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 ---
doc/README.efi | 237 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 237 insertions(+) create mode 100644 doc/README.efi
diff --git a/doc/README.efi b/doc/README.efi new file mode 100644 index 0000000..4706602 --- /dev/null +++ b/doc/README.efi @@ -0,0 +1,237 @@ +# +# Copyright (C) 2015 Google, Inc +# +# SPDX-License-Identifier: GPL-2.0+ +# + +U-Boot on EFI +============= +This document provides information about U-Boot running on top of EFI, either +as an application or just as a means of getting U-Boot onto a new platform. + + +In God's Name, Why? +------------------- +This is useful in several situations: + +- You have EFI running on a board but U-Boot does not natively support it +fully yet. You can boot into U-Boot from EFI and use that until U-Boot is +fully ported + +- You need to use an EFI implementation (e.g. UEFI) because your vendor +requires it in order to provide support + +- You plan to use Coreboot to boot into U-Boot but Coreboot support does +not currently exist for your platform. In the meantime you can use U-Boot +on EFI and then move to U-Boot on Coreboot when ready + +- You use EFI but want to experiment with a simpler alternative like U-Boot + + +Status +------ +Only x86 is supported at present. If you are using EFI on another +architecture you may want to reconsider. However, much of the code is generic +so could be ported. + +U-Boot supports running as an EFI application for 32-bit EFI only. This is +not very useful since only a serial port is provided. You can look around at +memory and type 'help' but that is about it. + +More usefully, U-Boot supports building itself as a payload for either 32-bit +or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once +started, U-Boot changes to 32-bit mode (currently) and takes over the +machine. You can use devices, boot a kernel, etc. + + +Build Instructions +------------------ +First choose a board that has EFI support and obtain an EFI implementation +for that board. It will be either 32-bit or 64-bit. + +To build U-Boot as an EFI application (32-bit EFI required), enable +CONFIG_EFI and CONFIG_ARCH_EFI. The efi-x86 config is set up for this. + +To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), adjust +an existing config to enable CONFIG_EFI, CONFIG_EFI_STUB and either +CONFIG_EFI_STUB_32BIT or CONFIG_EFI_STUB_64BIT. + +Then build U-Boot as normal, e.g. + + make qemu-x86_defconfig + make menuconfig (or make xconfig if you prefer) + # change the settings as above + make + +You will end up with one of these files: + + u-boot.efi - U-Boot EFI application + u-boot-payload.efi - U-Boot EFI payload application + + +Trying it out +------------- +Qemu is an emulator and it can emulate an x86 machine. You can run the +payload with something like this: + + mkdir /tmp/efi + cp /path/to/u-boot*.efi /tmp/efi + qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/ + +Add -nographic if you want to use the terminal for output. Once it starts +type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot.efi' to run +the application. 'bios.bin' is the EFI 'BIOS'. + +To try it on real hardware, put u-boot.efi on a suitable boot medium, such +as a USB stick. Then you can type something like this to start it: + + fs0:u-boot-payload.efi + +(or fs0:u-boot.efi for the application) + +This will start the payload, copy U-Boot into RAM and start U-Boot. Note +that EFI does not support booting a 64-bit application from a 32-bit +EFI (or vice versa). Also it will often fail to print an error message if +you get this wrong. + + +Inner workings +============== +Here follow a few implementation notes for those who want to fiddle with +this and perhaps contribute patches. + +The application and payload approaches sound similar but are in fact +implemented completely differently. + +EFI Application +--------------- +For the application the whole of U-Boot is built as a shared library. The +efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI +functions with efi_init(), sets up U-Boot global_data, allocates memory for +U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f() +and board_init_r()). + +Since U-Boot limits its memory access to the allocated regions very little +special code is needed. The CONFIG_ARCH_EFI option controls a few things +that need to change so 'git grep CONFIG_ARCH_EFI' may be instructive. +The CONFIG_EFI option controls more general EFI adjustments. + +The only available driver is the serial driver. This calls back into EFI +'boot services' to send and receive characters. Although it is implemented +as a serial driver the