One of the biggest challenges to getting started with embedded devices is that you
-can't just install a copy of Linux and expect to be able to compile a firmware.
+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 wouldn't have the basic set of tools needed to produce a firmware image.
+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
-incompatible with the hardware on your development machine, so in a process called
+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's not something that's
-regularly attempted and so there's a certain amount of mystery and black magic
-associated with it. In many cases when you're dealing with embedded devices you'll
+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's a time saving step but at the same time
-often means you'll be using a rather dated set of tools. Likewise, it's also common
+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.
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 doesn't contain any executables or even sources, it's an
+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's no work to be done trying to track down an unmodified
+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 doesn't
-just apply to the kernel, but to anything included with OpenWrt -- It's this one
+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}
+\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 co https://svn.openwrt.org/openwrt/trunk kamikaze
+$ svn checkout https://svn.openwrt.org/openwrt/trunk kamikaze
\end{Verbatim}
-Additionally, there's a trac interface on \href{https://dev.openwrt.org/}{https://dev.openwrt.org/}
-which can be used to monitor svn commits and browse the sources.
+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}
\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.
+used to build the firmware image, the compiler, and the C library.
The result of this is three new directories, \texttt{tool\_build}, which is a temporary
directory for building the target independent tools, \texttt{toolchain\_build\_\textit{<arch>}}
which is used for building the toolchain for a specific architecture, and
\texttt{staging\_dir\_\textit{<arch>}} where the resulting toolchain is installed.
-You won't need to do anything with the toolchain directory unless you intend to
+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}
trunk and can be obtained from subversion at the following location:
\begin{Verbatim}
-$ svn co https://svn.openwrt.org/openwrt/packages ../packages
+$ svn checkout https://svn.openwrt.org/openwrt/packages packages
\end{Verbatim}
Those packages can be used to extend the functionality of the build system and need to be
$ ln -s packages/net/nmap kamikaze/package/nmap
\end{Verbatim}
+To include all packages, issue the following command:
+
+\begin{Verbatim}
+$ ln -s packages/*/* kamikaze/package/
+\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 and contains the kernel config and patches
-to the kernel for a particular platform. There's also the "\texttt{target/image}" directory
+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\_\textit{<arch>}}"
\end{itemize}
After you've finished with the menu configuration, exit and when prompted, save your
-configuration changes. To begin compiling the firmware, type "\texttt{make}". By default
+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.
\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.
+
Looking at one of the package makefiles, you'd hardly recognize it as a makefile.
Through what can only be described as blatant disregard and abuse of the traditional
make format, the makefile has been transformed into an object oriented template which
\item \texttt{PKG\_NAME} \\
The name of the package, as seen via menuconfig and ipkg
\item \texttt{PKG\_VERSION} \\
- The upstream version number that we're downloading
+ The upstream version number that we are downloading
\item \texttt{PKG\_RELEASE} \\
The version of this package Makefile
\item \texttt{PKG\_SOURCE} \\
The filename of the original sources
\item \texttt{PKG\_SOURCE\_URL} \\
- Where to download the sources from (no trailing slash)
+ Where to download the sources from (no trailing slash), you can add multiple download sources by separating them with a \\ and a carriage return.
\item \texttt{PKG\_MD5SUM} \\
A checksum to validate the download
\item \texttt{PKG\_CAT} \\
The \texttt{PKG\_*} variables define where to download the package from;
\texttt{@SF} is a special keyword for downloading packages from sourceforge. There is also
-another keyword of \texttt{@GNU} for grabbing GNU source releases.
+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.
-The md5sum is used to verify the package was downloaded correctly and
+The md5sum (if present) is used to verify the package was downloaded correctly and
\texttt{PKG\_BUILD\_DIR} defines where to find the package after the sources are
uncompressed into \texttt{\$(BUILD\_DIR)}.
\item \texttt{SECTION} \\
The type of package (currently unused)
\item \texttt{CATEGORY} \\
- Which menu it appears in menuconfig
+ Which menu it appears in menuconfig: Network, Sound, Utilities, Multimedia ...
\item \texttt{TITLE} \\
A short description of the package
\item \texttt{URL} \\
\item \texttt{MAINTAINER} (optional) \\
Who to contact concerning the package
\item \texttt{DEPENDS} (optional) \\
- Which packages must be built/installed before this package
+ Which packages must be built/installed before this package. To reference a dependency defined in the
+ same Makefile, use \textit{<dependency name>}. If defined as an external package, use
+ \textit{+<dependency name>}. For a kernel version dependency use: \textit{@LINUX\_2\_<minor version>}
\end{itemize}
\textbf{\texttt{Package/\textit{<name>}/conffiles} (optional):} \\
\textbf{\texttt{Build/Configure} (optional):} \\
You can leave this undefined if the source doesn't use configure or has a
normal config script, otherwise you can put your own commands here or use
- "\texttt{\$(call Build/Configure/Default,\textit{<args>})}" as above to
- pass in additional arguments for a standard configure script.
