278 lines
10 KiB
ReStructuredText
278 lines
10 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=============================
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Adding a new board to LinuxSH
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=============================
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Paul Mundt <lethal@linux-sh.org>
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This document attempts to outline what steps are necessary to add support
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for new boards to the LinuxSH port under the new 2.5 and 2.6 kernels. This
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also attempts to outline some of the noticeable changes between the 2.4
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and the 2.5/2.6 SH backend.
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1. New Directory Structure
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==========================
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The first thing to note is the new directory structure. Under 2.4, most
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of the board-specific code (with the exception of stboards) ended up
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in arch/sh/kernel/ directly, with board-specific headers ending up in
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include/asm-sh/. For the new kernel, things are broken out by board type,
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companion chip type, and CPU type. Looking at a tree view of this directory
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hierarchy looks like the following:
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Board-specific code::
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.
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|-- arch
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| `-- sh
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| `-- boards
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| |-- adx
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| | `-- board-specific files
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| |-- bigsur
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| | `-- board-specific files
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| ... more boards here ...
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`-- include
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`-- asm-sh
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|-- adx
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| `-- board-specific headers
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|-- bigsur
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| `-- board-specific headers
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.. more boards here ...
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Next, for companion chips::
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.
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`-- arch
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`-- sh
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`-- cchips
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`-- hd6446x
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`-- hd64461
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`-- cchip-specific files
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... and so on. Headers for the companion chips are treated the same way as
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board-specific headers. Thus, include/asm-sh/hd64461 is home to all of the
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hd64461-specific headers.
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Finally, CPU family support is also abstracted::
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.
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|-- arch
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| `-- sh
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| |-- kernel
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| | `-- cpu
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| | |-- sh2
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| | | `-- SH-2 generic files
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| | |-- sh3
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| | | `-- SH-3 generic files
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| | `-- sh4
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| | `-- SH-4 generic files
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| `-- mm
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| `-- This is also broken out per CPU family, so each family can
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| have their own set of cache/tlb functions.
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`-- include
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`-- asm-sh
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|-- cpu-sh2
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| `-- SH-2 specific headers
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|-- cpu-sh3
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| `-- SH-3 specific headers
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`-- cpu-sh4
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`-- SH-4 specific headers
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It should be noted that CPU subtypes are _not_ abstracted. Thus, these still
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need to be dealt with by the CPU family specific code.
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2. Adding a New Board
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=====================
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The first thing to determine is whether the board you are adding will be
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isolated, or whether it will be part of a family of boards that can mostly
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share the same board-specific code with minor differences.
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In the first case, this is just a matter of making a directory for your
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board in arch/sh/boards/ and adding rules to hook your board in with the
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build system (more on this in the next section). However, for board families
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it makes more sense to have a common top-level arch/sh/boards/ directory
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and then populate that with sub-directories for each member of the family.
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Both the Solution Engine and the hp6xx boards are an example of this.
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After you have setup your new arch/sh/boards/ directory, remember that you
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should also add a directory in include/asm-sh for headers localized to this
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board (if there are going to be more than one). In order to interoperate
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seamlessly with the build system, it's best to have this directory the same
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as the arch/sh/boards/ directory name, though if your board is again part of
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a family, the build system has ways of dealing with this (via incdir-y
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overloading), and you can feel free to name the directory after the family
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member itself.
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There are a few things that each board is required to have, both in the
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arch/sh/boards and the include/asm-sh/ hierarchy. In order to better
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explain this, we use some examples for adding an imaginary board. For
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setup code, we're required at the very least to provide definitions for
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get_system_type() and platform_setup(). For our imaginary board, this
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might look something like::
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/*
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* arch/sh/boards/vapor/setup.c - Setup code for imaginary board
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*/
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#include <linux/init.h>
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const char *get_system_type(void)
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{
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return "FooTech Vaporboard";
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}
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int __init platform_setup(void)
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{
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/*
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* If our hardware actually existed, we would do real
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* setup here. Though it's also sane to leave this empty
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* if there's no real init work that has to be done for
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* this board.
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*/
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/* Start-up imaginary PCI ... */
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/* And whatever else ... */
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return 0;
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}
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Our new imaginary board will also have to tie into the machvec in order for it
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to be of any use.
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machvec functions fall into a number of categories:
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- I/O functions to IO memory (inb etc) and PCI/main memory (readb etc).
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- I/O mapping functions (ioport_map, ioport_unmap, etc).
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- a 'heartbeat' function.
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- PCI and IRQ initialization routines.
