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some more build system documentation

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docs/build.tex
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@ -1,5 +1,5 @@
One of the biggest challenges to getting started with embedded devices is that you
just can't install a copy of Linux and expect to be able to compile a firmware.
can't just install a copy of Linux and expect to be able to compile a firmware.
Even if you did remember to install a compiler and every development tool offered,
you still wouldn't have the basic set of tools needed to produce a firmware image.
The embedded device represents an entirely new hardware platform, which is
@ -9,25 +9,24 @@ your embedded platform, and then use it to compile a basic Linux distribution to
run on your device.
The process of creating a cross compiler can be tricky, it's not something that's
regularly attempted and so the there's a certain amount of mystery and black magic
regularly attempted and so there's a certain amount of mystery and black magic
associated with it. In many cases when you're dealing with embedded devices you'll
be provided with a binary copy of a compiler and basic libraries rather than
instructions for creating your own -- it's a time saving step but at the same time
often means you'll be using a rather dated set. Likewise, it's also common to be
provided with a patched copy of the Linux kernel from the board or chip vendor,
often means you'll be using a rather dated set of tools. Likewise, it's also common
to be provided with a patched copy of the Linux kernel from the board or chip vendor,
but this is also dated and it can be difficult to spot exactly what has been
changed to make the kernel run on the embedded platform.
modified to make the kernel run on the embedded platform.
\subsection{Building an image}
OpenWrt takes a different approach to building a firmware, downloading, patching
and compiling everything from scratch, including the cross compiler. Or to put it
OpenWrt takes a different approach to building a firmware; downloading, patching
and compiling everything from scratch, including the cross compiler. To put it
in simpler terms, OpenWrt doesn't contain any executables or even sources, it's an
automated system for downloading the sources, patching them to work with the given
platform and compiling them correctly for the platform. What this means is that
platform and compiling them correctly for that platform. What this means is that
just by changing the template, you can change any step in the process.
As an example, if a new kernel is released, a simple change to one of the Makefiles
will download the latest kernel, patch it to run on the embedded platform and produce
a new firmware image -- there's no work to be done trying to track down an unmodified
@ -37,7 +36,7 @@ just apply to the kernel, but to anything included with OpenWrt -- It's this one
simple understated concept which is what allows OpenWrt to stay on the bleeding edge
with the latest compilers, latest kernels and latest applications.
So let's take a look at OpenWrt and see how this all works
So let's take a look at OpenWrt and see how this all works.
\subsubsection{Download openwrt}
@ -46,7 +45,7 @@ This article refers to the "Kamikaze" branch of OpenWrt, which can be downloaded
subversion using the following command:
\begin{Verbatim}
svn co https://svn.openwrt.org/openwrt/trunk kamikaze
$ svn co https://svn.openwrt.org/openwrt/trunk kamikaze
\end{Verbatim}
Additionally, there's a trac interface on \href{https://dev.openwrt.org/}{https://dev.openwrt.org/}
@ -58,14 +57,14 @@ which can be used to monitor svn commits and browse the sources.
There are four key directories in the base:
\begin{itemize}
\item tools
\item toolchain
\item package
\item target
\item \texttt{tools}
\item \texttt{toolchain}
\item \texttt{package}
\item \texttt{target}
\end{itemize}
\texttt{tools} and \texttt{toolchain} refer to common tools which will be
used to build the firmware image and the compiler and c library.
used to build the firmware image, the compiler, and the c library.
