Adding a new board to LinuxSH
                 ================================
  
                 Paul Mundt <lethal@linux-sh.org>
  
  This document attempts to outline what steps are necessary to add support
  for new boards to the LinuxSH port under the new 2.5 and 2.6 kernels. This
  also attempts to outline some of the noticeable changes between the 2.4
  and the 2.5/2.6 SH backend.
  
  1. New Directory Structure
  ==========================
  
  The first thing to note is the new directory structure. Under 2.4, most
  of the board-specific code (with the exception of stboards) ended up
  in arch/sh/kernel/ directly, with board-specific headers ending up in
  include/asm-sh/. For the new kernel, things are broken out by board type,
  companion chip type, and CPU type. Looking at a tree view of this directory
  hierarchy looks like the following:
  
  Board-specific code:
  
  .
  |-- arch
  |   `-- sh
  |       `-- boards
  |           |-- adx
  |           |   `-- board-specific files
  |           |-- bigsur
  |           |   `-- board-specific files
  |           |
  |           ... more boards here ...
  |
  `-- include
      `-- asm-sh
          |-- adx
          |   `-- board-specific headers
          |-- bigsur
          |   `-- board-specific headers
          |
  	.. more boards here ...
  
  Next, for companion chips:
  .
  `-- arch
      `-- sh
          `-- cchips
              `-- hd6446x
                  `-- hd64461
                      `-- cchip-specific files
  
  ... and so on. Headers for the companion chips are treated the same way as
  board-specific headers. Thus, include/asm-sh/hd64461 is home to all of the
  hd64461-specific headers.
  
  Finally, CPU family support is also abstracted:
  .
  |-- arch
  |   `-- sh
  |       |-- kernel
  |       |   `-- cpu
  |       |       |-- sh2
  |       |       |   `-- SH-2 generic files
  |       |       |-- sh3
  |       |       |   `-- SH-3 generic files
  |       |       `-- sh4
  |       |           `-- SH-4 generic files
  |       `-- mm
  |           `-- This is also broken out per CPU family, so each family can
  |               have their own set of cache/tlb functions.
  |
  `-- include
      `-- asm-sh
          |-- cpu-sh2
          |   `-- SH-2 specific headers
          |-- cpu-sh3
          |   `-- SH-3 specific headers
          `-- cpu-sh4
              `-- SH-4 specific headers
  
  It should be noted that CPU subtypes are _not_ abstracted. Thus, these still
  need to be dealt with by the CPU family specific code.
  
  2. Adding a New Board
  =====================
  
  The first thing to determine is whether the board you are adding will be
  isolated, or whether it will be part of a family of boards that can mostly
  share the same board-specific code with minor differences.
  
  In the first case, this is just a matter of making a directory for your
  board in arch/sh/boards/ and adding rules to hook your board in with the
  build system (more on this in the next section). However, for board families
  it makes more sense to have a common top-level arch/sh/boards/ directory
  and then populate that with sub-directories for each member of the family.
  Both the Solution Engine and the hp6xx boards are an example of this.
  
  After you have setup your new arch/sh/boards/ directory, remember that you
  should also add a directory in include/asm-sh for headers localized to this
  board (if there are going to be more than one). In order to interoperate
  seamlessly with the build system, it's best to have this directory the same
  as the arch/sh/boards/ directory name, though if your board is again part of
  a family, the build system has ways of dealing with this (via incdir-y
  overloading), and you can feel free to name the directory after the family
  member itself.
  
  There are a few things that each board is required to have, both in the
  arch/sh/boards and the include/asm-sh/ hierarchy. In order to better
  explain this, we use some examples for adding an imaginary board. For
  setup code, we're required at the very least to provide definitions for
  get_system_type() and platform_setup(). For our imaginary board, this
  might look something like:
  
  /*
   * arch/sh/boards/vapor/setup.c - Setup code for imaginary board
   */
  #include <linux/init.h>
  #include <asm/rtc.h> /* for board_time_init() */
  
  const char *get_system_type(void)
  {
  	return "FooTech Vaporboard";
  }
  
  int __init platform_setup(void)
  {
    	/*
  	 * If our hardware actually existed, we would do real
  	 * setup here. Though it's also sane to leave this empty
  	 * if there's no real init work that has to be done for
  	 * this board.
  	 */
  
    	/* 
  	 * Presume all FooTech boards have the same broken timer,
  	 * and also presume that we've defined foo_timer_init to
  	 * do something useful.
  	 */
    	board_time_init = foo_timer_init;
  
  	/* Start-up imaginary PCI ... */
  
  	/* And whatever else ... */
  
  	return 0;
  }
  
  Our new imaginary board will also have to tie into the machvec in order for it
  to be of any use.
  
  machvec functions fall into a number of categories:
  
   - I/O functions to IO memory (inb etc) and PCI/main memory (readb etc).
   - I/O mapping functions (ioport_map, ioport_unmap, etc).
   - a 'heartbeat' function.
   - PCI and IRQ initialization routines.
   - Consistent allocators (for boards that need special allocators,
     particularly for allocating out of some board-specific SRAM for DMA
     handles).
  
