configure.txt 19 KB

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  1. // -*- mode:doc; -*-
  2. // vim: set syntax=asciidoc:
  3. [[configure]]
  4. == Buildroot configuration
  5. All the configuration options in +make *config+ have a help text
  6. providing details about the option.
  7. The +make *config+ commands also offer a search tool. Read the help
  8. message in the different frontend menus to know how to use it:
  9. * in _menuconfig_, the search tool is called by pressing +/+;
  10. * in _xconfig_, the search tool is called by pressing +Ctrl+ + +f+.
  11. The result of the search shows the help message of the matching items.
  12. In _menuconfig_, numbers in the left column provide a shortcut to the
  13. corresponding entry. Just type this number to directly jump to the
  14. entry, or to the containing menu in case the entry is not selectable due
  15. to a missing dependency.
  16. Although the menu structure and the help text of the entries should be
  17. sufficiently self-explanatory, a number of topics require additional
  18. explanation that cannot easily be covered in the help text and are
  19. therefore covered in the following sections.
  20. === Cross-compilation toolchain
  21. A compilation toolchain is the set of tools that allows you to compile
  22. code for your system. It consists of a compiler (in our case, +gcc+),
  23. binary utils like assembler and linker (in our case, +binutils+) and a
  24. C standard library (for example
  25. http://www.gnu.org/software/libc/libc.html[GNU Libc],
  26. http://www.uclibc-ng.org/[uClibc-ng]).
  27. The system installed on your development station certainly already has
  28. a compilation toolchain that you can use to compile an application
  29. that runs on your system. If you're using a PC, your compilation
  30. toolchain runs on an x86 processor and generates code for an x86
  31. processor. Under most Linux systems, the compilation toolchain uses
  32. the GNU libc (glibc) as the C standard library. This compilation
  33. toolchain is called the "host compilation toolchain". The machine on
  34. which it is running, and on which you're working, is called the "host
  35. system" footnote:[This terminology differs from what is used by GNU
  36. configure, where the host is the machine on which the application will
  37. run (which is usually the same as target)].
  38. The compilation toolchain is provided by your distribution, and
  39. Buildroot has nothing to do with it (other than using it to build a
  40. cross-compilation toolchain and other tools that are run on the
  41. development host).
  42. As said above, the compilation toolchain that comes with your system
  43. runs on and generates code for the processor in your host system. As
  44. your embedded system has a different processor, you need a
  45. cross-compilation toolchain - a compilation toolchain that runs on
  46. your _host system_ but generates code for your _target system_ (and
  47. target processor). For example, if your host system uses x86 and your
  48. target system uses ARM, the regular compilation toolchain on your host
  49. runs on x86 and generates code for x86, while the cross-compilation
  50. toolchain runs on x86 and generates code for ARM.
  51. Buildroot provides two solutions for the cross-compilation toolchain:
  52. * The *internal toolchain backend*, called +Buildroot toolchain+ in
  53. the configuration interface.
  54. * The *external toolchain backend*, called +External toolchain+ in
  55. the configuration interface.
  56. The choice between these two solutions is done using the +Toolchain
  57. Type+ option in the +Toolchain+ menu. Once one solution has been
  58. chosen, a number of configuration options appear, they are detailed in
  59. the following sections.
  60. [[internal-toolchain-backend]]
  61. ==== Internal toolchain backend
  62. The _internal toolchain backend_ is the backend where Buildroot builds
  63. by itself a cross-compilation toolchain, before building the userspace
  64. applications and libraries for your target embedded system.
  65. This backend supports several C libraries:
  66. http://www.uclibc-ng.org[uClibc-ng],
  67. http://www.gnu.org/software/libc/libc.html[glibc] and
  68. http://www.musl-libc.org[musl].