The result of this is three new directories, \texttt{tool\_build}, which is a temporary
directory for building the target independent tools, \texttt{toolchain\_build\_\textit{<arch>}}
which is used for building the toolchain for a specific architecture, and
@ -73,9 +72,29 @@ which is used for building the toolchain for a specific architecture, and
You won't need to do anything with the toolchain directory unless you intend to
add a new version of one of the components above.
\begin{itemize}
\item \texttt{tool\_build}
\item \texttt{toolchain\_build\_\textit{<arch>}}
\end{itemize}
\texttt{package} is for exactly that -- packages. In an OpenWrt firmware, almost everything
is an \texttt{.ipk}, a software package which can be added to the firmware to provide new
features or removed to save space.
features or removed to save space. Note that packages are also maintained outside of the main
trunk and can be obtained from subversion at the following location:
\begin{Verbatim}
$ svn co https://svn.openwrt.org/openwrt/packages ../packages
\end{Verbatim}
Those packages can be used to extend the functionality of the build system and need to be
symlinked into the main trunk. Once you do that, the packages will show up in the menu for
configuration. From kamikaze you would do something like this:
\begin{Verbatim}
$ ls
kamikaze packages
$ ln -s packages/net/nmap kamikaze/package/nmap
\end{Verbatim}
\texttt{target} refers to the embedded platform, this contains items which are specific to
a specific embedded platform. Of particular interest here is the "\texttt{target/linux}"
@ -87,6 +106,10 @@ Both the target and package steps will use the directory "\texttt{build\_\textit
as a temporary directory for compiling. Additionally, anything downloaded by the toolchain,
target or package steps will be placed in the "\texttt{dl}" directory.
\begin{itemize}
\item \texttt{build\_\textit{<arch>}}
\item \texttt{dl}
\end{itemize}
\subsubsection{Building OpenWrt}
@ -96,7 +119,11 @@ simple enough that an inexperienced end user can easily build his or her own cus
Running the command "\texttt{make menuconfig}" will bring up OpenWrt's configuration menu
screen, through this menu you can select which platform you're targeting, which versions of
the toolchain you want to use to build and what packages you want to install into the
firmware image. Similar to the linux kernel config, almost every option has three choices,
firmware image. Note that it will also check to make sure you have the basic dependencies for it
to run correctly. If that fails, you will need to install some more tools in your local environment
before you can begin.
Similar to the linux kernel config, almost every option has three choices,
\texttt{y/m/n} which are represented as follows:
\begin{itemize}
@ -137,7 +164,6 @@ and packages will be in the "\texttt{bin/packages}" directory.
\subsection{Creating packages}
One of the things that we've attempted to do with OpenWrt's template system is make it
incredibly easy to port software to OpenWrt. If you look at a typical package directory
in OpenWrt you'll find two things:
@ -159,49 +185,53 @@ simplifies the entire ordeal.
Here for example, is \texttt{package/bridge/Makefile}:
\begin{Verbatim}[frame=single,numbers=left]
#
# Copyright (C) 2006 OpenWrt.org
#
# This is free software, licensed under the GNU General Public License v2.
# See /LICENSE for more information.
#
# $Id: Makefile 5624 2006-11-23 00:29:07Z nbd $
include $(TOPDIR)/rules.mk
PKG_NAME:=bridge
PKG_VERSION:=1.0.6
PKG_RELEASE:=1
PKG_BUILD_DIR:=$(BUILD_DIR)/bridge-utils-$(PKG_VERSION)
PKG_SOURCE:=bridge-utils-$(PKG_VERSION).tar.gz
PKG_SOURCE_URL:=@SF/bridge
PKG_MD5SUM:=9b7dc52656f5cbec846a7ba3299f73bd
PKG_CAT:=zcat
PKG_BUILD_DIR:=$(BUILD_DIR)/bridge-utils-$(PKG_VERSION)
include $(INCLUDE_DIR)/package.mk
define Package/bridge
SECTION:=base
CATEGORY:=Network
DEFAULT:=y
SECTION:=net
CATEGORY:=Base system
TITLE:=Ethernet bridging configuration utility
DESCRIPTION:=\
Manage ethernet bridging: a way to connect networks together to \\\
form a larger network.
