add modified version of mbm's 'introduction to buildroot-ng' to the documentation
[openwrt/svn-archive/archive.git] / docs / build.tex
1 One of the biggest challenges to getting started with embedded devices is that you
2 just can't install a copy of Linux and expect to be able to compile a firmware.
3 Even if you did remember to install a compiler and every development tool offered,
4 you still wouldn't have the basic set of tools needed to produce a firmware image.
5 The embedded device represents an entirely new hardware platform, which is
6 incompatible with the hardware on your development machine, so in a process called
7 cross compiling you need to produce a new compiler capable of generating code for
8 your embedded platform, and then use it to compile a basic Linux distribution to
9 run on your device.
11 The process of creating a cross compiler can be tricky, it's not something that's
12 regularly attempted and so the there's a certain amount of mystery and black magic
13 associated with it. In many cases when you're dealing with embedded devices you'll
14 be provided with a binary copy of a compiler and basic libraries rather than
15 instructions for creating your own -- it's a time saving step but at the same time
16 often means you'll be using a rather dated set. Likewise, it's also common to be
17 provided with a patched copy of the Linux kernel from the board or chip vendor,
18 but this is also dated and it can be difficult to spot exactly what has been
19 changed to make the kernel run on the embedded platform.
21 \subsection{Building an image}
23 OpenWrt takes a different approach to building a firmware, downloading, patching
24 and compiling everything from scratch, including the cross compiler. Or to put it
25 in simpler terms, OpenWrt doesn't contain any executables or even sources, it's an
26 automated system for downloading the sources, patching them to work with the given
27 platform and compiling them correctly for the platform. What this means is that
28 just by changing the template, you can change any step in the process.
31 As an example, if a new kernel is released, a simple change to one of the Makefiles
32 will download the latest kernel, patch it to run on the embedded platform and produce
33 a new firmware image -- there's no work to be done trying to track down an unmodified
34 copy of the existing kernel to see what changes had been made, the patches are
35 already provided and the process ends up almost completely transparent. This doesn't
36 just apply to the kernel, but to anything included with OpenWrt -- It's this one
37 simple understated concept which is what allows OpenWrt to stay on the bleeding edge
38 with the latest compilers, latest kernels and latest applications.
40 So let's take a look at OpenWrt and see how this all works
43 \subsubsection{Download openwrt}
45 This article refers to the "Kamikaze" branch of OpenWrt, which can be downloaded via
46 subversion using the following command:
48 \begin{Verbatim}
49 svn co kamikaze
50 \end{Verbatim}
52 Additionally, there's a trac interface on \href{}{}
53 which can be used to monitor svn commits and browse the sources.
56 \subsubsection{The directory structure}
58 There are four key directories in the base:
60 \begin{itemize}
61 \item tools
62 \item toolchain
63 \item package
64 \item target
65 \end{itemize}
67 \texttt{tools} and \texttt{toolchain} refer to common tools which will be
68 used to build the firmware image and the compiler and c library.
69 The result of this is three new directories, \texttt{tool\_build}, which is a temporary
70 directory for building the target independent tools, \texttt{toolchain\_build\_\textit{<arch>}}
71 which is used for building the toolchain for a specific architecture, and
72 \texttt{staging\_dir\_\textit{<arch>}} where the resulting toolchain is installed.
73 You won't need to do anything with the toolchain directory unless you intend to
74 add a new version of one of the components above.
76 \texttt{package} is for exactly that -- packages. In an OpenWrt firmware, almost everything
77 is an \texttt{.ipk}, a software package which can be added to the firmware to provide new
78 features or removed to save space.
80 \texttt{target} refers to the embedded platform, this contains items which are specific to
81 a specific embedded platform. Of particular interest here is the "\texttt{target/linux}"
82 directory which is broken down by platform and contains the kernel config and patches
83 to the kernel for a particular platform. There's also the "\texttt{target/image}" directory
84 which describes how to package a firmware for a specific platform.
86 Both the target and package steps will use the directory "\texttt{build\_\textit{<arch>}}"
87 as a temporary directory for compiling. Additionally, anything downloaded by the toolchain,
88 target or package steps will be placed in the "\texttt{dl}" directory.
91 \subsubsection{Building OpenWrt}
93 While the OpenWrt build environment was intended mostly for developers, it also has to be
94 simple enough that an inexperienced end user can easily build his or her own customized firmware.