  69. Once you have selected this backend, a number of options appear. The
  70. most important ones allow to:
  71. * Change the version of the Linux kernel headers used to build the
  72. toolchain. This item deserves a few explanations. In the process of
  73. building a cross-compilation toolchain, the C library is being
  74. built. This library provides the interface between userspace
  75. applications and the Linux kernel. In order to know how to "talk"
  76. to the Linux kernel, the C library needs to have access to the
  77. _Linux kernel headers_ (i.e. the +.h+ files from the kernel), which
  78. define the interface between userspace and the kernel (system
  79. calls, data structures, etc.). Since this interface is backward
  80. compatible, the version of the Linux kernel headers used to build
  81. your toolchain do not need to match _exactly_ the version of the
  82. Linux kernel you intend to run on your embedded system. They only
  83. need to have a version equal or older to the version of the Linux
  84. kernel you intend to run. If you use kernel headers that are more
  85. recent than the Linux kernel you run on your embedded system, then
  86. the C library might be using interfaces that are not provided by
  87. your Linux kernel.
  88. * Change the version of the GCC compiler, binutils and the C library.
  89. * Select a number of toolchain options (uClibc only): whether the
  90. toolchain should have RPC support (used mainly for NFS),
  91. wide-char support, locale support (for internationalization),
  92. C++ support or thread support. Depending on which options you choose,
  93. the number of userspace applications and libraries visible in
  94. Buildroot menus will change: many applications and libraries require
  95. certain toolchain options to be enabled. Most packages show a comment
  96. when a certain toolchain option is required to be able to enable
  97. those packages. If needed, you can further refine the uClibc
  98. configuration by running +make uclibc-menuconfig+. Note however that
  99. all packages in Buildroot are tested against the default uClibc
  100. configuration bundled in Buildroot: if you deviate from this
  101. configuration by removing features from uClibc, some packages may no
  102. longer build.
  103. It is worth noting that whenever one of those options is modified,
  104. then the entire toolchain and system must be rebuilt. See
  105. xref:full-rebuild[].
  106. Advantages of this backend:
  107. * Well integrated with Buildroot
  108. * Fast, only builds what's necessary
  109. Drawbacks of this backend:
  110. * Rebuilding the toolchain is needed when doing +make clean+, which
  111. takes time. If you're trying to reduce your build time, consider
  112. using the _External toolchain backend_.
  113. [[external-toolchain-backend]]
  114. ==== External toolchain backend
  115. The _external toolchain backend_ allows to use existing pre-built
  116. cross-compilation toolchains. Buildroot knows about a number of
  117. well-known cross-compilation toolchains (from
  118. http://www.linaro.org[Linaro] for ARM,
  119. http://www.mentor.com/embedded-software/sourcery-tools/sourcery-codebench/editions/lite-edition/[Sourcery
  120. CodeBench] for ARM, x86, x86-64, PowerPC, MIPS and SuperH,
  121. https://blackfin.uclinux.org/gf/project/toolchain[Blackfin toolchains
  122. from Analog Devices], etc.) and is capable of downloading them
  123. automatically, or it can be pointed to a custom toolchain, either
  124. available for download or installed locally.
  125. Then, you have three solutions to use an external toolchain:
  126. * Use a predefined external toolchain profile, and let Buildroot
  127. download, extract and install the toolchain. Buildroot already knows
  128. about a few CodeSourcery, Linaro, Blackfin and Xilinx toolchains.
  129. Just select the toolchain profile in +Toolchain+ from the
  130. available ones. This is definitely the easiest solution.
  131. * Use a predefined external toolchain profile, but instead of having
  132. Buildroot download and extract the toolchain, you can tell Buildroot
  133. where your toolchain is already installed on your system. Just
  134. select the toolchain profile in +Toolchain+ through the available
  135. ones, unselect +Download toolchain automatically+, and fill the
  136. +Toolchain path+ text entry with the path to your cross-compiling
  137. toolchain.
  138. * Use a completely custom external toolchain. This is particularly
  139. useful for toolchains generated using crosstool-NG or with Buildroot
  140. itself. To do this, select the +Custom toolchain+ solution in the
  141. +Toolchain+ list. You need to fill the +Toolchain path+, +Toolchain
  142. prefix+ and +External toolchain C library+ options. Then, you have
  143. to tell Buildroot what your external toolchain supports. If your
  144. external toolchain uses the 'glibc' library, you only have to tell
  145. whether your toolchain supports C\++ or not and whether it has
  146. built-in RPC support. If your external toolchain uses the 'uClibc'
  147. library, then you have to tell Buildroot if it supports RPC,
  148. wide-char, locale, program invocation, threads and C++.
