u-boot_on_efi.rst 8.9 KB

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  1. .. SPDX-License-Identifier: GPL-2.0+
  2. .. Copyright (C) 2015 Google, Inc
  3. U-Boot on EFI
  4. =============
  5. This document provides information about U-Boot running on top of EFI, either
  6. as an application or just as a means of getting U-Boot onto a new platform.
  7. Motivation
  8. ----------
  9. Running U-Boot on EFI is useful in several situations:
  10. - You have EFI running on a board but U-Boot does not natively support it
  11. fully yet. You can boot into U-Boot from EFI and use that until U-Boot is
  12. fully ported
  13. - You need to use an EFI implementation (e.g. UEFI) because your vendor
  14. requires it in order to provide support
  15. - You plan to use coreboot to boot into U-Boot but coreboot support does
  16. not currently exist for your platform. In the meantime you can use U-Boot
  17. on EFI and then move to U-Boot on coreboot when ready
  18. - You use EFI but want to experiment with a simpler alternative like U-Boot
  19. Status
  20. ------
  21. Only x86 is supported at present. If you are using EFI on another architecture
  22. you may want to reconsider. However, much of the code is generic so could be
  23. ported.
  24. U-Boot supports running as an EFI application for 32-bit EFI only. This is
  25. not very useful since only a serial port is provided. You can look around at
  26. memory and type 'help' but that is about it.
  27. More usefully, U-Boot supports building itself as a payload for either 32-bit
  28. or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once
  29. started, U-Boot changes to 32-bit mode (currently) and takes over the
  30. machine. You can use devices, boot a kernel, etc.
  31. Build Instructions
  32. ------------------
  33. First choose a board that has EFI support and obtain an EFI implementation
  34. for that board. It will be either 32-bit or 64-bit. Alternatively, you can
  35. opt for using QEMU [1] and the OVMF [2], as detailed below.
  36. To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI
  37. and CONFIG_EFI_APP. The efi-x86_app config (efi-x86_app_defconfig) is set up
  38. for this. Just build U-Boot as normal, e.g.::
  39. make efi-x86_app_defconfig
  40. make
  41. To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), enable
  42. CONFIG_EFI, CONFIG_EFI_STUB, and select either CONFIG_EFI_STUB_32BIT or
  43. CONFIG_EFI_STUB_64BIT. The efi-x86_payload configs (efi-x86_payload32_defconfig
  44. and efi-x86_payload32_defconfig) are set up for this. Then build U-Boot as
  45. normal, e.g.::
  46. make efi-x86_payload32_defconfig (or efi-x86_payload64_defconfig)
  47. make
  48. You will end up with one of these files depending on what you build for:
  49. * u-boot-app.efi - U-Boot EFI application
  50. * u-boot-payload.efi - U-Boot EFI payload application
  51. Trying it out
  52. -------------
  53. QEMU is an emulator and it can emulate an x86 machine. Please make sure your
  54. QEMU version is 2.3.0 or above to test this. You can run the payload with
  55. something like this::
  56. mkdir /tmp/efi
  57. cp /path/to/u-boot*.efi /tmp/efi
  58. qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/
  59. Add -nographic if you want to use the terminal for output. Once it starts
  60. type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to
  61. run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a
  62. prebuilt EFI BIOS for QEMU or you can build one from source as well.
  63. To try it on real hardware, put u-boot-app.efi on a suitable boot medium,
  64. such as a USB stick. Then you can type something like this to start it::
  65. fs0:u-boot-payload.efi
  66. (or fs0:u-boot-app.efi for the application)
  67. This will start the payload, copy U-Boot into RAM and start U-Boot. Note
  68. that EFI does not support booting a 64-bit application from a 32-bit
  69. EFI (or vice versa). Also it will often fail to print an error message if
  70. you get this wrong.
  71. Inner workings
  72. --------------
  73. Here follow a few implementation notes for those who want to fiddle with
  74. this and perhaps contribute patches.
  75. The application and payload approaches sound similar but are in fact
  76. implemented completely differently.
  77. EFI Application
  78. ~~~~~~~~~~~~~~~
  79. For the application the whole of U-Boot is built as a shared library. The
  80. efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI
  81. functions with efi_init(), sets up U-Boot global_data, allocates memory for
  82. U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f()
  83. and board_init_r()).
  84. Since U-Boot limits its memory access to the allocated regions very little
  85. special code is needed. The CONFIG_EFI_APP option controls a few things
  86. that need to change so 'git grep CONFIG_EFI_APP' may be instructive.