console device is not necessarilly serial. If you +boot EFI with video output then the 'serial' device will operate on your +target devices's display instead and the device's USB keyboard will also +work if connected. If you have both serial and video output, then both +consoles will be active. Even though U-Boot does the same thing normally, +These are features of EFI, not U-Boot. + +Very little code is involved in implementing the EFI application feature. +U-Boot is highly portable. Most of the difficulty is in modifying the +Makefile settings to pass the right build flags. In particular there is very +little x86-specific code involved - you can find most of it in +arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave +enough) should be straightforward. + +Use the 'reset' command to get back to EFI. + +EFI Payload +----------- +The payload approach is a different kettle of fish. It works by building +U-Boot exactly as normal for your target board, then adding the entire +image (including device tree) into a small EFI stub application responsible +for booting it. The stub application is built as a normal EFI application +except that it has a lot of data attached to it. + +The stub application is implemented in lib/efi/efi_stub.c. The efi_main() +function is called by EFI. It is responsible for copying U-Boot from its +original location into memory, disabling EFI boot services and starting +U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc. + +The stub application is architecture-dependent. At present it has some +x86-specific code and a comment at the top of efi_stub.c describes this. + +While the stub application does allocate some memory from EFI this is not +used by U-Boot (the payload). In fact when U-Boot starts it has all of the +memory available to it and can operate as it pleases (but see the next +section). + +Tables +------ +The payload can pass information to U-Boot in the form of EFI tables. At +present this feature is used to pass the EFI memory map, an inordinately +large list of memory regions. You can use the 'efi mem all' command to +display this list. U-Boot uses the list to work out where to relocate +itself. + +Although U-Boot can use any memory it likes, EFI marks some memory as used +by 'run-time services', code that hangs around while U-Boot is running and +is even present when Linux is running. This is common on x86 and provides +a way for Linux to call back into the firmware to control things like CPU +fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It +will relocate itself to the top of the largest block of memory it can find +below 4GB. + +Interrupts +---------- +U-Boot drivers typically don't use interrupts. Since EFI enables interrupts +it is possible that an interrupt will fire that U-Boot cannot handle. This +seems to cause problems. For this reason the U-Boot payload runs with +interrupts disabled at present. + +32/64-bit +--------- +While the EFI application can in principle be built as either 32- or 64-bit, +only 32-bit is currently supported. This means that the application can only +be used with 32-bit EFI. + +The payload stub can be build as either 32- or 64-bits. Only a small amount +of code is built this way (see the extra- line in lib/efi/Makefile). +Everything else is built as a normal U-Boot, so is always 32-bit on x86 at +present. + +Future work +----------- +This work could be extended in a number of ways: + +- Add a generic x86 EFI payload configuration. At present you need to modify +an existing one, but mostly the low-level x86 code is disabled when booting +on EFI anyway, so a generic 'EFI' board could be created with a suitable set +of drivers enabled. + +- Add ARM support + +- Add 64-bit application support + +- Figure out how to solve the interrupt problem + +- Add more drivers to the application side (e.g. video, block devices, USB, +environment access). This would mostly be an academic exercise as a strong +use case is not readily apparent, but it might be fun. + +- Avoid turning off boot services in the stub. Instead allow U-Boot to make +use of boot services in case it wants to. It is unclear what it might want +though. + +Where is the code? +------------------ +lib/efi + payload stub, application, support code. Mostly arch-neutral + +arch/x86/lib/efi + helper functions for the fake DRAM init, etc. These can be used by + any board that runs as a payload. + +arch/x86/cpu/efi + x86 support code for running as an EFI application + +board/efi/efi-x86/efi.c + x86 board code for running as an EFI application + +common/cmd_efi.c + the 'efi' command + + +-- +Ben Stoltz, Simon Glass +Google, Inc +July 2015