docs/build.tex

   1 One of the biggest challenges to getting started with embedded devices is that you
   2 cannot just 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 would not 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 most of the time 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.
  10
  11 The process of creating a cross compiler can be tricky, it is not something that is
  12 regularly attempted and so there is a certain amount of mystery and black magic
  13 associated with it. In many cases when you are dealing with embedded devices you will
  14 be provided with a binary copy of a compiler and basic libraries rather than
  15 instructions for creating your own -- it is a time saving step but at the same time
  16 often means you will be using a rather dated set of tools. Likewise, it is also common
  17 to be 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 modified to make the kernel run on the embedded platform.
  20
  21 \subsection{Building an image}
  22
  23 OpenWrt takes a different approach to building a firmware; downloading, patching
  24 and compiling everything from scratch, including the cross compiler. To put it
  25 in simpler terms, OpenWrt does not contain any executables or even sources, it is an
  26 automated system for downloading the sources, patching them to work with the given
  27 platform and compiling them correctly for that platform. What this means is that
  28 just by changing the template, you can change any step in the process.
  29
  30 As an example, if a new kernel is released, a simple change to one of the Makefiles
  31 will download the latest kernel, patch it to run on the embedded platform and produce
  32 a new firmware image -- there is no work to be done trying to track down an unmodified
  33 copy of the existing kernel to see what changes had been made, the patches are
  34 already provided and the process ends up almost completely transparent. This does not
  35 just apply to the kernel, but to anything included with OpenWrt -- It is this one
  36 simple understated concept which is what allows OpenWrt to stay on the bleeding edge
  37 with the latest compilers, latest kernels and latest applications.
  38
  39 So let's take a look at OpenWrt and see how this all works.
  40
  41
  42 \subsubsection{Download openwrt}
  43
  44 This article refers to the "Kamikaze" branch of OpenWrt, which can be downloaded via
  45 subversion using the following command:
  46
  47 \begin{Verbatim}
  48 $ svn co https://svn.openwrt.org/openwrt/trunk kamikaze
  49 \end{Verbatim}
  50
  51 Additionally, ther is a trac interface on \href{https://dev.openwrt.org/}{https://dev.openwrt.org/}
  52 which can be used to monitor svn commits and browse the sources.
  53
  54
  55 \subsubsection{The directory structure}
  56
  57 There are four key directories in the base:
  58
  59 \begin{itemize}
  60     \item \texttt{tools}
  61     \item \texttt{toolchain}
  62     \item \texttt{package}
  63     \item \texttt{target}
  64 \end{itemize}
  65
  66 \texttt{tools} and \texttt{toolchain} refer to common tools which will be
  67 used to build the firmware image, the compiler, and the C library.
  68 The result of this is three new directories, \texttt{tool\_build}, which is a temporary
  69 directory for building the target independent tools, \texttt{toolchain\_build\_\textit{<arch>}}
  70 which is used for building the toolchain for a specific architecture, and
  71 \texttt{staging\_dir\_\textit{<arch>}} where the resulting toolchain is installed.
  72 You will not need to do anything with the toolchain directory unless you intend to
  73 add a new version of one of the components above.
