Start the deprecation process for generic board

We should move forward to remove the old board init code. Add a
prominent message to encourage maintainers to get started on this
work.

Signed-off-by: Simon Glass <sjg@chromium.org>

1 files changed, 189 insertions, 0 deletions

diff --git a/doc/README.generic-board b/doc/README.generic-board
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+#
+# (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