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-rw-r--r--docs/manual/configure.txt151
-rw-r--r--docs/manual/working-with.txt2
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diff --git a/docs/manual/configure.txt b/docs/manual/configure.txt
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index 000000000..2c7987dda
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@@ -0,0 +1,151 @@
+// -*- 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.
+
+/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.
diff --git a/docs/manual/working-with.txt b/docs/manual/working-with.txt
index 4ea9239e2..b62bacdcb 100644
--- a/docs/manual/working-with.txt
+++ b/docs/manual/working-with.txt
@@ -6,6 +6,8 @@ Working with Buildroot
This section explains how you can customize Buildroot to fit your
needs.
+include::configure.txt[]
+
include::make-tips.txt[]
include::customize.txt[]