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diff --git a/docs/manual/configure.txt b/docs/manual/configure.txt new file mode 100644 index 000000000..65e0eeb2f --- /dev/null +++ b/docs/manual/configure.txt @@ -0,0 +1,428 @@ +// -*- mode:doc -*- ; + +[[configure]] +Details on Buildroot configuration +---------------------------------- + +All the configuration options in +make *config+ have a help text +providing details about the option. However, a number of topics +require additional details that cannot easily be covered in the help +text and are there covered in the following sections. + +Cross-compilation toolchain +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +A compilation toolchain is the set of tools that allows you to compile +code for your system. It consists of a compiler (in our case, +gcc+), +binary utils like assembler and linker (in our case, +binutils+) and a +C standard library (for example +http://www.gnu.org/software/libc/libc.html[GNU Libc], +http://www.uclibc.org/[uClibc]). + +The system installed on your development station certainly already has +a compilation toolchain that you can use to compile an application +that runs on your system. If you're using a PC, your compilation +toolchain runs on an x86 processor and generates code for an x86 +processor. Under most Linux systems, the compilation toolchain uses +the GNU libc (glibc) as the C standard library. This compilation +toolchain is called the "host compilation toolchain". The machine on +which it is running, and on which you're working, is called the "host +system" footnote:[This terminology differs from what is used by GNU +configure, where the host is the machine on which the application will +run (which is usually the same as target)]. + +The compilation toolchain is provided by your distribution, and +Buildroot has nothing to do with it (other than using it to build a +cross-compilation toolchain and other tools that are run on the +development host). + +As said above, the compilation toolchain that comes with your system +runs on and generates code for the processor in your host system. As +your embedded system has a different processor, you need a +cross-compilation toolchain - a compilation toolchain that runs on +your _host system_ but generates code for your _target system_ (and +target processor). For example, if your host system uses x86 and your +target system uses ARM, the regular compilation toolchain on your host +runs on x86 and generates code for x86, while the cross-compilation +toolchain runs on x86 and generates code for ARM. + +Buildroot provides different solutions to build, or use existing +cross-compilation toolchains: + + * The *internal toolchain backend*, called +Buildroot toolchain+ in + the configuration interface. + + * The *external toolchain backend*, called +External toolchain+ in + the configuration interface. + + * The *Crosstool-NG toolchain backend*, called +Crosstool-NG + toolchain+ in the configuration interface. + +The choice between these three solutions is done using the +Toolchain +Type+ option in the +Toolchain+ menu. Once one solution has been +chosen, a number of configuration options appear, they are detailed in +the following sections. + +Internal toolchain backend +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The _internal toolchain backend_ is the backend where Buildroot builds +by itself a cross-compilation toolchain, before building the userspace +applications and libraries for your target embedded system. + +This backend is the historical backend of Buildroot, and is limited to +the usage of the http://www.uclibc.org[uClibc C library] (i.e, the +_glibc_ and _eglibc_ C libraries are not supported by this backend, +see the _External toolchain backend_ and _Crosstool-NG toolchain +backend_ for solutions to use either _glibc_ or _eglibc_). + +Once you have selected this backend, a number of options appear. The +most important ones allow to: + + * Change the version of the Linux kernel headers used to build the + toolchain. This item deserves a few explanations. In the process of + building a cross-compilation toolchain, the C library is being + built. This library provides the interface between userspace + applications and the Linux kernel. In order to know how to "talk" + to the Linux kernel, the C library needs to have access to the + _Linux kernel headers_ (i.e, the +.h+ files from the kernel), which + define the interface between userspace and the kernel (system + calls, data structures, etc.). Since this interface is backward + compatible, the version of the Linux kernel headers used to build + your toolchain do not need to match _exactly_ the version of the + Linux kernel you intend to run on your embedded system. They only + need to have a version equal or older to the version of the Linux + kernel you intend to run. If you use kernel headers that are more + recent than the Linux kernel you run on your embedded system, then + the C library might be using interfaces that are not provided by + your Linux kernel. + + * Change the version and the configuration of the uClibc C + library. The default options are usually fine. However, if you + really need to specifically customize the configuration of your + uClibc C library, you can pass a specific configuration file + here. Or alternatively, you can run the +make uclibc-menuconfig+ + command to get access to uClibc's