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 checkout https://svn.openwrt.org/openwrt/trunk kamikaze
  49 \end{Verbatim}
  50
  51 Additionally, there 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 source repository.
  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 checkout 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 \texttt{target} refers to the embedded platform, this contains items which are specific to
 106 a specific embedded platform. Of particular interest here is the "\texttt{target/linux}"
 107 directory which is broken down by platform \textit{<arch>} and contains the patches to the
 108 kernel, profile config, for a particular platform. There's also the "\texttt{target/image}" directory
 109 which describes how to package a firmware for a specific platform.
 110
 111 Both the target and package steps will use the directory "\texttt{build\_\textit{<arch>}}"
 112 as a temporary directory for compiling. Additionally, anything downloaded by the toolchain,
 113 target or package steps will be placed in the "\texttt{dl}" directory.
 114
 115 \begin{itemize}
 116     \item \texttt{build\_\textit{<arch>}}
 117     \item \texttt{dl}
 118 \end{itemize}
 119
 120 \subsubsection{Building OpenWrt}
 121
 122 While the OpenWrt build environment was intended mostly for developers, it also has to be
 123 simple enough that an inexperienced end user can easily build his or her own customized firmware.
 124
 125 Running the command "\texttt{make menuconfig}" will bring up OpenWrt's configuration menu
 126 screen, through this menu you can select which platform you're targeting, which versions of
 127 the toolchain you want to use to build and what packages you want to install into the
 128 firmware image. Note that it will also check to make sure you have the basic dependencies for it
 129 to run correctly.  If that fails, you will need to install some more tools in your local environment
 130 before you can begin.
 131
 132 Similar to the linux kernel config, almost every option has three choices,
 133 \texttt{y/m/n} which are represented as follows:
 134
 135 \begin{itemize}
 136     \item{\texttt{<*>} (pressing y)} \\
 137         This will be included in the firmware image
 138     \item{\texttt{<M>} (pressing m)} \\
 139         This will be compiled but not included (for later install)
 140     \item{\texttt{< >} (pressing n)} \\
 141         This will not be compiled
 142 \end{itemize}
 143
 144 After you've finished with the menu configuration, exit and when prompted, save your
 145 configuration changes.
 146
 147 If you want, you can also modify the kernel config for the selected target system.
 148 simply run "\texttt{make kernel\_menuconfig}" and the build system will unpack the kernel sources
 149 (if necessary), run menuconfig inside of the kernel tree, and then copy the kernel config
 150 to \texttt{target/linux/\textit{<platform>}/config} so that it is preserved over
 151 "\texttt{make clean}" calls.
 152
 153 To begin compiling the firmware, type "\texttt{make}". By default
 154 OpenWrt will only display a high level overview of the compile process and not each individual
 155 command.
 156
 157 \subsubsection{Example:}
 158
 159 \begin{Verbatim}
 160 make[2] toolchain/install
 161 make[3] -C toolchain install
 162 make[2] target/compile
 163 make[3] -C target compile
 164 make[4] -C target/utils prepare
 165
 166 [...]
 167 \end{Verbatim}
 168
 169 This makes it easier to monitor which step it's actually compiling and reduces the amount
 170 of noise caused by the compile output. To see the full output, run the command
 171 "\texttt{make V=99}".
 172
 173 During the build process, buildroot will download all sources to the "\texttt{dl}"
 174 directory and will start patching and compiling them in the "\texttt{build\_\textit{<arch>}}"
 175 directory. When finished, the resulting firmware will be in the "\texttt{bin}" directory
 176 and packages will be in the "\texttt{bin/packages}" directory.
 177
 178
 179 \subsection{Creating packages}
 180
 181 One of the things that we've attempted to do with OpenWrt's template system is make it
 182 incredibly easy to port software to OpenWrt. If you look at a typical package directory
 183 in OpenWrt you'll find two things:
 184
 185 \begin{itemize}
 186     \item \texttt{package/\textit{<name>}/Makefile}
 187     \item \texttt{package/\textit{<name>}/patches}
 188     \item \texttt{package/\textit{<name>}/files}
 189 \end{itemize}
 190
 191 The patches directory is optional and typically contains bug fixes or optimizations to
 192 reduce the size of the executable. The package makefile is the important item, provides
 193 the steps actually needed to download and compile the package.
 194
 195 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.
 196
 197 Looking at one of the package makefiles, you'd hardly recognize it as a makefile.
 198 Through what can only be described as blatant disregard and abuse of the traditional
 199 make format, the makefile has been transformed into an object oriented template which
 200 simplifies the entire ordeal.