  74
  75 \begin{itemize}
  76     \item \texttt{tool\_build}
  77     \item \texttt{toolchain\_build\_\textit{<arch>}}
  78 \end{itemize}
  79
  80 \texttt{package} is for exactly that -- packages. In an OpenWrt firmware, almost everything
  81 is an \texttt{.ipk}, a software package which can be added to the firmware to provide new
  82 features or removed to save space. Note that packages are also maintained outside of the main
  83 trunk and can be obtained from subversion at the following location:
  84
  85 \begin{Verbatim}
  86 $ svn co https://svn.openwrt.org/openwrt/packages ../packages
  87 \end{Verbatim}
  88
  89 Those packages can be used to extend the functionality of the build system and need to be
  90 symlinked into the main trunk. Once you do that, the packages will show up in the menu for
  91 configuration. From kamikaze you would do something like this:
  92
  93 \begin{Verbatim}
  94 $ ls
  95 kamikaze  packages
  96 $ ln -s packages/net/nmap kamikaze/package/nmap
  97 \end{Verbatim}
  98
  99 To include all packages, issue the following command:
 100
 101 \begin{Verbatim}
 102 $ ln -s packages/*/* kamikaze/package/
 103 \end{Verbatim}
 104
 105
 106 \texttt{target} refers to the embedded platform, this contains items which are specific to
 107 a specific embedded platform. Of particular interest here is the "\texttt{target/linux}"
 108 directory which is broken down by platform and contains the kernel config and patches
 109 to the kernel for a particular platform. There's also the "\texttt{target/image}" directory
 110 which describes how to package a firmware for a specific platform.
 111
 112 Both the target and package steps will use the directory "\texttt{build\_\textit{<arch>}}"
 113 as a temporary directory for compiling. Additionally, anything downloaded by the toolchain,
 114 target or package steps will be placed in the "\texttt{dl}" directory.
 115
 116 \begin{itemize}
 117     \item \texttt{build\_\textit{<arch>}}
 118     \item \texttt{dl}
 119 \end{itemize}
 120
 121 \subsubsection{Building OpenWrt}
 122
 123 While the OpenWrt build environment was intended mostly for developers, it also has to be
 124 simple enough that an inexperienced end user can easily build his or her own customized firmware.
 125
 126 Running the command "\texttt{make menuconfig}" will bring up OpenWrt's configuration menu
 127 screen, through this menu you can select which platform you're targeting, which versions of
 128 the toolchain you want to use to build and what packages you want to install into the
 129 firmware image. Note that it will also check to make sure you have the basic dependencies for it
 130 to run correctly.  If that fails, you will need to install some more tools in your local environment
 131 before you can begin.
 132
 133 Similar to the linux kernel config, almost every option has three choices,
 134 \texttt{y/m/n} which are represented as follows:
 135
 136 \begin{itemize}
 137     \item{\texttt{<*>} (pressing y)} \\
 138         This will be included in the firmware image
 139     \item{\texttt{<M>} (pressing m)} \\
 140         This will be compiled but not included (for later install)
 141     \item{\texttt{< >} (pressing n)} \\
 142         This will not be compiled
 143 \end{itemize}
 144
 145 After you've finished with the menu configuration, exit and when prompted, save your
 146 configuration changes.
 147
 148 If you want, you can also modify the kernel config for the selected target system.
 149 simply run "\texttt{make kernel\_menuconfig}" and the build system will unpack the kernel sources
 150 (if necessary), run menuconfig inside of the kernel tree, and then copy the kernel config
 151 to \texttt{target/linux/\textit{<platform>}/config} so that it is preserved over
 152 "\texttt{make clean}" calls.
 153
 154 To begin compiling the firmware, type "\texttt{make}". By default
 155 OpenWrt will only display a high level overview of the compile process and not each individual
 156 command.
 157
 158 \subsubsection{Example:}
 159
 160 \begin{Verbatim}
 161 make[2] toolchain/install
 162 make[3] -C toolchain install
 163 make[2] target/compile
 164 make[3] -C target compile
 165 make[4] -C target/utils prepare
 166
 167 [...]
 168 \end{Verbatim}
 169
 170 This makes it easier to monitor which step it's actually compiling and reduces the amount
 171 of noise caused by the compile output. To see the full output, run the command
 172 "\texttt{make V=99}".
 173
 174 During the build process, buildroot will download all sources to the "\texttt{dl}"
 175 directory and will start patching and compiling them in the "\texttt{build\_\textit{<arch>}}"
 176 directory. When finished, the resulting firmware will be in the "\texttt{bin}" directory
 177 and packages will be in the "\texttt{bin/packages}" directory.
