.. SPDX-License-Identifier: GPL-2.0

=============================
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>

    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.
            */

            /* 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.