 201
 202 Here for example, is \texttt{package/bridge/Makefile}:
 203
 204 \begin{Verbatim}[frame=single,numbers=left]
 205 #
 206 # Copyright (C) 2006 OpenWrt.org
 207 #
 208 # This is free software, licensed under the GNU General Public License v2.
 209 # See /LICENSE for more information.
 210 #
 211 # $Id: Makefile 5624 2006-11-23 00:29:07Z nbd $
 212
 213 include $(TOPDIR)/rules.mk
 214
 215 PKG_NAME:=bridge
 216 PKG_VERSION:=1.0.6
 217 PKG_RELEASE:=1
 218
 219 PKG_SOURCE:=bridge-utils-$(PKG_VERSION).tar.gz
 220 PKG_SOURCE_URL:=@SF/bridge
 221 PKG_MD5SUM:=9b7dc52656f5cbec846a7ba3299f73bd
 222 PKG_CAT:=zcat
 223
 224 PKG_BUILD_DIR:=$(BUILD_DIR)/bridge-utils-$(PKG_VERSION)
 225
 226 include $(INCLUDE_DIR)/package.mk
 227
 228 define Package/bridge
 229   SECTION:=net
 230   CATEGORY:=Base system
 231   TITLE:=Ethernet bridging configuration utility
 232   DESCRIPTION:=\
 233     Manage ethernet bridging: a way to connect networks together to \\\
 234     form a larger network.
 235   URL:=http://bridge.sourceforge.net/
 236 endef
 237
 238 define Build/Configure
 239     $(call Build/Configure/Default, \
 240         --with-linux-headers="$(LINUX_DIR)" \
 241     )
 242 endef
 243
 244 define Package/bridge/install
 245     $(INSTALL_DIR) $(1)/usr/sbin
 246     $(INSTALL_BIN) $(PKG_BUILD_DIR)/brctl/brctl $(1)/usr/sbin/
 247 endef
 248
 249 $(eval $(call BuildPackage,bridge))
 250 \end{Verbatim}
 251
 252 As you can see, there's not much work to be done; everything is hidden in other makefiles
 253 and abstracted to the point where you only need to specify a few variables.
 254
 255 \begin{itemize}
 256     \item \texttt{PKG\_NAME} \\
 257         The name of the package, as seen via menuconfig and ipkg
 258     \item \texttt{PKG\_VERSION} \\
 259         The upstream version number that we are downloading
 260     \item \texttt{PKG\_RELEASE} \\
 261         The version of this package Makefile
 262     \item \texttt{PKG\_SOURCE} \\
 263         The filename of the original sources
 264     \item \texttt{PKG\_SOURCE\_URL} \\
 265         Where to download the sources from (no trailing slash), you can add multiple download sources by separating them with a \\ and a carriage return.
 266     \item \texttt{PKG\_MD5SUM} \\
 267         A checksum to validate the download
 268     \item \texttt{PKG\_CAT} \\
 269         How to decompress the sources (zcat, bzcat, unzip)
 270     \item \texttt{PKG\_BUILD\_DIR} \\
 271         Where to compile the package
 272 \end{itemize}
 273
 274 The \texttt{PKG\_*} variables define where to download the package from;
 275 \texttt{@SF} is a special keyword for downloading packages from sourceforge. There is also
 276 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.
 277
 278 The md5sum (if present) is used to verify the package was downloaded correctly and
 279 \texttt{PKG\_BUILD\_DIR} defines where to find the package after the sources are
 280 uncompressed into \texttt{\$(BUILD\_DIR)}.
 281
 282 At the bottom of the file is where the real magic happens, "BuildPackage" is a macro
 283 set up by the earlier include statements. BuildPackage only takes one argument directly --
 284 the name of the package to be built, in this case "\texttt{bridge}". All other information
 285 is taken from the define blocks. This is a way of providing a level of verbosity, it's
 286 inherently clear what the contents of the \texttt{description} template in
 287 \texttt{Package/bridge} is, which wouldn't be the case if we passed this information
 288 directly as the Nth argument to \texttt{BuildPackage}.
