangs around while U-Boot is running and 172is even present when Linux is running. This is common on x86 and provides 173a way for Linux to call back into the firmware to control things like CPU 174fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It 175will relocate itself to the top of the largest block of memory it can find 176below 4GB. 177 178Interrupts 179~~~~~~~~~~ 180U-Boot drivers typically don't use interrupts. Since EFI enables interrupts 181it is possible that an interrupt will fire that U-Boot cannot handle. This 182seems to cause problems. For this reason the U-Boot payload runs with 183interrupts disabled at present. 184 18532/64-bit 186~~~~~~~~~ 187While the EFI application can in principle be built as either 32- or 64-bit, 188only 32-bit is currently supported. This means that the application can only 189be used with 32-bit EFI. 190 191The payload stub can be build as either 32- or 64-bits. Only a small amount 192of code is built this way (see the extra- line in lib/efi/Makefile). 193Everything else is built as a normal U-Boot, so is always 32-bit on x86 at 194present. 195 196Future work 197----------- 198This work could be extended in a number of ways: 199 200- Add ARM support 201 202- Add 64-bit application support 203 204- Figure out how to solve the interrupt problem 205 206- Add more drivers to the application side (e.g. video, block devices, USB, 207 environment access). This would mostly be an academic exercise as a strong 208 use case is not readily apparent, but it might be fun. 209 210- Avoid turning off boot services in the stub. Instead allow U-Boot to make 211 use of boot services in case it wants to. It is unclear what it might want 212 though. 213 214Where is the code? 215------------------ 216lib/efi 217 payload stub, application, support code. Mostly arch-neutral 218 219arch/x86/cpu/efi 220 x86 support code for running as an EFI application and payload 221 222board/efi/efi-x86_app/efi.c 223 x86 board code for running as an EFI application 224 225board/efi/efi-x86_payload 226 generic x86 EFI payload board support code 227 228common/cmd_efi.c 229 the 'efi' command 230 231-- 232Ben Stoltz, Simon Glass 233Google, Inc 234July 2015 235 236* [1] http://www.qemu.org 237* [2] https://github.com/tianocore/tianocore.github.io/wiki/OVMF 238