kumquat-buildroot/docs/manual/configure.txt

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[[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 two solutions for the cross-compilation toolchain:

 * The *internal toolchain backend*, called +Buildroot toolchain+ in
   the configuration interface.

 * The *external toolchain backend*, called +External toolchain+ in
   the configuration interface.

The choice between these two 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]]
==== 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 supports several C libraries:
http://www.uclibc.org[uClibc], the
http://www.gnu.org/software/libc/libc.html[glibc] and
http://www.eglibc.org[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 of the GCC compiler, binutils and the C library.

 * Select a number of toolchain options (uClibc only): 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 or 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. If needed, you can further
   refine the uClibc configuration by running +make
   uclibc-menuconfig+. Note however 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.

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_.

[[external-toolchain-backend]]
==== 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 Analog Devices], 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 or with Buildroot
  itself. 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 or SDK generated by OpenEmbedded or
Yocto, because these toolchains are not pure toolchains (i.e. just the
compiler, binutils, the C and C++ libraries). Instead these toolchains
come 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.

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 recommendation 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.

=== /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 eudev*. This method also
   relies on the _devtmpfs_ virtual filesystem detailed above, but
   adds the +eudev+ userspace daemon on top of it. +eudev+ 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.
   +eudev+ is a standalone version of +udev+, the original userspace
   daemon used in most desktop Linux distributions, which is now part
   of Systemd. For more details, see http://en.wikipedia.org/wiki/Udev.

The Buildroot developers recommendation 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.

Note that if +systemd+ is chosen as init system, /dev management will
be performed by the +udev+ program provided by +systemd+.

=== 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+, +udev+
   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.