efi: Add a README to explain how things work

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>

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_EFI_APP. 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-app.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-app.efi' to
+run the application. 'bios.bin' is the EFI 'BIOS'.
+
+To try it on real hardware, put u-boot-app.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-app.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_EFI_APP option controls a few things
+that need to change so 'git grep CONFIG_EFI_APP' 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