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One of the biggest challenges to getting started with embedded devices is that you |
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just can't install a copy of Linux and expect to be able to compile a firmware. |
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Even if you did remember to install a compiler and every development tool offered, |
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you still wouldn't have the basic set of tools needed to produce a firmware image. |
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The embedded device represents an entirely new hardware platform, which is |
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incompatible with the hardware on your development machine, so in a process called |
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cross compiling you need to produce a new compiler capable of generating code for |
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your embedded platform, and then use it to compile a basic Linux distribution to |
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run on your device. |
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The process of creating a cross compiler can be tricky, it's not something that's |
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regularly attempted and so the there's a certain amount of mystery and black magic |
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associated with it. In many cases when you're dealing with embedded devices you'll |
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be provided with a binary copy of a compiler and basic libraries rather than |
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instructions for creating your own -- it's a time saving step but at the same time |
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often means you'll be using a rather dated set. Likewise, it's also common to be |
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provided with a patched copy of the Linux kernel from the board or chip vendor, |
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but this is also dated and it can be difficult to spot exactly what has been |
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changed to make the kernel run on the embedded platform. |
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\subsection{Building an image} |
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OpenWrt takes a different approach to building a firmware, downloading, patching |
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and compiling everything from scratch, including the cross compiler. Or to put it |
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in simpler terms, OpenWrt doesn't contain any executables or even sources, it's an |
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automated system for downloading the sources, patching them to work with the given |
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platform and compiling them correctly for the platform. What this means is that |
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just by changing the template, you can change any step in the process. |
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As an example, if a new kernel is released, a simple change to one of the Makefiles |
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will download the latest kernel, patch it to run on the embedded platform and produce |
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a new firmware image -- there's no work to be done trying to track down an unmodified |
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copy of the existing kernel to see what changes had been made, the patches are |
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already provided and the process ends up almost completely transparent. This doesn't |
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just apply to the kernel, but to anything included with OpenWrt -- It's this one |
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simple understated concept which is what allows OpenWrt to stay on the bleeding edge |
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with the latest compilers, latest kernels and latest applications. |
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So let's take a look at OpenWrt and see how this all works |
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\subsubsection{Download openwrt} |
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This article refers to the "Kamikaze" branch of OpenWrt, which can be downloaded via |
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subversion using the following command: |
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\begin{Verbatim} |
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svn co https://svn.openwrt.org/openwrt/trunk kamikaze |
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\end{Verbatim} |
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Additionally, there's a trac interface on \href{https://dev.openwrt.org/}{https://dev.openwrt.org/} |
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which can be used to monitor svn commits and browse the sources. |
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\subsubsection{The directory structure} |
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There are four key directories in the base: |
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\begin{itemize} |
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\item tools |
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\item toolchain |
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\item package |
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\item target |
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\end{itemize} |
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|
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\texttt{tools} and \texttt{toolchain} refer to common tools which will be |
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used to build the firmware image and the compiler and c library. |
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The result of this is three new directories, \texttt{tool\_build}, which is a temporary |
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directory for building the target independent tools, \texttt{toolchain\_build\_\textit{<arch>}} |
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which is used for building the toolchain for a specific architecture, and |
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\texttt{staging\_dir\_\textit{<arch>}} where the resulting toolchain is installed. |
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You won't need to do anything with the toolchain directory unless you intend to |
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add a new version of one of the components above. |
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\texttt{package} is for exactly that -- packages. In an OpenWrt firmware, almost everything |
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is an \texttt{.ipk}, a software package which can be added to the firmware to provide new |
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features or removed to save space. |
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|
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\texttt{target} refers to the embedded platform, this contains items which are specific to |
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a specific embedded platform. Of particular interest here is the "\texttt{target/linux}" |
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directory which is broken down by platform and contains the kernel config and patches |
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to the kernel for a particular platform. There's also the "\texttt{target/image}" directory |
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which describes how to package a firmware for a specific platform. |
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Both the target and package steps will use the directory "\texttt{build\_\textit{<arch>}}" |
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as a temporary directory for compiling. Additionally, anything downloaded by the toolchain, |
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target or package steps will be placed in the "\texttt{dl}" directory. |
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\subsubsection{Building OpenWrt} |
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While the OpenWrt build environment was intended mostly for developers, it also has to be |
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simple enough that an inexperienced end user can easily build his or her own customized firmware. |
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Running the command "\texttt{make menuconfig}" will bring up OpenWrt's configuration menu |
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screen, through this menu you can select which platform you're targeting, which versions of |
