+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.
+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
+incompatible with the hardware on your development machine, so in a process called
+cross compiling you need to produce a new compiler capable of generating code for
+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
+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,
+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.
+
+\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
+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
+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
+copy of the existing kernel to see what changes had been made, the patches are
+already provided and the process ends up almost completely transparent. This doesn't
+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
+
+
+\subsubsection{Download openwrt}
+
+This article refers to the "Kamikaze" branch of OpenWrt, which can be downloaded via
+subversion using the following command:
+
+\begin{Verbatim}
+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/}
+which can be used to monitor svn commits and browse the sources.
+
+
+\subsubsection{The directory structure}
+
+There are four key directories in the base:
+
+\begin{itemize}
+ \item tools
+ \item toolchain
+ \item package
+ \item 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.
+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
+\texttt{staging\_dir\_\textit{<arch>}} where the resulting toolchain is installed.
+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.
+
+\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.
+
+\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}"
+directory which is broken down by platform and contains the kernel config and patches
+to the kernel for a particular platform. There's also the "\texttt{target/image}" directory
+which describes how to package a firmware for a specific platform.
+
+Both the target and package steps will use the directory "\texttt{build\_\textit{<arch>}}"
+as a temporary directory for compiling. Additionally, anything downloaded by the toolchain,
+target or package steps will be placed in the "\texttt{dl}" directory.
+
+
+\subsubsection{Building OpenWrt}
+
+While the OpenWrt build environment was intended mostly for developers, it also has to be
+simple enough that an inexperienced end user can easily build his or her own customized firmware.
+
+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,
+\texttt{y/m/n} which are represented as follows:
+
+\begin{itemize}
+ \item{\texttt{<*>} (pressing y)} \\
+ This will be included in the firmware image
+ \item{\texttt{<M>} (pressing m)} \\
+ This will be compiled but not included (for later install)
+ \item{\texttt{< >} (pressing n)} \\
+ This will not be compiled
+\end{itemize}
+
+After you've finished with the menu configuration, exit and when prompted, save your
+configuration changes. To begin compiling the firmware, type "\texttt{make}". By default
+OpenWrt will only display a high level overview of the compile process and not each individual
+command.
+
+\subsubsection{Example:}
+
+\begin{Verbatim}
+make[2] toolchain/install
+make[3] -C toolchain install
+make[2] target/compile
+make[3] -C target compile
+make[4] -C target/utils prepare
+
+[...]
+\end{Verbatim}
+
+This makes it easier to monitor which step it's actually compiling and reduces the amount
+of noise caused by the compile output. To see the full output, run the command
+"\texttt{make V=99}".
+
+During the build process, buildroot will download all sources to the "\texttt{dl}"
+directory and will start patching and compiling them in the "\texttt{build\_\textit{<arch>}}"
+directory. When finished, the resulting firmware will be in the "\texttt{bin}" directory
+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:
+
+\begin{itemize}
+ \item \texttt{package/\textit{<name>}/Makefile}
+ \item \texttt{package/\textit{<name>}/patches}
+\end{itemize}
+
+The patches directory is optional and typically contains bug fixes or optimizations to
+reduce the size of the executable. The package makefile is the important item, provides
+the steps actually needed to download and compile the package.
+
+Looking at one of the package makefiles, you'd hardly recognize it as a makefile.
+Through what can only be described as blatant disregard and abuse of the traditional
+make format, the makefile has been transformed into an object oriented template which
+simplifies the entire ordeal.
+
+Here for example, is \texttt{package/bridge/Makefile}:
+
+\begin{Verbatim}[frame=single,numbers=left]
+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
+
+include $(INCLUDE_DIR)/package.mk
+
+define Package/bridge
+ SECTION:=base
+ CATEGORY:=Network
+ DEFAULT:=y
+ TITLE:=Ethernet bridging configuration utility
+ 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))
+endef
+
+define Package/bridge/install
+ install -m0755 -d $(1)/usr/sbin
+ install -m0755 $(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.