configuration interface. Note + that all packages in Buildroot are tested against the default + uClibc configuration bundled in Buildroot: if you deviate from this + configuration by removing features from uClibc, some packages may + no longer build. + + * Change the version of the GCC compiler and binutils. + + * Select a number of toolchain options: whether the toolchain should + have largefile support (i.e support for files larger than 2 GB on + 32 bits systems), IPv6 support, RPC support (used mainly for NFS), + wide-char support, locale support (for internationalization), C++ + support, thread support. Depending on which options you choose, the + number of userspace applications and libraries visible in Buildroot + menus will change: many applications and libraries require certain + toolchain options to be enabled. Most packages show a comment when + a certain toolchain option is required to be able to enable those + packages. + +It is worth noting that whenever one of those options is modified, +then the entire toolchain and system must be rebuilt. See +xref:full-rebuild[]. + +Advantages of this backend: + +* Well integrated with Buildroot +* Fast, only builds what's necessary + +Drawbacks of this backend: + +* Rebuilding the toolchain is needed when doing +make clean+, which + takes time. If you're trying to reduce your build time, consider + using the _External toolchain backend_. +* Limited to the _uClibc_ C library. + +External toolchain backend +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The _external toolchain backend_ allows to use existing pre-built +cross-compilation toolchains. Buildroot knows about a number of +well-known cross-compilation toolchains (from +http://www.linaro.org[Linaro] for ARM, +http://www.mentor.com/embedded-software/sourcery-tools/sourcery-codebench/editions/lite-edition/[Sourcery +CodeBench] for ARM, x86, x86-64, PowerPC, MIPS and SuperH, +https://blackfin.uclinux.org/gf/project/toolchain[Blackfin toolchains +from ADI], http://git.xilinx.com/[Xilinx toolchains for Microblaze], +etc.) and is capable of downloading them automatically, or it can be +pointed to a custom toolchain, either available for download or +installed locally. + +Then, you have three solutions to use an external toolchain: + +* Use a predefined external toolchain profile, and let Buildroot + download, extract and install the toolchain. Buildroot already knows + about a few CodeSourcery, Linaro, Blackfin and Xilinx toolchains. + Just select the toolchain profile in +Toolchain+ from the + available ones. This is definitely the easiest solution. + +* Use a predefined external toolchain profile, but instead of having + Buildroot download and extract the toolchain, you can tell Buildroot + where your toolchain is already installed on your system. Just + select the toolchain profile in +Toolchain+ through the available + ones, unselect +Download toolchain automatically+, and fill the + +Toolchain path+ text entry with the path to your cross-compiling + toolchain. + +* Use a completely custom external toolchain. This is particularly + useful for toolchains generated using crosstool-NG. To do this, + select the +Custom toolchain+ solution in the +Toolchain+ list. You + need to fill the +Toolchain path+, +Toolchain prefix+ and +External + toolchain C library+ options. Then, you have to tell Buildroot what + your external toolchain supports. If your external toolchain uses + the 'glibc' library, you only have to tell whether your toolchain + supports C\+\+ or not and whether it has built-in RPC support. If + your external toolchain uses the 'uClibc' + library, then you have to tell Buildroot if it supports largefile, + IPv6, RPC, wide-char, locale, program invocation, threads and + C++. At the beginning of the execution, Buildroot will tell you if + the selected options do not match the toolchain configuration. + +Our external toolchain support has been tested with toolchains from +CodeSourcery and Linaro, toolchains generated by +http://crosstool-ng.org[crosstool-NG], and toolchains generated by +Buildroot itself. In general, all toolchains that support the +'sysroot' feature should work. If not, do not hesitate to contact the +developers. + +We do not support toolchains from the +http://www.denx.de/wiki/DULG/ELDK[ELDK] of Denx, for two reasons: + +* The ELDK does not contain a pure toolchain (i.e just the compiler, + binutils, the C and C++ libraries), but a toolchain that comes with + a very large set of pre-compiled libraries and programs. Therefore, + Buildroot cannot import the 'sysroot' of the toolchain, as it would + contain hundreds of megabytes of pre-compiled libraries that are + normally built by Buildroot. + +* The ELDK toolchains have a completely non-standard custom mechanism + to handle multiple library variants. Instead of using the standard + GCC 'multilib' mechanism, the ARM ELDK uses different symbolic links + to the compiler to differentiate between library variants (for ARM + soft-float and ARM VFP), and the PowerPC ELDK compiler uses a + +CROSS_COMPILE+ environment variable. This non-standard behaviour + makes it difficult to support ELDK in Buildroot. + +We also do not support using the distribution toolchain (i.e the +gcc/binutils/C library installed by your distribution) as the +toolchain to build software for the target. This is because your +distribution toolchain is not a "pure" toolchain (i.e only with the +C/C++ library), so we cannot import it properly into the Buildroot +build environment. So even if you are building a system for a x86 or +x86_64 target, you have to generate a cross-compilation toolchain with +Buildroot or crosstool-NG. + +If you want to generate a custom toolchain for your project, that can +be used as an external toolchain in Buildroot, our recommandation is +definitely to build it with http://crosstool-ng.org[crosstool-NG]. We +recommend to build the toolchain separately from Buildroot, and then +_import_ it in Buildroot using the external toolchain backend. + +Advantages of this backend: + +* Allows to use well-known and well-tested cross-compilation + toolchains. + +* Avoids the build time of the cross-compilation toolchain, which is + often very significant in the overall build time of an embedded + Linux system. + +* Not limited to uClibc: glibc and eglibc toolchains are supported. + +Drawbacks of this backend: + +* If your pre-built external toolchain has a bug, may be hard to get a + fix from the toolchain vendor, unless you build your external + toolchain by yourself using Crosstool-NG. + +Crosstool-NG toolchain backend +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The _Crosstool-NG toolchain backend_ integrates the +http://crosstool-ng.org[Crosstool-NG] project with +Buildroot. Crosstool-NG is a highly-configurable, versatile and +well-maintained tool to build cross-compilation toolchains. + +If you select the +Crosstool-NG toolchain+ option in +Toolchain Type+, +then you will be offered to: + + * Choose which C library you want to use. Crosstool-NG supports the + three most important C libraries used in Linux systems: glibc, + eglibc and uClibc + + * Choose a custom Crosstool-NG configuration file. Buildroot has its + own default configuration file (one per C library choice), but you + can provide your own. Another option is to run +make + ctng-menuconfig+ to get access to the Crosstool-NG configuration + interface. However, note that all Buildroot packages have only been + tested with the default Crosstool-NG configurations. + + * Choose a number of toolchain options (rather limited if glibc or + eglibc are used, or numerous if uClibc is used) + +When you will start the Buildroot build process, Buildroot will +download and install the Crosstool-NG tool, build and install its +required dependencies, and then run Crosstool-NG with the provided +configuration. + +Advantages of this backend: + +* Not limited to uClibc: glibc and eglibc are supported. + +* Vast possibilities of toolchain configuration. + +Drawbacks of this backend: + +* Crosstool-NG is not perfectly integrated with Buildroot. For + example, Crosstool-NG has its own download infrastructure, not + integrated with the one in Buildroot (for example a Buildroot +make + source+ will not download all the source code tarballs needed by + Crosstool-NG). + +* The toolchain is completely rebuilt from scratch if you do a +make + clean+. + +/dev management +~~~~~~~~~~~~~~~ + +On a Linux system, the +/dev+ directory contains special files, called +_device files_, that allow userspace applications to access the +hardware devices managed by the Linux kernel. Without these _device +files_, your userspace applications would not be able to use the +hardware devices, even if they are properly recognized by the Linux +kernel. + +Under +System configuration+, +/dev management+, Buildroot offers four +different solutions to handle the +/dev+ directory : + + * The first solution is *Static using device table*. This is the old + classical way of handling device files in Linux. With this method, + the device files are persistently stored in the root filesystem + (i.e they persist accross reboots), and there is nothing that will + automatically create and remove those device files when hardware + devices are added or removed from the system. Buildroot therefore + creates a standard set of device files using a _device table_, the + default one being stored in +system/device_table_dev.txt+ in the + Buildroot source code. This file is processed when Buildroot + generates the final root filesystem image, and the _device files_ + are therefore not visible in the +output/target+ directory. The + +BR2_ROOTFS_STATIC_DEVICE_TABLE+ option allows to change the + default device table used by Buildroot, or to add an additional + device table, so that additional _device files_ are created by + Buildroot during the build. So, if you use this method, and a + _device file_ is missing in your system, you can for example create + a +board/<yourcompany>/<yourproject>/device_table_dev.txt+ file + that contains the description of your additional _device files_, + and then you can set +BR2_ROOTFS_STATIC_DEVICE_TABLE+ to + +system/device_table_dev.txt + board/<yourcompany>/<yourproject>/device_table_dev.txt+. For more + details about the format of the device table file, see + xref:makedev-syntax[]. + + * The second solution is *Dynamic using devtmpfs only*. _devtmpfs_ is + a virtual filesystem inside the Linux kernel that has been + introduced in kernel 2.6.32 (if you use an older kernel, it is not + possible to use this option). When mounted in +/dev+, this virtual + filesystem will automatically make _device files_ appear and + disappear as hardware devices are added and removed from the + system. This filesystem is not persistent