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- Consistent allocators (for boards that need special allocators,
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particularly for allocating out of some board-specific SRAM for DMA
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handles).
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There are machvec functions added and removed over time, so always be sure to
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consult include/asm-sh/machvec.h for the current state of the machvec.
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The kernel will automatically wrap in generic routines for undefined function
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pointers in the machvec at boot time, as machvec functions are referenced
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unconditionally throughout most of the tree. Some boards have incredibly
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sparse machvecs (such as the dreamcast and sh03), whereas others must define
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virtually everything (rts7751r2d).
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Adding a new machine is relatively trivial (using vapor as an example):
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If the board-specific definitions are quite minimalistic, as is the case for
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the vast majority of boards, simply having a single board-specific header is
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sufficient.
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- add a new file include/asm-sh/vapor.h which contains prototypes for
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any machine specific IO functions prefixed with the machine name, for
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example vapor_inb. These will be needed when filling out the machine
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vector.
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Note that these prototypes are generated automatically by setting
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__IO_PREFIX to something sensible. A typical example would be::
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#define __IO_PREFIX vapor
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#include <asm/io_generic.h>
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somewhere in the board-specific header. Any boards being ported that still
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have a legacy io.h should remove it entirely and switch to the new model.
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- Add machine vector definitions to the board's setup.c. At a bare minimum,
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this must be defined as something like::
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struct sh_machine_vector mv_vapor __initmv = {
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.mv_name = "vapor",
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};
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ALIAS_MV(vapor)
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- finally add a file arch/sh/boards/vapor/io.c, which contains definitions of
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the machine specific io functions (if there are enough to warrant it).
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3. Hooking into the Build System
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================================
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Now that we have the corresponding directories setup, and all of the
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board-specific code is in place, it's time to look at how to get the
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whole mess to fit into the build system.
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Large portions of the build system are now entirely dynamic, and merely
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require the proper entry here and there in order to get things done.
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The first thing to do is to add an entry to arch/sh/Kconfig, under the
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"System type" menu::
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config SH_VAPOR
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bool "Vapor"
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help
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select Vapor if configuring for a FooTech Vaporboard.
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next, this has to be added into arch/sh/Makefile. All boards require a
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machdir-y entry in order to be built. This entry needs to be the name of
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the board directory as it appears in arch/sh/boards, even if it is in a
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sub-directory (in which case, all parent directories below arch/sh/boards/
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need to be listed). For our new board, this entry can look like::
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machdir-$(CONFIG_SH_VAPOR) += vapor
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provided that we've placed everything in the arch/sh/boards/vapor/ directory.
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Next, the build system assumes that your include/asm-sh directory will also
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be named the same. If this is not the case (as is the case with multiple
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boards belonging to a common family), then the directory name needs to be
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implicitly appended to incdir-y. The existing code manages this for the
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Solution Engine and hp6xx boards, so see these for an example.
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Once that is taken care of, it's time to add an entry for the mach type.
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This is done by adding an entry to the end of the arch/sh/tools/mach-types
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list. The method for doing this is self explanatory, and so we won't waste
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space restating it here. After this is done, you will be able to use
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implicit checks for your board if you need this somewhere throughout the
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common code, such as::
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/* Make sure we're on the FooTech Vaporboard */
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if (!mach_is_vapor())
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return -ENODEV;
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also note that the mach_is_boardname() check will be implicitly forced to
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lowercase, regardless of the fact that the mach-types entries are all
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uppercase. You can read the script if you really care, but it's pretty ugly,
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so you probably don't want to do that.
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Now all that's left to do is providing a defconfig for your new board. This
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way, other people who end up with this board can simply use this config
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for reference instead of trying to guess what settings are supposed to be
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used on it.
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Also, as soon as you have copied over a sample .config for your new board
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(assume arch/sh/configs/vapor_defconfig), you can also use this directly as a
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build target, and it will be implicitly listed as such in the help text.
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Looking at the 'make help' output, you should now see something like:
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Architecture specific targets (sh):
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======================= =============================================
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zImage Compressed kernel image (arch/sh/boot/zImage)
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adx_defconfig Build for adx
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cqreek_defconfig Build for cqreek
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dreamcast_defconfig Build for dreamcast
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...
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vapor_defconfig Build for vapor
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======================= =============================================
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which then allows you to do::
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$ make ARCH=sh CROSS_COMPILE=sh4-linux- vapor_defconfig vmlinux
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which will in turn copy the defconfig for this board, run it through
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oldconfig (prompting you for any new options since the time of creation),
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and start you on your way to having a functional kernel for your new
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board.
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