96 Running the command "\texttt{make menuconfig}" will bring up OpenWrt's configuration menu
97 screen, through this menu you can select which platform you're targeting, which versions of
98 the toolchain you want to use to build and what packages you want to install into the
99 firmware image. Similar to the linux kernel config, almost every option has three choices,
100 \texttt{y/m/n} which are represented as follows:
102 \begin{itemize}
103 \item{\texttt{<*>} (pressing y)} \\
104 This will be included in the firmware image
105 \item{\texttt{<M>} (pressing m)} \\
106 This will be compiled but not included (for later install)
107 \item{\texttt{< >} (pressing n)} \\
108 This will not be compiled
109 \end{itemize}
111 After you've finished with the menu configuration, exit and when prompted, save your
112 configuration changes. To begin compiling the firmware, type "\texttt{make}". By default
113 OpenWrt will only display a high level overview of the compile process and not each individual
114 command.
116 \subsubsection{Example:}
118 \begin{Verbatim}
119 make[2] toolchain/install
120 make[3] -C toolchain install
121 make[2] target/compile
122 make[3] -C target compile
123 make[4] -C target/utils prepare
125 [...]
126 \end{Verbatim}
128 This makes it easier to monitor which step it's actually compiling and reduces the amount
129 of noise caused by the compile output. To see the full output, run the command
130 "\texttt{make V=99}".
132 During the build process, buildroot will download all sources to the "\texttt{dl}"
133 directory and will start patching and compiling them in the "\texttt{build\_\textit{<arch>}}"
134 directory. When finished, the resulting firmware will be in the "\texttt{bin}" directory
135 and packages will be in the "\texttt{bin/packages}" directory.
138 \subsection{Creating packages}
141 One of the things that we've attempted to do with OpenWrt's template system is make it
142 incredibly easy to port software to OpenWrt. If you look at a typical package directory
143 in OpenWrt you'll find two things:
145 \begin{itemize}
146 \item \texttt{package/\textit{<name>}/Makefile}
147 \item \texttt{package/\textit{<name>}/patches}
148 \end{itemize}
150 The patches directory is optional and typically contains bug fixes or optimizations to
151 reduce the size of the executable. The package makefile is the important item, provides
152 the steps actually needed to download and compile the package.
154 Looking at one of the package makefiles, you'd hardly recognize it as a makefile.
155 Through what can only be described as blatant disregard and abuse of the traditional
156 make format, the makefile has been transformed into an object oriented template which
157 simplifies the entire ordeal.
159 Here for example, is \texttt{package/bridge/Makefile}:
161 \begin{Verbatim}[frame=single,numbers=left]
162 include $(TOPDIR)/
164 PKG_NAME:=bridge
165 PKG_VERSION:=1.0.6
168 PKG_BUILD_DIR:=$(BUILD_DIR)/bridge-utils-$(PKG_VERSION)
169 PKG_SOURCE:=bridge-utils-$(PKG_VERSION).tar.gz
170 PKG_SOURCE_URL:=@SF/bridge
171 PKG_MD5SUM:=9b7dc52656f5cbec846a7ba3299f73bd
172 PKG_CAT:=zcat
174 include $(INCLUDE_DIR)/
176 define Package/bridge
177 SECTION:=base
178 CATEGORY:=Network
179 DEFAULT:=y
180 TITLE:=Ethernet bridging configuration utility
181 URL:=
182 endef
184 define Package/bridge/description
185 Ethernet bridging configuration utility
186 Manage ethernet bridging; a way to connect networks together
187 to form a larger network.
188 endef
190 define Build/Configure
191 $(call Build/Configure/Default, \
192 --with-linux-headers=$(LINUX_DIR))
193 endef
195 define Package/bridge/install
196 install -m0755 -d $(1)/usr/sbin
197 install -m0755 $(PKG_BUILD_DIR)/brctl/brctl \
198 $(1)/usr/sbin/
199 endef
201 $(eval $(call BuildPackage,bridge))
202 \end{Verbatim}
205 As you can see, there's not much work to be done; everything is hidden in other makefiles
206 and abstracted to the point where you only need to specify a few variables.
208 \begin{itemize}
209 \item \texttt{PKG\_NAME} \\
210 The name of the package, as seen via menuconfig and ipkg
211 \item \texttt{PKG\_VERSION} \\
212 The upstream version number that we're downloading
213 \item \texttt{PKG\_RELEASE} \\
214 The version of this package Makefile
215 \item \texttt{PKG\_BUILD\_DIR} \\
216 Where to compile the package
217 \item \texttt{PKG\_SOURCE} \\
218 The filename of the original sources
219 \item \texttt{PKG\_SOURCE\_URL} \\
220 Where to download the sources from
221 \item \texttt{PKG\_MD5SUM} \\
222 A checksum to validate the download
223 \item \texttt{PKG\_CAT} \\
224 How to decompress the sources (zcat, bzcat, unzip)
225 \end{itemize}
227 The \texttt{PKG\_*} variables define where to download the package from;
228 \texttt{@SF} is a special keyword for downloading packages from sourceforge.
