152 lines
8.0 KiB
Plaintext
152 lines
8.0 KiB
Plaintext
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// -*- mode:doc -*- ;
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[[configure]]
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Details on Buildroot configuration
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----------------------------------
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All the configuration options in +make *config+ have a help text
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providing details about the option. However, a number of topics
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require additional details that cannot easily be covered in the help
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text and are there covered in the following sections.
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/dev management
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~~~~~~~~~~~~~~~
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On a Linux system, the +/dev+ directory contains special files, called
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_device files_, that allow userspace applications to access the
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hardware devices managed by the Linux kernel. Without these _device
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files_, your userspace applications would not be able to use the
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hardware devices, even if they are properly recognized by the Linux
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kernel.
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Under +System configuration+, +/dev management+, Buildroot offers four
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different solutions to handle the +/dev+ directory :
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* The first solution is *Static using device table*. This is the old
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classical way of handling device files in Linux. With this method,
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the device files are persistently stored in the root filesystem
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(i.e they persist accross reboots), and there is nothing that will
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automatically create and remove those device files when hardware
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devices are added or removed from the system. Buildroot therefore
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creates a standard set of device files using a _device table_, the
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default one being stored in +system/device_table_dev.txt+ in the
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Buildroot source code. This file is processed when Buildroot
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generates the final root filesystem image, and the _device files_
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are therefore not visible in the +output/target+ directory. The
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+BR2_ROOTFS_STATIC_DEVICE_TABLE+ option allows to change the
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default device table used by Buildroot, or to add an additional
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device table, so that additional _device files_ are created by
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Buildroot during the build. So, if you use this method, and a
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_device file_ is missing in your system, you can for example create
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a +board/<yourcompany>/<yourproject>/device_table_dev.txt+ file
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that contains the description of your additional _device files_,
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and then you can set +BR2_ROOTFS_STATIC_DEVICE_TABLE+ to
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+system/device_table_dev.txt
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board/<yourcompany>/<yourproject>/device_table_dev.txt+. For more
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details about the format of the device table file, see
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xref:makedev-syntax[].
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* The second solution is *Dynamic using devtmpfs only*. _devtmpfs_ is
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a virtual filesystem inside the Linux kernel that has been
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introduced in kernel 2.6.32 (if you use an older kernel, it is not
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possible to use this option). When mounted in +/dev+, this virtual
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filesystem will automatically make _device files_ appear and
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disappear as hardware devices are added and removed from the
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system. This filesystem is not persistent accross reboots: it is
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filled dynamically by the kernel. Using _devtmpfs_ requires the
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following kernel configuration options to be enabled:
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+CONFIG_DEVTMPFS+ and +CONFIG_DEVTMPFS_MOUNT+. When Buildroot is in
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charge of building the Linux kernel for your embedded device, it
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makes sure that those two options are enabled. However, if you
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build your Linux kernel outside of Buildroot, then it is your
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responsability to enable those two options (if you fail to do so,
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your Buildroot system will not boot).
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* The third solution is *Dynamic using mdev*. This method also relies
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on the _devtmpfs_ virtual filesystem detailed above (so the
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requirement to have +CONFIG_DEVTMPFS+ and +CONFIG_DEVTMPFS_MOUNT+
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enabled in the kernel configuration still apply), but adds the
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+mdev+ userspace utility on top of it. +mdev+ is a program part of
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Busybox that the kernel will call every time a device is added or
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removed. Thanks to the +/etc/mdev.conf+ configuration file, +mdev+
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can be configured to for example, set specific permissions or
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ownership on a device file, call a script or application whenever a
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device appears or disappear, etc. Basically, it allows _userspace_
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to react on device addition and removal events. +mdev+ can for
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example be used to automatically load kernel modules when devices
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appear on the system. +mdev+ is also important if you have devices
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that require a firmware, as it will be responsible for pushing the
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firmware contents to the kernel. +mdev+ is a lightweight
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implementation (with fewer features) of +udev+. For more details
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about +mdev+ and the syntax of its configuration file, see
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http://git.busybox.net/busybox/tree/docs/mdev.txt.
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* The fourth solution is *Dynamic using udev*. This method also
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relies on the _devtmpfs_ virtual filesystem detailed above, but
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adds the +udev+ userspace daemon on top of it. +udev+ is a daemon
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that runs in the background, and gets called by the kernel when a
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device gets added or removed from the system. It is a more
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heavyweight solution than +mdev+, but provides higher flexibility
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and is sometimes mandatory for some system components (systemd for
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example). +udev+ is the mechanism used in most desktop Linux
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distributions. For more details about +udev+, see
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http://en.wikipedia.org/wiki/Udev.
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The Buildroot developers recommandation is to start with the *Dynamic
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using devtmpfs only* solution, until you have the need for userspace
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to be notified when devices are added/removed, or if firmwares are
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needed, in which case *Dynamic using mdev* is usually a good solution.
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init system
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~~~~~~~~~~~
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The _init_ program is the first userspace program started by the
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kernel (it carries the PID number 1), and is responsible for starting
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the userspace services and programs (for example: web server,
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graphical applications, other network servers, etc.).
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Buildroot allows to use three different types of init systems, which
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can be chosen from +System configuration+, +Init system+:
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* The first solution is *Busybox*. Amongst many programs, Busybox has
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an implementation of a basic +init+ program, which is sufficient
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for most embedded systems. Enabling the +BR2_INIT_BUSYBOX+ will
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ensure Busybox will build and install its +init+ program. This is
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the default solution in Buildroot. The Busybox +init+ program will
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read the +/etc/inittab+ file at boot to know what to do. The syntax
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of this file can be found in
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http://git.busybox.net/busybox/tree/examples/inittab (note that
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Busybox +inittab+ syntax is special: do not use a random +inittab+
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documentation from the Internet to learn about Busybox
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+inittab+). The default +inittab+ in Buildroot is stored in
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+system/skeleton/etc/inittab+. Apart from mounting a few important
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filesystems, the main job the default inittab does is to start the
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+/etc/init.d/rcS+ shell script, and start a +getty+ program (which
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provides a login prompt).
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* The second solution is *systemV*. This solution uses the old
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traditional _sysvinit_ program, packed in Buildroot in
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+package/sysvinit+. This was the solution used in most desktop
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Linux distributions, until they switched to more recent
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alternatives such as Upstart or Systemd. +sysvinit+ also works with
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an +inittab+ file (which has a slightly different syntax than the
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one from Busybox). The default +inittab+ installed with this init
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solution is located in +package/sysvinit/inittab+.
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* The third solution is *systemd*. +systemd+ is the new generation
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init system for Linux. It does far more than traditional _init_
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programs: aggressive parallelization capabilities, uses socket and
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D-Bus activation for starting services, offers on-demand starting
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of daemons, keeps track of processes using Linux control groups,
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supports snapshotting and restoring of the system state,
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etc. +systemd+ will be useful on relatively complex embedded
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systems, for example the ones requiring D-Bus and services
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communicating between each other. It is worth noting that +systemd+
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brings a fairly big number of large dependencies: +dbus+, +glib+
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and more. For more details about +systemd+, see
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http://www.freedesktop.org/wiki/Software/systemd.
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The solution recommended by Buildroot developers is to use the
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*Busybox init* as it is sufficient for most embedded
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systems. *systemd* can be used for more complex situations.
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