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diff --git a/doc/README.generic-board b/doc/README.generic-board new file mode 100644 index 00000000000..50d3a26849d --- /dev/null +++ b/doc/README.generic-board @@ -0,0 +1,189 @@ +# +# (C) Copyright 2014 Google, Inc +# Simon Glass <sjg@chromium.org> +# +# SPDX-License-Identifier: GPL-2.0+ +# + +DEPRECATION NOTICE FOR arch/<arch>/lib/board.c + +For board maintainers: Please submit patches for boards you maintain before +July 2014, to make them use generic board. + +For architecture maintainers: Please submit patches to remove your +architecture-specific board.c file before October 2014. + + +Background +---------- + +U-Boot has tranditionally had a board.c file for each architecture. This has +introduced quite a lot of duplication, with each architecture tending to do +initialisation slightly differently. To address this, a new 'generic board +init' feature was introduced a year ago in March 2013 (further motivation is +provided in the cover letter below). + + +What has changed? +----------------- + +The main change is that the arch/<arch>/lib/board.c file is being removed in +favour of common/board_f.c (for pre-relocation init) and common/board_r.c +(for post-relocation init). + +Related to this, the global_data and bd_t structures now have a core set of +fields which are common to all architectures. Architecture-specific fields +have been moved to separate structures. + + +Supported Arcthitectures +------------------------ + +If you are unlucky then your architecture may not support generic board. +The following architectures are supported at the time of writing: + + arc + arm + powerpc + sandbox + x86 + +If your architecture is not supported, you need to adjust your +arch/<arch>/config.mk file to include: + + __HAVE_ARCH_GENERIC_BOARD := y + +and test it with a suitable board, as follows. + + +Adding Support for your Board +----------------------------- + +To enable generic board for your board, define CONFIG_SYS_GENERIC_BOARD in +your board config header file. + +Test that U-Boot still functions correctly on your board, and fix any +problems you find. Don't be surprised if there are no problems - generic +board has had a reasonable amount of testing with common boards. + + +DeadLine +-------- + +Please don't take this the wrong way - there is no intent to make your life +miserable, and we have the greatest respect and admiration for U-Boot users. +However, with any migration there has to be a period where the old way is +deprecated and removed. Every patch to the deprecated code introduces a +potential breakage in the new unused code. Therefore: + +Boards or architectures not converted over to general board by the +end of 2014 may be forcibly changed over (potentially causing run-time +breakage) or removed. + + + +Further Background +------------------ + +The full text of the original generic board series is reproduced below. + +--8<------------- + +This series creates a generic board.c implementation which contains +the essential functions of the major arch/xxx/lib/board.c files. + +What is the motivation for this change? + +1. There is a lot of repeated code in the board.c files. Any change to +things like setting up the baud rate requires a change in 10 separate +places. + +2. Since there are 10 separate files, adding a new feature which requires +initialisation is painful since it must be independently added in 10 +places. + +3. As time goes by the architectures naturely diverge since there is limited +pressure to compare features or even CONFIG options against simiilar things +in other board.c files. + +4. New architectures must implement all the features all over again, and +sometimes in subtley different ways. This places an unfair burden on getting +a new architecture fully functional and running with U-Boot. + +5. While it is a bit of a tricky change, I believe it is worthwhile and +achievable. There is no requirement that all code be common, only that +the code that is common should be located in common/board.c rather than +arch/xxx/lib/board.c. + +All the functions of board_init_f() and board_init_r() are broken into +separate function calls so that they can easily be included or excluded +for a particular architecture. It also makes it easier to adopt Graeme's +initcall proposal when it is ready. + +http://lists.denx.de/pipermail/u-boot/2012-January/114499.html + +This series removes the dependency on generic relocation. So relocation +happens as one big chunk and is still completely arch-specific. See the +relocation series for a proposed solution to this for ARM: + +http://lists.denx.de/pipermail/u-boot/2011-December/112928.html + +or Graeme's recent x86 series v2: + +http://lists.denx.de/pipermail/u-boot/2012-January/114467.html + +Instead of moving over a whole architecture, this series takes the approach +of simply enabling generic board support for an architecture. It is then up +to each board to opt in by defining CONFIG_SYS_GENERIC_BOARD in the board +config file. If this is not done, then the code will be generated as +before. This allows both sets of code to co-exist until we are comfortable +with the generic approach, and enough boards run. + +ARM is a relatively large board.c file and one which I can test, therefore +I think it is a good target for this series. On the other hand, x86 is +relatively small and simple, but different enough that it introduces a +few issues to be solved. So I have chosen both ARM and x86 for this series. +After a suggestion from Wolfgang I have added PPC also. This is the +largest and most feature-full board, so hopefully we have all bases +covered in this RFC. + +A generic global_data structure is also required. This might upset a few +people. Here is my basic reasoning: most fields are the same, all +architectures include and need it, most global_data.h files already have +#ifdefs to select fields for a particular SOC, so it is hard to +see why architecures are different in this area. We can perhaps add a +way to put architecture-specific fields into a separate header file, but +for now I have judged that to be counter-productive. + +Similarly we need a generic bd_info structure, since generic code will +be accessing it. I have done this in the same way as global_data and the +same comments apply. + +There was dicussion on the list about passing gd_t around as a parameter +to pre-relocation init functions. I think this makes sense, but it can +be done as a separate change, and this series does not require it. + +While this series needs to stand on its own (as with the link script +cleanup series and the generic relocation series) the goal is the +unification of the board init code. So I hope we can address issues with +this in mind, rather than focusing too narrowly on particular ARM, x86 or +PPC issues. + +I have run-tested ARM on Tegra Seaboard only. To try it out, define +CONFIG_SYS_GENERIC_BOARD in your board file and rebuild. Most likely on +x86 and PPC at least it will hang, but if you are lucky it will print +something first :-) + +I have run this though MAKEALL with CONFIG_SYS_GENERIC_BOARD on for all +ARM, PPC and x86 boards. There are a few failures due to errors in +the board config, which I have sent patches for. The main issue is +just the difference between __bss_end and __bss_end__. + +Note: the first group of commits are required for this series to build, +but could be separated out if required. I have included them here for +convenience. + +------------->8-- + +Simon Glass, sjg@chromium.org +March 2014 |