 289
 290 \texttt{BuildPackage} uses the following defines:
 291
 292 \textbf{\texttt{Package/\textit{<name>}}:} \\
 293     \texttt{\textit{<name>}} matches the argument passed to buildroot, this describes
 294     the package the menuconfig and ipkg entries. Within \texttt{Package/\textit{<name>}}
 295     you can define the following variables:
 296
 297     \begin{itemize}
 298         \item \texttt{SECTION} \\
 299             The type of package (currently unused)
 300         \item \texttt{CATEGORY} \\
 301             Which menu it appears in menuconfig: Network, Sound, Utilities, Multimedia ...
 302         \item \texttt{TITLE} \\
 303             A short description of the package
 304         \item \texttt{URL} \\
 305             Where to find the original software
 306         \item \texttt{MAINTAINER} (optional) \\
 307             Who to contact concerning the package
 308         \item \texttt{DEPENDS} (optional) \\
 309             Which packages must be built/installed before this package. To reference a dependency defined in the
 310                         same Makefile, use \textit{<dependency name>}. If defined as an external package, use
 311                         \textit{+<dependency name>}. For a kernel version dependency use: \textit{@LINUX\_2\_<minor version>}
 312     \end{itemize}
 313
 314 \textbf{\texttt{Package/\textit{<name>}/conffiles} (optional):} \\
 315    A list of config files installed by this package, one file per line.
 316
 317 \textbf{\texttt{Build/Prepare} (optional):} \\
 318    A set of commands to unpack and patch the sources. You may safely leave this
 319    undefined.
 320
 321 \textbf{\texttt{Build/Configure} (optional):} \\
 322    You can leave this undefined if the source doesn't use configure or has a
 323    normal config script, otherwise you can put your own commands here or use
 324    "\texttt{\$(call Build/Configure/Default,\textit{<first list of arguments, second list>})}" as above to
 325    pass in additional arguments for a standard configure script. The first list of arguments will be passed
 326    to the configure script like that: \texttt{--arg 1} \texttt{--arg 2}. The second list contains arguments that should be
 327    defined before running the configure script such as autoconf or compiler specific variables.
 328
 329    To make it easier to modify the configure command line, you can either extend or completely override the following variables:
 330    \begin{itemize}
 331      \item \texttt{CONFIGURE\_ARGS} \\
 332              Contains all command line arguments (format: \texttt{--arg 1} \texttt{--arg 2})
 333      \item \texttt{CONFIGURE\_VARS} \\
 334              Contains all environment variables that are passed to ./configure (format: \texttt{NAME="value"})
 335    \end{itemize}
 336
 337 \textbf{\texttt{Build/Compile} (optional):} \\
 338    How to compile the source; in most cases you should leave this undefined.
 339
 340    As with \texttt{Build/Configure} there are two variables that allow you to override
 341    the make command line environment variables and flags:
 342    \begin{itemize}
 343      \item \texttt{MAKE\_FLAGS} \\
 344            Contains all command line arguments (typically variable overrides like \texttt{NAME="value"}
 345          \item \texttt{MAKE\_VARS} \\
 346            Contains all environment variables that are passed to the make command
 347    \end{itemize}
 348
 349 \textbf{\texttt{Package/\textit{<name>}/install}:} \\
 350    A set of commands to copy files out of the compiled source and into the ipkg
 351    which is represented by the \texttt{\$(1)} directory. Note that there are currently
 352    4 defined install macros:
 353    \begin{itemize}
 354        \item \texttt{INSTALL\_DIR} \\
 355            install -d -m0755
 356        \item \texttt{INSTALL\_BIN} \\
 357            install -m0755
 358        \item \texttt{INSTALL\_DATA} \\
 359            install -m0644
 360        \item \texttt{INSTALL\_CONF} \\
 361            install -m0600
 362    \end{itemize}
 363
 364 The reason that some of the defines are prefixed by "\texttt{Package/\textit{<name>}}"
 365 and others are simply "\texttt{Build}" is because of the possibility of generating
 366 multiple packages from a single source. OpenWrt works under the assumption of one
 367 source per package Makefile, but you can split that source into as many packages as
 368 desired. Since you only need to compile the sources once, there's one global set of
 369 "\texttt{Build}" defines, but you can add as many "Package/<name>" defines as you want
 370 by adding extra calls to \texttt{BuildPackage} -- see the dropbear package for an example.
 371
 372 After you have created your \texttt{package/\textit{<name>}/Makefile}, the new package
 373 will automatically show in the menu the next time you run "make menuconfig" and if selected
 374 will be built automatically the next time "\texttt{make}" is run.
 375
 376 \subsection{Creating kernel modules packages}
 377
 378 The OpenWrt distribution makes the distinction between two kind of kernel modules, those coming along with the mainline kernel, and the others available as a separate project. We will see later that a common template is used for both of them.
 379
 380 For kernel modules that are part of the mainline kernel source, the makefiles are located in \textit{package/kernel/modules/*.mk} and they appear under the section "Kernel modules"
 381
 382 For external kernel modules, you can add them to the build system just like if they were software packages by defining a KernelPackage section in the package makefile.