+
+\begin{itemize}
+ \item \texttt{PKG\_NAME} \\
+ The name of the package, as seen via menuconfig and ipkg
+ \item \texttt{PKG\_VERSION} \\
+ 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} \\
+ Where to download the sources from
+ \item \texttt{PKG\_MD5SUM} \\
+ A checksum to validate the download
+ \item \texttt{PKG\_CAT} \\
+ How to decompress the sources (zcat, bzcat, unzip)
+\end{itemize}
+
+The \texttt{PKG\_*} variables define where to download the package from;
+\texttt{@SF} is a special keyword for downloading packages from sourceforge.
+The md5sum is used to verify the package was downloaded correctly and
+\texttt{PKG\_BUILD\_DIR} defines where to find the package after the sources are
+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 --
+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
+\texttt{Package/bridge} is, which wouldn't be the case if we passed this information
+directly as the Nth argument to \texttt{BuildPackage}.
+
+\texttt{BuildPackage} uses the following defines:
+
+\textbf{\texttt{Package/\textit{<name>}}:} \\
+ \texttt{\textit{<name>}} matches the argument passed to buildroot, this describes
+ the package the menuconfig and ipkg entries. Within \texttt{Package/\textit{<name>}}
+ you can define the following variables:
+
+ \begin{itemize}
+ \item \texttt{SECTION} \\
+ The type of package (currently unused)
+ \item \texttt{CATEGORY} \\
+ Which menu it appears in menuconfig
+ \item \texttt{TITLE} \\
+ A short description of the package
+ \item \texttt{URL} \\
+ Where to find the original software
+ \item \texttt{MAINTAINER} (optional) \\
+ Who to contact concerning the package
+ \item \texttt{DEPENDS} (optional) \\
+ Which packages must be built/installed before this package
+ \end{itemize}
+
+\textbf{\texttt{Package/\textit{<name>}/conffiles} (optional):} \\
+ A list of config files installed by this package, one file per line.
+
+\textbf{\texttt{Build/Prepare} (optional):} \\
+ A set of commands to unpack and patch the sources. You may safely leave this
+ undefined.
+
+\textbf{\texttt{Build/Configure} (optional):} \\
+ You can leave this undefined if the source doesn't use configure or has a
+ normal config script, otherwise you can put your own commands here or use
+ "\texttt{\$(call Build/Configure/Default,\textit{<args>})}" as above to
+ pass in additional arguments for a standard configure script.
+
+\textbf{\texttt{Build/Compile} (optional):} \\
+ How to compile the source; in most cases you should leave this undefined.
+
+\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.
+
+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
+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.
+
+After you've created your \texttt{package/\textit{<name>}/Makefile}, the new package
+will automatically show in the menu the next time you run "make menuconfig" and if selected
+will be built automatically the next time "\texttt{make}" is run.
+
+\subsubsection{Troubleshooting}
+
+If you find your package doesn't show up in menuconfig, try the following command to
+see if you get the correct description:
+
+\begin{Verbatim}
+ TOPDIR=$PWD make -C package/<name> DUMP=1 V=99
+\end{Verbatim}
+
+If you're just having trouble getting your package to compile, there's a few
+shortcuts you can take. Instead of waiting for make to get to your package, you can
+run one of the following:
+
+\begin{itemize}
+ \item \texttt{make package/\textit{<name>}-clean V=99}
+ \item \texttt{make package/\textit{<name>}-install V=99}
+\end{itemize}
+
+Another nice trick is that if the source directory under \texttt{build\_\textit{<arch>}}
+is newer than the package directory, it won't clobber it by unpacking the sources again.
+If you were working on a patch you could simply edit the sources under the
+\texttt{build\_\textit{<arch>}/\textit{<source>}} directory and run the install command above,
+when satisfied, copy the patched sources elsewhere and diff them with the unpatched
+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.
+
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