accross reboots: it is + filled dynamically by the kernel. Using _devtmpfs_ requires the + following kernel configuration options to be enabled: + +CONFIG_DEVTMPFS+ and +CONFIG_DEVTMPFS_MOUNT+. When Buildroot is in + charge of building the Linux kernel for your embedded device, it + makes sure that those two options are enabled. However, if you + build your Linux kernel outside of Buildroot, then it is your + responsability to enable those two options (if you fail to do so, + your Buildroot system will not boot). + + * The third solution is *Dynamic using mdev*. This method also relies + on the _devtmpfs_ virtual filesystem detailed above (so the + requirement to have +CONFIG_DEVTMPFS+ and +CONFIG_DEVTMPFS_MOUNT+ + enabled in the kernel configuration still apply), but adds the + +mdev+ userspace utility on top of it. +mdev+ is a program part of + Busybox that the kernel will call every time a device is added or + removed. Thanks to the +/etc/mdev.conf+ configuration file, +mdev+ + can be configured to for example, set specific permissions or + ownership on a device file, call a script or application whenever a + device appears or disappear, etc. Basically, it allows _userspace_ + to react on device addition and removal events. +mdev+ can for + example be used to automatically load kernel modules when devices + appear on the system. +mdev+ is also important if you have devices + that require a firmware, as it will be responsible for pushing the + firmware contents to the kernel. +mdev+ is a lightweight + implementation (with fewer features) of +udev+. For more details + about +mdev+ and the syntax of its configuration file, see + http://git.busybox.net/busybox/tree/docs/mdev.txt. + + * The fourth solution is *Dynamic using udev*. This method also + relies on the _devtmpfs_ virtual filesystem detailed above, but + adds the +udev+ userspace daemon on top of it. +udev+ is a daemon + that runs in the background, and gets called by the kernel when a + device gets added or removed from the system. It is a more + heavyweight solution than +mdev+, but provides higher flexibility + and is sometimes mandatory for some system components (systemd for + example). +udev+ is the mechanism used in most desktop Linux + distributions. For more details about +udev+, see + http://en.wikipedia.org/wiki/Udev. + +The Buildroot developers recommandation is to start with the *Dynamic +using devtmpfs only* solution, until you have the need for userspace +to be notified when devices are added/removed, or if firmwares are +needed, in which case *Dynamic using mdev* is usually a good solution. + +init system +~~~~~~~~~~~ + +The _init_ program is the first userspace program started by the +kernel (it carries the PID number 1), and is responsible for starting +the userspace services and programs (for example: web server, +graphical applications, other network servers, etc.). + +Buildroot allows to use three different types of init systems, which +can be chosen from +System configuration+, +Init system+: + + * The first solution is *Busybox*. Amongst many programs, Busybox has + an implementation of a basic +init+ program, which is sufficient + for most embedded systems. Enabling the +BR2_INIT_BUSYBOX+ will + ensure Busybox will build and install its +init+ program. This is + the default solution in Buildroot. The Busybox +init+ program will + read the +/etc/inittab+ file at boot to know what to do. The syntax + of this file can be found in + http://git.busybox.net/busybox/tree/examples/inittab (note that + Busybox +inittab+ syntax is special: do not use a random +inittab+ + documentation from the Internet to learn about Busybox + +inittab+). The default +inittab+ in Buildroot is stored in + +system/skeleton/etc/inittab+. Apart from mounting a few important + filesystems, the main job the default inittab does is to start the + +/etc/init.d/rcS+ shell script, and start a +getty+ program (which + provides a login prompt). + + * The second solution is *systemV*. This solution uses the old + traditional _sysvinit_ program, packed in Buildroot in + +package/sysvinit+. This was the solution used in most desktop + Linux distributions, until they switched to more recent + alternatives such as Upstart or Systemd. +sysvinit+ also works with + an +inittab+ file (which has a slightly different syntax than the + one from Busybox). The default +inittab+ installed with this init + solution is located in +package/sysvinit/inittab+. + + * The third solution is *systemd*. +systemd+ is the new generation + init system for Linux. It does far more than traditional _init_ + programs: aggressive parallelization capabilities, uses socket and + D-Bus activation for starting services, offers on-demand starting + of daemons, keeps track of processes using Linux control groups, + supports snapshotting and restoring of the system state, + etc. +systemd+ will be useful on relatively complex embedded + systems, for example the ones requiring D-Bus and services + communicating between each other. It is worth noting that +systemd+ + brings a fairly big number of large dependencies: +dbus+, +glib+ + and more. For more details about +systemd+, see + http://www.freedesktop.org/wiki/Software/systemd. + +The solution recommended by Buildroot developers is to use the +*Busybox init* as it is sufficient for most embedded +systems. *systemd* can be used for more complex situations. |