 178
 179
 180 \subsection{Creating packages}
 181
 182 One of the things that we've attempted to do with OpenWrt's template system is make it
 183 incredibly easy to port software to OpenWrt. If you look at a typical package directory
 184 in OpenWrt you'll find two things:
 185
 186 \begin{itemize}
 187     \item \texttt{package/\textit{<name>}/Makefile}
 188     \item \texttt{package/\textit{<name>}/patches}
 189         \item \texttt{package/\textit{<name>}/files}
 190 \end{itemize}
 191
 192 The patches directory is optional and typically contains bug fixes or optimizations to
 193 reduce the size of the executable. The package makefile is the important item, provides
 194 the steps actually needed to download and compile the package.
 195
 196 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.
 197
 198 Looking at one of the package makefiles, you'd hardly recognize it as a makefile.
 199 Through what can only be described as blatant disregard and abuse of the traditional
 200 make format, the makefile has been transformed into an object oriented template which
 201 simplifies the entire ordeal.
 202
 203 Here for example, is \texttt{package/bridge/Makefile}:
 204
 205 \begin{Verbatim}[frame=single,numbers=left]
 206 #
 207 # Copyright (C) 2006 OpenWrt.org
 208 #
 209 # This is free software, licensed under the GNU General Public License v2.
 210 # See /LICENSE for more information.
 211 #
 212 # $Id: Makefile 5624 2006-11-23 00:29:07Z nbd $
 213
 214 include $(TOPDIR)/rules.mk
 215
 216 PKG_NAME:=bridge
 217 PKG_VERSION:=1.0.6
 218 PKG_RELEASE:=1
 219
 220 PKG_SOURCE:=bridge-utils-$(PKG_VERSION).tar.gz
 221 PKG_SOURCE_URL:=@SF/bridge
 222 PKG_MD5SUM:=9b7dc52656f5cbec846a7ba3299f73bd
 223 PKG_CAT:=zcat
 224
 225 PKG_BUILD_DIR:=$(BUILD_DIR)/bridge-utils-$(PKG_VERSION)
 226
 227 include $(INCLUDE_DIR)/package.mk
 228
 229 define Package/bridge
 230   SECTION:=net
 231   CATEGORY:=Base system
 232   TITLE:=Ethernet bridging configuration utility
 233   DESCRIPTION:=\
 234     Manage ethernet bridging: a way to connect networks together to \\\
 235     form a larger network.
 236   URL:=http://bridge.sourceforge.net/
 237 endef
 238
 239 define Build/Configure
 240     $(call Build/Configure/Default, \
 241         --with-linux-headers="$(LINUX_DIR)" \
 242     )
 243 endef
 244
 245 define Package/bridge/install
 246     $(INSTALL_DIR) $(1)/usr/sbin
 247     $(INSTALL_BIN) $(PKG_BUILD_DIR)/brctl/brctl $(1)/usr/sbin/
 248 endef
 249
 250 $(eval $(call BuildPackage,bridge))
 251 \end{Verbatim}
 252
 253 As you can see, there's not much work to be done; everything is hidden in other makefiles
 254 and abstracted to the point where you only need to specify a few variables.
 255
 256 \begin{itemize}
 257     \item \texttt{PKG\_NAME} \\
 258         The name of the package, as seen via menuconfig and ipkg
 259     \item \texttt{PKG\_VERSION} \\
 260         The upstream version number that we are downloading
 261     \item \texttt{PKG\_RELEASE} \\
 262         The version of this package Makefile
 263     \item \texttt{PKG\_SOURCE} \\
 264         The filename of the original sources
 265     \item \texttt{PKG\_SOURCE\_URL} \\
 266         Where to download the sources from (no trailing slash), you can add multiple download sources by separating them with a \\ and a carriage return.
