Add some documentation on the EFI implementation in U-Boot. Signed-off-by: Ben Stoltz <stoltz@google.com> Signed-off-by: Simon Glass <sjg@chromium.org> Reviewed-by: Bin Meng <bmeng.cn@gmail.com>master
Simon Glass
10 years ago
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# |
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# Copyright (C) 2015 Google, Inc |
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# |
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# SPDX-License-Identifier: GPL-2.0+ |
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# |
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U-Boot on EFI |
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============= |
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This document provides information about U-Boot running on top of EFI, either |
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as an application or just as a means of getting U-Boot onto a new platform. |
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In God's Name, Why? |
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------------------- |
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This is useful in several situations: |
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- You have EFI running on a board but U-Boot does not natively support it |
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fully yet. You can boot into U-Boot from EFI and use that until U-Boot is |
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fully ported |
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- You need to use an EFI implementation (e.g. UEFI) because your vendor |
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requires it in order to provide support |
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- You plan to use coreboot to boot into U-Boot but coreboot support does |
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not currently exist for your platform. In the meantime you can use U-Boot |
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on EFI and then move to U-Boot on coreboot when ready |
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- You use EFI but want to experiment with a simpler alternative like U-Boot |
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Status |
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------ |
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Only x86 is supported at present. If you are using EFI on another architecture |
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you may want to reconsider. However, much of the code is generic so could be |
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ported. |
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U-Boot supports running as an EFI application for 32-bit EFI only. This is |
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not very useful since only a serial port is provided. You can look around at |
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memory and type 'help' but that is about it. |
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More usefully, U-Boot supports building itself as a payload for either 32-bit |
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or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once |
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started, U-Boot changes to 32-bit mode (currently) and takes over the |
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machine. You can use devices, boot a kernel, etc. |
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Build Instructions |
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------------------ |
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First choose a board that has EFI support and obtain an EFI implementation |
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for that board. It will be either 32-bit or 64-bit. |
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To build U-Boot as an EFI application (32-bit EFI required), enable |
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CONFIG_EFI and CONFIG_EFI_APP. The efi-x86 config is set up for this. |
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To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), adjust |
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an existing config to enable CONFIG_EFI, CONFIG_EFI_STUB and either |
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CONFIG_EFI_STUB_32BIT or CONFIG_EFI_STUB_64BIT. |
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Then build U-Boot as normal, e.g. |
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make qemu-x86_defconfig |
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make menuconfig (or make xconfig if you prefer) |
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# change the settings as above |
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make |
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You will end up with one of these files: |
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u-boot-app.efi - U-Boot EFI application |
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u-boot-payload.efi - U-Boot EFI payload application |
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Trying it out |
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------------- |
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Qemu is an emulator and it can emulate an x86 machine. You can run the |
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payload with something like this: |
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mkdir /tmp/efi |
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cp /path/to/u-boot*.efi /tmp/efi |
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qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/ |
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Add -nographic if you want to use the terminal for output. Once it starts |
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type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to |
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run the application. 'bios.bin' is the EFI 'BIOS'. |
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To try it on real hardware, put u-boot-app.efi on a suitable boot medium, |
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such as a USB stick. Then you can type something like this to start it: |
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fs0:u-boot-payload.efi |
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(or fs0:u-boot-app.efi for the application) |
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This will start the payload, copy U-Boot into RAM and start U-Boot. Note |
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that EFI does not support booting a 64-bit application from a 32-bit |
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EFI (or vice versa). Also it will often fail to print an error message if |
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you get this wrong. |
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Inner workings |
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============== |
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Here follow a few implementation notes for those who want to fiddle with |
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this and perhaps contribute patches. |
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The application and payload approaches sound similar but are in fact |
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implemented completely differently. |
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EFI Application |
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--------------- |
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For the application the whole of U-Boot is built as a shared library. The |
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efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI |
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functions with efi_init(), sets up U-Boot global_data, allocates memory for |
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U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f() |
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and board_init_r()). |
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Since U-Boot limits its memory access to the allocated regions very little |
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special code is needed. The CONFIG_EFI_APP option controls a few things |
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that need to change so 'git grep CONFIG_EFI_APP' may be instructive. |
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The CONFIG_EFI option controls more general EFI adjustments. |
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The only available driver is the serial driver. This calls back into EFI |
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'boot services' to send and receive characters. Although it is implemented |
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as a serial driver the console device is not necessarilly serial. If you |
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boot EFI with video output then the 'serial' device will operate on your |