229 The md5sum is used to verify the package was downloaded correctly and
230 \texttt{PKG\_BUILD\_DIR} defines where to find the package after the sources are
231 uncompressed into \texttt{\$(BUILD\_DIR)}.
233 At the bottom of the file is where the real magic happens, "BuildPackage" is a macro
234 setup by the earlier include statements. BuildPackage only takes one argument directly --
235 the name of the package to be built, in this case "\texttt{bridge}". All other information
236 is taken from the define blocks. This is a way of providing a level of verbosity, it's
237 inherently clear what the contents of the \texttt{description} template in
238 \texttt{Package/bridge} is, which wouldn't be the case if we passed this information
239 directly as the Nth argument to \texttt{BuildPackage}.
241 \texttt{BuildPackage} uses the following defines:
243 \textbf{\texttt{Package/\textit{<name>}}:} \\
244 \texttt{\textit{<name>}} matches the argument passed to buildroot, this describes
245 the package the menuconfig and ipkg entries. Within \texttt{Package/\textit{<name>}}
246 you can define the following variables:
248 \begin{itemize}
249 \item \texttt{SECTION} \\
250 The type of package (currently unused)
251 \item \texttt{CATEGORY} \\
252 Which menu it appears in menuconfig
253 \item \texttt{TITLE} \\
254 A short description of the package
255 \item \texttt{URL} \\
256 Where to find the original software
257 \item \texttt{MAINTAINER} (optional) \\
258 Who to contact concerning the package
259 \item \texttt{DEPENDS} (optional) \\
260 Which packages must be built/installed before this package
261 \end{itemize}
263 \textbf{\texttt{Package/\textit{<name>}/conffiles} (optional):} \\
264 A list of config files installed by this package, one file per line.
266 \textbf{\texttt{Build/Prepare} (optional):} \\
267 A set of commands to unpack and patch the sources. You may safely leave this
268 undefined.
270 \textbf{\texttt{Build/Configure} (optional):} \\
271 You can leave this undefined if the source doesn't use configure or has a
272 normal config script, otherwise you can put your own commands here or use
273 "\texttt{\$(call Build/Configure/Default,\textit{<args>})}" as above to
274 pass in additional arguments for a standard configure script.
276 \textbf{\texttt{Build/Compile} (optional):} \\
277 How to compile the source; in most cases you should leave this undefined.
279 \textbf{\texttt{Package/\textit{<name>}/install}:} \\
280 A set of commands to copy files out of the compiled source and into the ipkg
281 which is represented by the \texttt{\$(1)} directory.
283 The reason that some of the defines are prefixed by "\texttt{Package/\textit{<name>}}"
284 and others are simply "\texttt{Build}" is because of the possibility of generating
285 multiple packages from a single source. OpenWrt works under the assumption of one
286 source per package makefile, but you can split that source into as many packages as
287 desired. Since you only need to compile the sources once, there's one global set of
288 "\texttt{Build}" defines, but you can add as many "Package/<name>" defines as you want
289 by adding extra calls to \texttt{BuildPackage} -- see the dropbear package for an example.
291 After you've created your \texttt{package/\textit{<name>}/Makefile}, the new package
292 will automatically show in the menu the next time you run "make menuconfig" and if selected
293 will be built automatically the next time "\texttt{make}" is run.
295 \subsubsection{Troubleshooting}
297 If you find your package doesn't show up in menuconfig, try the following command to
298 see if you get the correct description:
300 \begin{Verbatim}
301 TOPDIR=$PWD make -C package/<name> DUMP=1 V=99
302 \end{Verbatim}
304 If you're just having trouble getting your package to compile, there's a few
305 shortcuts you can take. Instead of waiting for make to get to your package, you can
306 run one of the following:
308 \begin{itemize}
309 \item \texttt{make package/\textit{<name>}-clean V=99}
310 \item \texttt{make package/\textit{<name>}-install V=99}
311 \end{itemize}
313 Another nice trick is that if the source directory under \texttt{build\_\textit{<arch>}}
314 is newer than the package directory, it won't clobber it by unpacking the sources again.
315 If you were working on a patch you could simply edit the sources under the
316 \texttt{build\_\textit{<arch>}/\textit{<source>}} directory and run the install command above,
317 when satisfied, copy the patched sources elsewhere and diff them with the unpatched
318 sources. A warning though - if you go modify anything under \texttt{package/\textit{<name>}}
319 it will remove the old sources and unpack a fresh copy.