openwrt/upstream - upstream openwrt

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One of the biggest challenges to getting started with embedded devices is that you cannot 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 would not have the basic set of tools needed to produce a firmware image. The embedded device represents an entirely new hardware platform, which is most of the time 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 is not something that is regularly attempted and so there is a certain amount of mystery and black magic associated with it. In many cases when you are dealing with embedded devices you will be provided with a binary copy of a compiler and basic libraries rather than instructions for creating your own -- it is a time saving step but at the same time often means you will be using a rather dated set of tools. Likewise, it is 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 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. To put it in simpler terms, OpenWrt does not contain any executables or even sources, it is an automated system for downloading the sources, patching them to work with the given 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 is 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 does not just apply to the kernel, but to anything included with OpenWrt -- It is 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 checkout svn://svn.openwrt.org/openwrt/trunk kamikaze \end{Verbatim} Additionally, there is a trac interface on \href{https://dev.openwrt.org/}{https://dev.openwrt.org/} which can be used to monitor svn commits and browse the source repository. \subsubsection{The directory structure} There are four key directories in the base: \begin{itemize} \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, the compiler, and the C library. The result of this is three new directories, \texttt{build\_dir/host}, which is a temporary directory for building the target independent tools, \texttt{build\_dir/toolchain-\textit{<arch>}*} which is used for building the toolchain for a specific architecture, and \texttt{staging\_dir/toolchain-\textit{<arch>}*} where the resulting toolchain is installed. You will not 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{build\_dir/host} \item \texttt{build\_dir/toolchain-\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. Note that packages are also maintained outside of the main trunk and can be obtained from subversion using the package feeds system: \begin{Verbatim} $ ./scripts/feeds update \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} $ ./scripts/feeds search nmap Search results in feed 'packages': nmap Network exploration and/or security auditing utility $ ./scripts/feeds install nmap \end{Verbatim} To include all packages, issue the following command: \begin{Verbatim} $ make package/symlinks \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}" directory which is broken down by platform \textit{<arch>} and contains the patches to the kernel, profile config, 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\_dir/\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. \begin{itemize} \item \texttt{build\_dir/\textit{<arch>}} \item \texttt{dl} \end{itemize} \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. 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} \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. If you want, you can also modify the kernel config for the selected target system. simply run "\texttt{make kernel\_menuconfig}" and the build system will unpack the kernel sources (if necessary), run menuconfig inside of the kernel tree, and then copy the kernel config to \texttt{target/linux/\textit{<platform>}/config} so that it is preserved over "\texttt{make clean}" calls. 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\_dir/\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} \item \texttt{package/\textit{<name>}/files} \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. The files directory is also optional and typicall contains package specific startup scripts or default configuration files that can be used out of the box with OpenWrt.


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