 383
 384 Here for instance the Makefile for the I2C subsytem kernel modules :
 385
 386 \begin{Verbatim}[frame=single,numbers=left]
 387 #
 388 # Copyright (C) 2006 OpenWrt.org
 389 #
 390 # This is free software, licensed under the GNU General Public License v2.
 391 # See /LICENSE for more information.
 392 #
 393 # $Id $
 394
 395 I2CMENU:=I2C Bus
 396
 397 define KernelPackage/i2c-core
 398   TITLE:=I2C support
 399   DESCRIPTION:=Kernel modules for i2c support
 400   SUBMENU:=$(I2CMENU)
 401   KCONFIG:=$(CONFIG_I2C_CORE) \
 402         $(CONFIG_I2C_DEV)
 403   FILES:=$(MODULES_DIR)/kernel/drivers/i2c/*.$(LINUX_KMOD_SUFFIX)
 404   AUTOLOAD:=$(call AutoLoad,50,i2c-core i2c-dev)
 405 endef
 406 $(eval $(call KernelPackage,i2c-core))
 407 \end{Verbatim}
 408
 409 To group kernel modules under a common description in menuconfig, you might want to define a \textit{<description>MENU} variable on top of the kernel modules makefile.
 410
 411 \begin{itemize}
 412     \item \texttt{TITLE} \\
 413         The name of the module as seen via menuconfig
 414     \item \texttt{DESCRIPTION} \\
 415         The description as seen via help in menuconfig
 416     \item \texttt{SUBMENU} \\
 417         The sub menu under which this package will be seen
 418     \item \texttt{KCONFIG} \\
 419         Kernel configuration option dependency. For external modules, remove it.
 420     \item \texttt{FILES} \\
 421         Files you want to inlude to this kernel module package, separate with spaces.
 422     \item \texttt{AUTOLOAD} \\
 423         Modules that will be loaded automatically on boot, the order you write them is the order they would be loaded.
 424 \end{itemize}
 425
 426 After you have created your \texttt{package/kernel/modules/\textit{<name>}.mk}, the new kernel modules package
 427 will automatically show in the menu under "Kernel modules" next time you run "make menuconfig" and if selected
 428 will be built automatically the next time "\texttt{make}" is run.
 429
 430 \subsection{Conventions}
 431
 432 There are a couple conventions to follow regarding packages:
 433
 434 \begin{itemize}
 435     \item \texttt{files}
 436     \begin{enumerate}
 437         \item configuration files follow the convention \\
 438         \texttt{\textit{<name>}.conf}
 439         \item init files follow the convention \\
 440         \texttt{\textit{<name>}.init}
 441     \end{enumerate}
 442     \item \texttt{patches}
 443     \begin{enumerate}
 444         \item patches are numerically prefixed and named related to what they do
 445     \end{enumerate}
 446 \end{itemize}
 447
 448 \subsection{Troubleshooting}
 449
 450 If you find your package doesn't show up in menuconfig, try the following command to
 451 see if you get the correct description:
 452
 453 \begin{Verbatim}
 454   TOPDIR=$PWD make -C package/<name> DUMP=1 V=99
 455 \end{Verbatim}
 456
 457 If you're just having trouble getting your package to compile, there's a few
 458 shortcuts you can take. Instead of waiting for make to get to your package, you can
 459 run one of the following:
 460
 461 \begin{itemize}
 462     \item \texttt{make package/\textit{<name>}-clean V=99}
 463     \item \texttt{make package/\textit{<name>}-install V=99}
 464 \end{itemize}
 465
 466 Another nice trick is that if the source directory under \texttt{build\_\textit{<arch>}}
 467 is newer than the package directory, it won't clobber it by unpacking the sources again.
 468 If you were working on a patch you could simply edit the sources under the
 469 \texttt{build\_\textit{<arch>}/\textit{<source>}} directory and run the install command above,
 470 when satisfied, copy the patched sources elsewhere and diff them with the unpatched
 471 sources. A warning though - if you go modify anything under \texttt{package/\textit{<name>}}
 472 it will remove the old sources and unpack a fresh copy.
 473
 474 Other useful targets include:
 475
 476 \begin{itemize}
 477     \item \texttt{make package/\textit{<name>}-prepare V=99}
 478     \item \texttt{make package/\textit{<name>}-compile V=99}
 479     \item \texttt{make package/\textit{<name>}-configure V=99}
 480 \end{itemize}
 481