 267     \item \texttt{PKG\_MD5SUM} \\
 268         A checksum to validate the download
 269     \item \texttt{PKG\_CAT} \\
 270         How to decompress the sources (zcat, bzcat, unzip)
 271     \item \texttt{PKG\_BUILD\_DIR} \\
 272         Where to compile the package
 273 \end{itemize}
 274
 275 The \texttt{PKG\_*} variables define where to download the package from;
 276 \texttt{@SF} is a special keyword for downloading packages from sourceforge. There is also
 277 another keyword of \texttt{@GNU} for grabbing GNU source releases. If any of the above mentionned download source fails, the OpenWrt mirrors will be used as source.
 278
 279 The md5sum (if present) is used to verify the package was downloaded correctly and
 280 \texttt{PKG\_BUILD\_DIR} defines where to find the package after the sources are
 281 uncompressed into \texttt{\$(BUILD\_DIR)}.
 282
 283 At the bottom of the file is where the real magic happens, "BuildPackage" is a macro
 284 set up by the earlier include statements. BuildPackage only takes one argument directly --
 285 the name of the package to be built, in this case "\texttt{bridge}". All other information
 286 is taken from the define blocks. This is a way of providing a level of verbosity, it's
 287 inherently clear what the contents of the \texttt{description} template in
 288 \texttt{Package/bridge} is, which wouldn't be the case if we passed this information
 289 directly as the Nth argument to \texttt{BuildPackage}.
 290
 291 \texttt{BuildPackage} uses the following defines:
 292
 293 \textbf{\texttt{Package/\textit{<name>}}:} \\
 294     \texttt{\textit{<name>}} matches the argument passed to buildroot, this describes
 295     the package the menuconfig and ipkg entries. Within \texttt{Package/\textit{<name>}}
 296     you can define the following variables:
 297
 298     \begin{itemize}
 299         \item \texttt{SECTION} \\
 300             The type of package (currently unused)
 301         \item \texttt{CATEGORY} \\
 302             Which menu it appears in menuconfig: Network, Sound, Utilities, Multimedia ...
 303         \item \texttt{TITLE} \\
 304             A short description of the package
 305         \item \texttt{URL} \\
 306             Where to find the original software
 307         \item \texttt{MAINTAINER} (optional) \\
 308             Who to contact concerning the package
 309         \item \texttt{DEPENDS} (optional) \\
 310             Which packages must be built/installed before this package. To reference a dependency defined in the
 311                         same Makefile, use \textit{<dependency name>}. If defined as an external package, use
 312                         \textit{+<dependency name>}. For a kernel version dependency use: \textit{@LINUX\_2\_<minor version>}
 313     \end{itemize}
 314
 315 \textbf{\texttt{Package/\textit{<name>}/conffiles} (optional):} \\
 316    A list of config files installed by this package, one file per line.
 317
 318 \textbf{\texttt{Build/Prepare} (optional):} \\
 319    A set of commands to unpack and patch the sources. You may safely leave this
 320    undefined.
 321
 322 \textbf{\texttt{Build/Configure} (optional):} \\
 323    You can leave this undefined if the source doesn't use configure or has a
 324    normal config script, otherwise you can put your own commands here or use
 325    "\texttt{\$(call Build/Configure/Default,\textit{<first list of arguments, second list>})}" as above to
 326    pass in additional arguments for a standard configure script. The first list of arguments will be passed
 327    to the configure script like that: \texttt{--arg 1} \texttt{--arg 2}. The second list contains arguments that should be
 328    defined before running the configure script such as autoconf or compiler specific variables.