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the toolchain you want to use to build and what packages you want to install into the |
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firmware image. Similar to the linux kernel config, almost every option has three choices, |
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\texttt{y/m/n} which are represented as follows: |
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\begin{itemize} |
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\item{\texttt{<*>} (pressing y)} \\ |
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This will be included in the firmware image |
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\item{\texttt{<M>} (pressing m)} \\ |
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This will be compiled but not included (for later install) |
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\item{\texttt{< >} (pressing n)} \\ |
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This will not be compiled |
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\end{itemize} |
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After you've finished with the menu configuration, exit and when prompted, save your |
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configuration changes. To begin compiling the firmware, type "\texttt{make}". By default |
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OpenWrt will only display a high level overview of the compile process and not each individual |
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command. |
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\subsubsection{Example:} |
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\begin{Verbatim} |
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make[2] toolchain/install |
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make[3] -C toolchain install |
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make[2] target/compile |
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make[3] -C target compile |
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make[4] -C target/utils prepare |
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[...] |
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\end{Verbatim} |
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This makes it easier to monitor which step it's actually compiling and reduces the amount |
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of noise caused by the compile output. To see the full output, run the command |
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"\texttt{make V=99}". |
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During the build process, buildroot will download all sources to the "\texttt{dl}" |
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directory and will start patching and compiling them in the "\texttt{build\_\textit{<arch>}}" |
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directory. When finished, the resulting firmware will be in the "\texttt{bin}" directory |
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and packages will be in the "\texttt{bin/packages}" directory. |
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\subsection{Creating packages} |
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One of the things that we've attempted to do with OpenWrt's template system is make it |
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incredibly easy to port software to OpenWrt. If you look at a typical package directory |
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in OpenWrt you'll find two things: |
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\begin{itemize} |
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\item \texttt{package/\textit{<name>}/Makefile} |
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\item \texttt{package/\textit{<name>}/patches} |
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\end{itemize} |
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The patches directory is optional and typically contains bug fixes or optimizations to |
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reduce the size of the executable. The package makefile is the important item, provides |
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the steps actually needed to download and compile the package. |
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Looking at one of the package makefiles, you'd hardly recognize it as a makefile. |
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Through what can only be described as blatant disregard and abuse of the traditional |
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make format, the makefile has been transformed into an object oriented template which |
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simplifies the entire ordeal. |
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Here for example, is \texttt{package/bridge/Makefile}: |
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\begin{Verbatim}[frame=single,numbers=left] |
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include $(TOPDIR)/rules.mk |
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PKG_NAME:=bridge |
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PKG_VERSION:=1.0.6 |
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PKG_RELEASE:=1 |
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PKG_BUILD_DIR:=$(BUILD_DIR)/bridge-utils-$(PKG_VERSION) |
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PKG_SOURCE:=bridge-utils-$(PKG_VERSION).tar.gz |
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PKG_SOURCE_URL:=@SF/bridge |
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PKG_MD5SUM:=9b7dc52656f5cbec846a7ba3299f73bd |
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PKG_CAT:=zcat |
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include $(INCLUDE_DIR)/package.mk |
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define Package/bridge |
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SECTION:=base |
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CATEGORY:=Network |
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DEFAULT:=y |
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TITLE:=Ethernet bridging configuration utility |
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URL:=http://bridge.sourceforge.net/ |
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endef |
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define Package/bridge/description |
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Ethernet bridging configuration utility |
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Manage ethernet bridging; a way to connect networks together |
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to form a larger network. |
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endef |
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define Build/Configure |
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$(call Build/Configure/Default, \ |
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--with-linux-headers=$(LINUX_DIR)) |
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endef |
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define Package/bridge/install |
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install -m0755 -d $(1)/usr/sbin |
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install -m0755 $(PKG_BUILD_DIR)/brctl/brctl \ |
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$(1)/usr/sbin/ |
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endef |
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$(eval $(call BuildPackage,bridge)) |
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\end{Verbatim} |
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As you can see, there's not much work to be done; everything is hidden in other makefiles |
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and abstracted to the point where you only need to specify a few variables. |
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\begin{itemize} |
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\item \texttt{PKG\_NAME} \\ |
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The name of the package, as seen via menuconfig and ipkg |
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\item \texttt{PKG\_VERSION} \\ |
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The upstream version number that we're downloading |
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\item \texttt{PKG\_RELEASE} \\ |
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The version of this package Makefile |
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\item \texttt{PKG\_BUILD\_DIR} \\ |
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Where to compile the package |
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\item \texttt{PKG\_SOURCE} \\ |
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The filename of the original sources |
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\item \texttt{PKG\_SOURCE\_URL} \\ |
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Where to download the sources from |