+ "\texttt{\$(call Build/Configure/Default,\textit{<first list of arguments, second list>})}" as above to
+ pass in additional arguments for a standard configure script. The first list of arguments will be passed
+ to the configure script like that: \texttt{--arg 1} \texttt{--arg 2}. The second list contains arguments that should be
+ defined before running the configure script such as autoconf or compiler specific variables.
+
+ To make it easier to modify the configure command line, you can either extend or completely override the following variables:
+ \begin{itemize}
+ \item \texttt{CONFIGURE\_ARGS} \\
+ Contains all command line arguments (format: \texttt{--arg 1} \texttt{--arg 2})
+ \item \texttt{CONFIGURE\_VARS} \\
+ Contains all environment variables that are passed to ./configure (format: \texttt{NAME="value"})
+ \end{itemize}
\textbf{\texttt{Build/Compile} (optional):} \\
How to compile the source; in most cases you should leave this undefined.
+
+ As with \texttt{Build/Configure} there are two variables that allow you to override
+ the make command line environment variables and flags:
+ \begin{itemize}
+ \item \texttt{MAKE\_FLAGS} \\
+ Contains all command line arguments (typically variable overrides like \texttt{NAME="value"}
+ \item \texttt{MAKE\_VARS} \\
+ Contains all environment variables that are passed to the make command
+ \end{itemize}
\textbf{\texttt{Package/\textit{<name>}/install}:} \\
A set of commands to copy files out of the compiled source and into the ipkg
which is represented by the \texttt{\$(1)} directory. Note that there are currently
- 3 defined install macros:
+ 4 defined install macros:
\begin{itemize}
\item \texttt{INSTALL\_DIR} \\
install -d -m0755
install -m0755
\item \texttt{INSTALL\_DATA} \\
install -m0644
+ \item \texttt{INSTALL\_CONF} \\
+ install -m0600
\end{itemize}
The reason that some of the defines are prefixed by "\texttt{Package/\textit{<name>}}"
"\texttt{Build}" defines, but you can add as many "Package/<name>" defines as you want
by adding extra calls to \texttt{BuildPackage} -- see the dropbear package for an example.
-After you've created your \texttt{package/\textit{<name>}/Makefile}, the new package
+After you have created your \texttt{package/\textit{<name>}/Makefile}, the new package
will automatically show in the menu the next time you run "make menuconfig" and if selected
will be built automatically the next time "\texttt{make}" is run.
+\subsection{Creating kernel modules packages}
+
+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.
+
+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"
+
+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.
+
+Here for instance the Makefile for the I2C subsytem kernel modules :
+
+\begin{Verbatim}[frame=single,numbers=left]
+#
+# Copyright (C) 2006 OpenWrt.org
+#
+# This is free software, licensed under the GNU General Public License v2.
+# See /LICENSE for more information.
+#
+# $Id $
+
+I2CMENU:=I2C Bus
+
+define KernelPackage/i2c-core
+ TITLE:=I2C support
+ DESCRIPTION:=Kernel modules for i2c support
+ SUBMENU:=$(I2CMENU)
+ KCONFIG:=$(CONFIG_I2C_CORE) \
+ $(CONFIG_I2C_DEV)
+ FILES:=$(MODULES_DIR)/kernel/drivers/i2c/*.$(LINUX_KMOD_SUFFIX)
+ AUTOLOAD:=$(call AutoLoad,50,i2c-core i2c-dev)
+endef
+$(eval $(call KernelPackage,i2c-core))
+\end{Verbatim}
+
+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.
+
+\begin{itemize}
+ \item \texttt{TITLE} \\
+ The name of the module as seen via menuconfig
+ \item \texttt{DESCRIPTION} \\
+ The description as seen via help in menuconfig
+ \item \texttt{SUBMENU} \\
+ The sub menu under which this package will be seen
+ \item \texttt{KCONFIG} \\
+ Kernel configuration option dependency. For external modules, remove it.
+ \item \texttt{FILES} \\
+ Files you want to inlude to this kernel module package, separate with spaces.
+ \item \texttt{AUTOLOAD} \\
+ Modules that will be loaded automatically on boot, the order you write them is the order they would be loaded.
+\end{itemize}
+
+After you have created your \texttt{package/kernel/modules/\textit{<name>}.mk}, the new kernel modules package
+will automatically show in the menu under "Kernel modules" next time you run "make menuconfig" and if selected
+will be built automatically the next time "\texttt{make}" is run.
\subsection{Conventions}
projects / openwrt.git / blobdiff