  87. The CONFIG_EFI option controls more general EFI adjustments.
  88. The only available driver is the serial driver. This calls back into EFI
  89. 'boot services' to send and receive characters. Although it is implemented
  90. as a serial driver the console device is not necessarilly serial. If you
  91. boot EFI with video output then the 'serial' device will operate on your
  92. target devices's display instead and the device's USB keyboard will also
  93. work if connected. If you have both serial and video output, then both
  94. consoles will be active. Even though U-Boot does the same thing normally,
  95. These are features of EFI, not U-Boot.
  96. Very little code is involved in implementing the EFI application feature.
  97. U-Boot is highly portable. Most of the difficulty is in modifying the
  98. Makefile settings to pass the right build flags. In particular there is very
  99. little x86-specific code involved - you can find most of it in
  100. arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave
  101. enough) should be straightforward.
  102. Use the 'reset' command to get back to EFI.
  103. EFI Payload
  104. ~~~~~~~~~~~
  105. The payload approach is a different kettle of fish. It works by building
  106. U-Boot exactly as normal for your target board, then adding the entire
  107. image (including device tree) into a small EFI stub application responsible
  108. for booting it. The stub application is built as a normal EFI application
  109. except that it has a lot of data attached to it.
  110. The stub application is implemented in lib/efi/efi_stub.c. The efi_main()
  111. function is called by EFI. It is responsible for copying U-Boot from its
  112. original location into memory, disabling EFI boot services and starting
  113. U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc.
  114. The stub application is architecture-dependent. At present it has some
  115. x86-specific code and a comment at the top of efi_stub.c describes this.
  116. While the stub application does allocate some memory from EFI this is not
  117. used by U-Boot (the payload). In fact when U-Boot starts it has all of the
  118. memory available to it and can operate as it pleases (but see the next
  119. section).
  120. Tables
  121. ~~~~~~
  122. The payload can pass information to U-Boot in the form of EFI tables. At
  123. present this feature is used to pass the EFI memory map, an inordinately
  124. large list of memory regions. You can use the 'efi mem all' command to
  125. display this list. U-Boot uses the list to work out where to relocate
  126. itself.
  127. Although U-Boot can use any memory it likes, EFI marks some memory as used
  128. by 'run-time services', code that hangs around while U-Boot is running and
  129. is even present when Linux is running. This is common on x86 and provides
  130. a way for Linux to call back into the firmware to control things like CPU
  131. fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It
  132. will relocate itself to the top of the largest block of memory it can find
  133. below 4GB.
  134. Interrupts
  135. ~~~~~~~~~~
  136. U-Boot drivers typically don't use interrupts. Since EFI enables interrupts
  137. it is possible that an interrupt will fire that U-Boot cannot handle. This
  138. seems to cause problems. For this reason the U-Boot payload runs with
  139. interrupts disabled at present.
  140. 32/64-bit
  141. ~~~~~~~~~
  142. While the EFI application can in principle be built as either 32- or 64-bit,
  143. only 32-bit is currently supported. This means that the application can only
  144. be used with 32-bit EFI.
  145. The payload stub can be build as either 32- or 64-bits. Only a small amount
  146. of code is built this way (see the extra- line in lib/efi/Makefile).
  147. Everything else is built as a normal U-Boot, so is always 32-bit on x86 at
  148. present.
  149. Future work
  150. -----------
  151. This work could be extended in a number of ways:
  152. - Add ARM support
  153. - Add 64-bit application support
  154. - Figure out how to solve the interrupt problem
  155. - Add more drivers to the application side (e.g. video, block devices, USB,
  156. environment access). This would mostly be an academic exercise as a strong
  157. use case is not readily apparent, but it might be fun.
  158. - Avoid turning off boot services in the stub. Instead allow U-Boot to make
  159. use of boot services in case it wants to. It is unclear what it might want
  160. though.
  161. Where is the code?
  162. ------------------
  163. lib/efi
  164. payload stub, application, support code. Mostly arch-neutral
  165. arch/x86/cpu/efi
  166. x86 support code for running as an EFI application and payload
  167. board/efi/efi-x86_app/efi.c
  168. x86 board code for running as an EFI application
  169. board/efi/efi-x86_payload
  170. generic x86 EFI payload board support code
  171. common/cmd_efi.c
  172. the 'efi' command
  173. --
  174. Ben Stoltz, Simon Glass
  175. Google, Inc
  176. July 2015
  177. * [1] http://www.qemu.org
  178. * [2] http://www.tianocore.org/ovmf/