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target devices's display instead and the device's USB keyboard will also |
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work if connected. If you have both serial and video output, then both |
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consoles will be active. Even though U-Boot does the same thing normally, |
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These are features of EFI, not U-Boot. |
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Very little code is involved in implementing the EFI application feature. |
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U-Boot is highly portable. Most of the difficulty is in modifying the |
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Makefile settings to pass the right build flags. In particular there is very |
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little x86-specific code involved - you can find most of it in |
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arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave |
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enough) should be straightforward. |
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Use the 'reset' command to get back to EFI. |
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EFI Payload |
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----------- |
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The payload approach is a different kettle of fish. It works by building |
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U-Boot exactly as normal for your target board, then adding the entire |
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image (including device tree) into a small EFI stub application responsible |
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for booting it. The stub application is built as a normal EFI application |
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except that it has a lot of data attached to it. |
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The stub application is implemented in lib/efi/efi_stub.c. The efi_main() |
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function is called by EFI. It is responsible for copying U-Boot from its |
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original location into memory, disabling EFI boot services and starting |
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U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc. |
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The stub application is architecture-dependent. At present it has some |
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x86-specific code and a comment at the top of efi_stub.c describes this. |
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While the stub application does allocate some memory from EFI this is not |
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used by U-Boot (the payload). In fact when U-Boot starts it has all of the |
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memory available to it and can operate as it pleases (but see the next |
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section). |
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Tables |
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------ |
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The payload can pass information to U-Boot in the form of EFI tables. At |
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present this feature is used to pass the EFI memory map, an inordinately |
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large list of memory regions. You can use the 'efi mem all' command to |
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display this list. U-Boot uses the list to work out where to relocate |
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itself. |
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Although U-Boot can use any memory it likes, EFI marks some memory as used |
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by 'run-time services', code that hangs around while U-Boot is running and |
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is even present when Linux is running. This is common on x86 and provides |
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a way for Linux to call back into the firmware to control things like CPU |
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fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It |
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will relocate itself to the top of the largest block of memory it can find |
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below 4GB. |
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Interrupts |
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---------- |
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U-Boot drivers typically don't use interrupts. Since EFI enables interrupts |
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it is possible that an interrupt will fire that U-Boot cannot handle. This |
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seems to cause problems. For this reason the U-Boot payload runs with |
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interrupts disabled at present. |
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32/64-bit |
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--------- |
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While the EFI application can in principle be built as either 32- or 64-bit, |
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only 32-bit is currently supported. This means that the application can only |
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be used with 32-bit EFI. |
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The payload stub can be build as either 32- or 64-bits. Only a small amount |
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of code is built this way (see the extra- line in lib/efi/Makefile). |
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Everything else is built as a normal U-Boot, so is always 32-bit on x86 at |
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present. |
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Future work |
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----------- |
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This work could be extended in a number of ways: |
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- Add a generic x86 EFI payload configuration. At present you need to modify |
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an existing one, but mostly the low-level x86 code is disabled when booting |
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on EFI anyway, so a generic 'EFI' board could be created with a suitable set |
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of drivers enabled. |
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- Add ARM support |
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- Add 64-bit application support |
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- Figure out how to solve the interrupt problem |
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- Add more drivers to the application side (e.g. video, block devices, USB, |
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environment access). This would mostly be an academic exercise as a strong |
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use case is not readily apparent, but it might be fun. |
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- Avoid turning off boot services in the stub. Instead allow U-Boot to make |
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use of boot services in case it wants to. It is unclear what it might want |
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though. |
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Where is the code? |
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------------------ |
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lib/efi |
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payload stub, application, support code. Mostly arch-neutral |
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arch/x86/lib/efi |
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helper functions for the fake DRAM init, etc. These can be used by |
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any board that runs as a payload. |
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arch/x86/cpu/efi |
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x86 support code for running as an EFI application |
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board/efi/efi-x86/efi.c |
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x86 board code for running as an EFI application |
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common/cmd_efi.c |
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the 'efi' command |
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-- |
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Ben Stoltz, Simon Glass |
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Google, Inc |
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July 2015 |
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Reference in new issue