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\item \texttt{PKG\_MD5SUM} \\ |
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A checksum to validate the download |
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\item \texttt{PKG\_CAT} \\ |
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How to decompress the sources (zcat, bzcat, unzip) |
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\end{itemize} |
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The \texttt{PKG\_*} variables define where to download the package from; |
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\texttt{@SF} is a special keyword for downloading packages from sourceforge. |
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The md5sum is used to verify the package was downloaded correctly and |
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\texttt{PKG\_BUILD\_DIR} defines where to find the package after the sources are |
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uncompressed into \texttt{\$(BUILD\_DIR)}. |
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At the bottom of the file is where the real magic happens, "BuildPackage" is a macro |
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setup by the earlier include statements. BuildPackage only takes one argument directly -- |
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the name of the package to be built, in this case "\texttt{bridge}". All other information |
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is taken from the define blocks. This is a way of providing a level of verbosity, it's |
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inherently clear what the contents of the \texttt{description} template in |
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\texttt{Package/bridge} is, which wouldn't be the case if we passed this information |
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directly as the Nth argument to \texttt{BuildPackage}. |
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\texttt{BuildPackage} uses the following defines: |
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\textbf{\texttt{Package/\textit{<name>}}:} \\ |
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\texttt{\textit{<name>}} matches the argument passed to buildroot, this describes |
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the package the menuconfig and ipkg entries. Within \texttt{Package/\textit{<name>}} |
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you can define the following variables: |
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\begin{itemize} |
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\item \texttt{SECTION} \\ |
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The type of package (currently unused) |
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\item \texttt{CATEGORY} \\ |
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Which menu it appears in menuconfig |
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\item \texttt{TITLE} \\ |
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A short description of the package |
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\item \texttt{URL} \\ |
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Where to find the original software |
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\item \texttt{MAINTAINER} (optional) \\ |
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Who to contact concerning the package |
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\item \texttt{DEPENDS} (optional) \\ |
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Which packages must be built/installed before this package |
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\end{itemize} |
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\textbf{\texttt{Package/\textit{<name>}/conffiles} (optional):} \\ |
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A list of config files installed by this package, one file per line. |
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\textbf{\texttt{Build/Prepare} (optional):} \\ |
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A set of commands to unpack and patch the sources. You may safely leave this |
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undefined. |
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\textbf{\texttt{Build/Configure} (optional):} \\ |
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You can leave this undefined if the source doesn't use configure or has a |
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normal config script, otherwise you can put your own commands here or use |
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"\texttt{\$(call Build/Configure/Default,\textit{<args>})}" as above to |
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pass in additional arguments for a standard configure script. |
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\textbf{\texttt{Build/Compile} (optional):} \\ |
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How to compile the source; in most cases you should leave this undefined. |
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\textbf{\texttt{Package/\textit{<name>}/install}:} \\ |
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A set of commands to copy files out of the compiled source and into the ipkg |
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which is represented by the \texttt{\$(1)} directory. |
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The reason that some of the defines are prefixed by "\texttt{Package/\textit{<name>}}" |
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and others are simply "\texttt{Build}" is because of the possibility of generating |
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multiple packages from a single source. OpenWrt works under the assumption of one |
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source per package makefile, but you can split that source into as many packages as |
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desired. Since you only need to compile the sources once, there's one global set of |
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"\texttt{Build}" defines, but you can add as many "Package/<name>" defines as you want |
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by adding extra calls to \texttt{BuildPackage} -- see the dropbear package for an example. |
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After you've created your \texttt{package/\textit{<name>}/Makefile}, the new package |
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will automatically show in the menu the next time you run "make menuconfig" and if selected |
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will be built automatically the next time "\texttt{make}" is run. |
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\subsubsection{Troubleshooting} |
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If you find your package doesn't show up in menuconfig, try the following command to |
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see if you get the correct description: |
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\begin{Verbatim} |
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TOPDIR=$PWD make -C package/<name> DUMP=1 V=99 |
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\end{Verbatim} |
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If you're just having trouble getting your package to compile, there's a few |
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shortcuts you can take. Instead of waiting for make to get to your package, you can |
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run one of the following: |
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\begin{itemize} |
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\item \texttt{make package/\textit{<name>}-clean V=99} |
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\item \texttt{make package/\textit{<name>}-install V=99} |
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\end{itemize} |
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Another nice trick is that if the source directory under \texttt{build\_\textit{<arch>}} |
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is newer than the package directory, it won't clobber it by unpacking the sources again. |
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If you were working on a patch you could simply edit the sources under the |
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\texttt{build\_\textit{<arch>}/\textit{<source>}} directory and run the install command above, |
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when satisfied, copy the patched sources elsewhere and diff them with the unpatched |
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sources. A warning though - if you go modify anything under \texttt{package/\textit{<name>}} |
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it will remove the old sources and unpack a fresh copy. |
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Loading…
Reference in new issue