  There are machvec functions added and removed over time, so always be sure to
  consult include/asm-sh/machvec.h for the current state of the machvec.
  
  The kernel will automatically wrap in generic routines for undefined function
  pointers in the machvec at boot time, as machvec functions are referenced
  unconditionally throughout most of the tree. Some boards have incredibly
  sparse machvecs (such as the dreamcast and sh03), whereas others must define
  virtually everything (rts7751r2d).
  
  Adding a new machine is relatively trivial (using vapor as an example):
  
  If the board-specific definitions are quite minimalistic, as is the case for
  the vast majority of boards, simply having a single board-specific header is
  sufficient.
  
   - add a new file include/asm-sh/vapor.h which contains prototypes for
     any machine specific IO functions prefixed with the machine name, for
     example vapor_inb. These will be needed when filling out the machine
     vector.
  
     Note that these prototypes are generated automatically by setting
     __IO_PREFIX to something sensible. A typical example would be:
  
  	#define __IO_PREFIX vapor
     	#include <asm/io_generic.h>
  
     somewhere in the board-specific header. Any boards being ported that still
     have a legacy io.h should remove it entirely and switch to the new model.
  
   - Add machine vector definitions to the board's setup.c. At a bare minimum,
     this must be defined as something like:
  
  	struct sh_machine_vector mv_vapor __initmv = {
  		.mv_name = "vapor",
  	};
  	ALIAS_MV(vapor)
  
   - finally add a file arch/sh/boards/vapor/io.c, which contains definitions of
     the machine specific io functions (if there are enough to warrant it).
  
  3. Hooking into the Build System
  ================================
  
  Now that we have the corresponding directories setup, and all of the
  board-specific code is in place, it's time to look at how to get the
  whole mess to fit into the build system.
  
  Large portions of the build system are now entirely dynamic, and merely
  require the proper entry here and there in order to get things done.
  
  The first thing to do is to add an entry to arch/sh/Kconfig, under the
  "System type" menu:
  
  config SH_VAPOR
  	bool "Vapor"
  	help
  	  select Vapor if configuring for a FooTech Vaporboard.
  
  next, this has to be added into arch/sh/Makefile. All boards require a
  machdir-y entry in order to be built. This entry needs to be the name of
  the board directory as it appears in arch/sh/boards, even if it is in a
  sub-directory (in which case, all parent directories below arch/sh/boards/
  need to be listed). For our new board, this entry can look like:
  
  machdir-$(CONFIG_SH_VAPOR)	+= vapor
  
  provided that we've placed everything in the arch/sh/boards/vapor/ directory.
  
  Next, the build system assumes that your include/asm-sh directory will also
  be named the same. If this is not the case (as is the case with multiple
  boards belonging to a common family), then the directory name needs to be
  implicitly appended to incdir-y. The existing code manages this for the
  Solution Engine and hp6xx boards, so see these for an example.
  
  Once that is taken care of, it's time to add an entry for the mach type.
  This is done by adding an entry to the end of the arch/sh/tools/mach-types
  list. The method for doing this is self explanatory, and so we won't waste
  space restating it here. After this is done, you will be able to use
  implicit checks for your board if you need this somewhere throughout the
  common code, such as:
  
  	/* Make sure we're on the FooTech Vaporboard */
  	if (!mach_is_vapor())
  		return -ENODEV;
  
  also note that the mach_is_boardname() check will be implicitly forced to
  lowercase, regardless of the fact that the mach-types entries are all
  uppercase. You can read the script if you really care, but it's pretty ugly,
  so you probably don't want to do that.
  
  Now all that's left to do is providing a defconfig for your new board. This
  way, other people who end up with this board can simply use this config
  for reference instead of trying to guess what settings are supposed to be
  used on it.
  
  Also, as soon as you have copied over a sample .config for your new board
  (assume arch/sh/configs/vapor_defconfig), you can also use this directly as a
  build target, and it will be implicitly listed as such in the help text.
  
  Looking at the 'make help' output, you should now see something like:
  
  Architecture specific targets (sh):
    zImage                  - Compressed kernel image (arch/sh/boot/zImage)
    adx_defconfig           - Build for adx
    cqreek_defconfig        - Build for cqreek
    dreamcast_defconfig     - Build for dreamcast
  ...
    vapor_defconfig         - Build for vapor
  
  which then allows you to do:
  
  $ make ARCH=sh CROSS_COMPILE=sh4-linux- vapor_defconfig vmlinux
  
  which will in turn copy the defconfig for this board, run it through
  oldconfig (prompting you for any new options since the time of creation),
  and start you on your way to having a functional kernel for your new
  board.