URL:=http://bridge.sourceforge.net/
endef
define Package/bridge/description
Ethernet bridging configuration utility
Manage ethernet bridging; a way to connect networks together
to form a larger network.
endef
define Build/Configure
$(call Build/Configure/Default, \
--with-linux-headers=$(LINUX_DIR))
$(call Build/Configure/Default, \
--with-linux-headers="$(LINUX_DIR)" \
)
endef
define Package/bridge/install
install -m0755 -d $(1)/usr/sbin
install -m0755 $(PKG_BUILD_DIR)/brctl/brctl \
$(1)/usr/sbin/
$(INSTALL_DIR) $(1)/usr/sbin
$(INSTALL_BIN) $(PKG_BUILD_DIR)/brctl/brctl $(1)/usr/sbin/
endef
$(eval $(call BuildPackage,bridge))
\end{Verbatim}
As you can see, there's not much work to be done; everything is hidden in other makefiles
and abstracted to the point where you only need to specify a few variables.
@ -212,8 +242,6 @@ and abstracted to the point where you only need to specify a few variables.
The upstream version number that we're downloading
\item \texttt{PKG\_RELEASE} \\
The version of this package Makefile
\item \texttt{PKG\_BUILD\_DIR} \\
Where to compile the package
\item \texttt{PKG\_SOURCE} \\
The filename of the original sources
\item \texttt{PKG\_SOURCE\_URL} \\
@ -222,6 +250,8 @@ and abstracted to the point where you only need to specify a few variables.
A checksum to validate the download
\item \texttt{PKG\_CAT} \\
How to decompress the sources (zcat, bzcat, unzip)
\item \texttt{PKG\_BUILD\_DIR} \\
Where to compile the package
\end{itemize}
The \texttt{PKG\_*} variables define where to download the package from;
@ -231,7 +261,7 @@ The md5sum is used to verify the package was downloaded correctly and
uncompressed into \texttt{\$(BUILD\_DIR)}.
At the bottom of the file is where the real magic happens, "BuildPackage" is a macro
setup by the earlier include statements. BuildPackage only takes one argument directly --
set up by the earlier include statements. BuildPackage only takes one argument directly --
the name of the package to be built, in this case "\texttt{bridge}". All other information
is taken from the define blocks. This is a way of providing a level of verbosity, it's
inherently clear what the contents of the \texttt{description} template in
@ -278,12 +308,21 @@ directly as the Nth argument to \texttt{BuildPackage}.
\textbf{\texttt{Package/\textit{<name>}/install}:} \\
A set of commands to copy files out of the compiled source and into the ipkg
which is represented by the \texttt{\$(1)} directory.
which is represented by the \texttt{\$(1)} directory. Note that there are currently
3 defined install macros:
\begin{itemize}
\item \texttt{INSTALL\_DIR} \\
install -d -m0755
\item \texttt{INSTALL\_BIN} \\
install -m0755
\item \texttt{INSTALL\_DATA} \\
install -m0644
\end{itemize}
The reason that some of the defines are prefixed by "\texttt{Package/\textit{<name>}}"
and others are simply "\texttt{Build}" is because of the possibility of generating
multiple packages from a single source. OpenWrt works under the assumption of one
source per package makefile, but you can split that source into as many packages as
source per package Makefile, but you can split that source into as many packages as
desired. Since you only need to compile the sources once, there's one global set of
"\texttt{Build}" defines, but you can add as many "Package/<name>" defines as you want
by adding extra calls to \texttt{BuildPackage} -- see the dropbear package for an example.
@ -318,3 +357,11 @@ when satisfied, copy the patched sources elsewhere and diff them with the unpatc
sources. A warning though - if you go modify anything under \texttt{package/\textit{<name>}}
it will remove the old sources and unpack a fresh copy.
Other useful targets include:
\begin{itemize}
\item \texttt{make package/\textit{<name>}-prepare V=99}
\item \texttt{make package/\textit{<name>}-compile V=99}
\item \texttt{make package/\textit{<name>}-configure V=99}
\end{itemize}

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