 329
 330    To make it easier to modify the configure command line, you can either extend or completely override the following variables:
 331    \begin{itemize}
 332      \item \texttt{CONFIGURE\_ARGS} \\
 333              Contains all command line arguments (format: \texttt{--arg 1} \texttt{--arg 2})
 334      \item \texttt{CONFIGURE\_VARS} \\
 335              Contains all environment variables that are passed to ./configure (format: \texttt{NAME="value"})
 336    \end{itemize}
 337
 338 \textbf{\texttt{Build/Compile} (optional):} \\
 339    How to compile the source; in most cases you should leave this undefined.
 340
 341 \textbf{\texttt{Package/\textit{<name>}/install}:} \\
 342    A set of commands to copy files out of the compiled source and into the ipkg
 343    which is represented by the \texttt{\$(1)} directory. Note that there are currently
 344    4 defined install macros:
 345    \begin{itemize}
 346        \item \texttt{INSTALL\_DIR} \\
 347            install -d -m0755
 348        \item \texttt{INSTALL\_BIN} \\
 349            install -m0755
 350        \item \texttt{INSTALL\_DATA} \\
 351            install -m0644
 352        \item \texttt{INSTALL\_CONF} \\
 353            install -m0600
 354    \end{itemize}
 355
 356 The reason that some of the defines are prefixed by "\texttt{Package/\textit{<name>}}"
 357 and others are simply "\texttt{Build}" is because of the possibility of generating
 358 multiple packages from a single source. OpenWrt works under the assumption of one
 359 source per package Makefile, but you can split that source into as many packages as
 360 desired. Since you only need to compile the sources once, there's one global set of
 361 "\texttt{Build}" defines, but you can add as many "Package/<name>" defines as you want
 362 by adding extra calls to \texttt{BuildPackage} -- see the dropbear package for an example.
 363
 364 After you have created your \texttt{package/\textit{<name>}/Makefile}, the new package
 365 will automatically show in the menu the next time you run "make menuconfig" and if selected
 366 will be built automatically the next time "\texttt{make}" is run.
 367
 368
 369 \subsection{Conventions}
 370
 371 There are a couple conventions to follow regarding packages:
 372
 373 \begin{itemize}
 374     \item \texttt{files}
 375     \begin{enumerate}
 376         \item configuration files follow the convention \\
 377         \texttt{\textit{<name>}.conf}
 378         \item init files follow the convention \\
 379         \texttt{\textit{<name>}.init}
 380     \end{enumerate}
 381     \item \texttt{patches}
 382     \begin{enumerate}
 383         \item patches are numerically prefixed and named related to what they do
 384     \end{enumerate}
 385 \end{itemize}
 386
 387 \subsection{Troubleshooting}
 388
 389 If you find your package doesn't show up in menuconfig, try the following command to
 390 see if you get the correct description:
 391
 392 \begin{Verbatim}
 393   TOPDIR=$PWD make -C package/<name> DUMP=1 V=99
 394 \end{Verbatim}
 395
 396 If you're just having trouble getting your package to compile, there's a few
 397 shortcuts you can take. Instead of waiting for make to get to your package, you can
 398 run one of the following:
 399
 400 \begin{itemize}
 401     \item \texttt{make package/\textit{<name>}-clean V=99}
 402     \item \texttt{make package/\textit{<name>}-install V=99}
 403 \end{itemize}
 404
 405 Another nice trick is that if the source directory under \texttt{build\_\textit{<arch>}}
 406 is newer than the package directory, it won't clobber it by unpacking the sources again.
 407 If you were working on a patch you could simply edit the sources under the
 408 \texttt{build\_\textit{<arch>}/\textit{<source>}} directory and run the install command above,
 409 when satisfied, copy the patched sources elsewhere and diff them with the unpatched
 410 sources. A warning though - if you go modify anything under \texttt{package/\textit{<name>}}
 411 it will remove the old sources and unpack a fresh copy.
 412
 413 Other useful targets include:
 414
 415 \begin{itemize}
 416     \item \texttt{make package/\textit{<name>}-prepare V=99}
 417     \item \texttt{make package/\textit{<name>}-compile V=99}
 418     \item \texttt{make package/\textit{<name>}-configure V=99}
 419 \end{itemize}
 420