  149. At the beginning of the execution, Buildroot will tell you if
  150. the selected options do not match the toolchain configuration.
  151. Our external toolchain support has been tested with toolchains from
  152. CodeSourcery and Linaro, toolchains generated by
  153. http://crosstool-ng.org[crosstool-NG], and toolchains generated by
  154. Buildroot itself. In general, all toolchains that support the
  155. 'sysroot' feature should work. If not, do not hesitate to contact the
  156. developers.
  157. We do not support toolchains or SDK generated by OpenEmbedded or
  158. Yocto, because these toolchains are not pure toolchains (i.e. just the
  159. compiler, binutils, the C and C++ libraries). Instead these toolchains
  160. come with a very large set of pre-compiled libraries and
  161. programs. Therefore, Buildroot cannot import the 'sysroot' of the
  162. toolchain, as it would contain hundreds of megabytes of pre-compiled
  163. libraries that are normally built by Buildroot.
  164. We also do not support using the distribution toolchain (i.e. the
  165. gcc/binutils/C library installed by your distribution) as the
  166. toolchain to build software for the target. This is because your
  167. distribution toolchain is not a "pure" toolchain (i.e. only with the
  168. C/C++ library), so we cannot import it properly into the Buildroot
  169. build environment. So even if you are building a system for a x86 or
  170. x86_64 target, you have to generate a cross-compilation toolchain with
  171. Buildroot or crosstool-NG.
  172. If you want to generate a custom toolchain for your project, that can
  173. be used as an external toolchain in Buildroot, our recommendation is
  174. definitely to build it with http://crosstool-ng.org[crosstool-NG]. We
  175. recommend to build the toolchain separately from Buildroot, and then
  176. _import_ it in Buildroot using the external toolchain backend.
  177. Advantages of this backend:
  178. * Allows to use well-known and well-tested cross-compilation
  179. toolchains.
  180. * Avoids the build time of the cross-compilation toolchain, which is
  181. often very significant in the overall build time of an embedded
  182. Linux system.
  183. * Not limited to uClibc: glibc and eglibc toolchains are supported.
  184. Drawbacks of this backend:
  185. * If your pre-built external toolchain has a bug, may be hard to get a
  186. fix from the toolchain vendor, unless you build your external
  187. toolchain by yourself using Crosstool-NG.
  188. ===== External toolchain wrapper
  189. When using an external toolchain, Buildroot generates a wrapper program,
  190. that transparently passes the appropriate options (according to the
  191. configuration) to the external toolchain programs. In case you need to
  192. debug this wrapper to check exactly what arguments are passed, you can
  193. set the environment variable +BR2_DEBUG_WRAPPER+ to either one of:
  194. * +0+, empty or not set: no debug
  195. * +1+: trace all arguments on a single line
  196. * +2+: trace one argument per line
  197. === /dev management
  198. On a Linux system, the +/dev+ directory contains special files, called
  199. _device files_, that allow userspace applications to access the
  200. hardware devices managed by the Linux kernel. Without these _device
  201. files_, your userspace applications would not be able to use the
  202. hardware devices, even if they are properly recognized by the Linux
  203. kernel.
  204. Under +System configuration+, +/dev management+, Buildroot offers four
  205. different solutions to handle the +/dev+ directory :
  206. * The first solution is *Static using device table*. This is the old
  207. classical way of handling device files in Linux. With this method,
  208. the device files are persistently stored in the root filesystem
  209. (i.e. they persist across reboots), and there is nothing that will
  210. automatically create and remove those device files when hardware
  211. devices are added or removed from the system. Buildroot therefore
  212. creates a standard set of device files using a _device table_, the
  213. default one being stored in +system/device_table_dev.txt+ in the
  214. Buildroot source code. This file is processed when Buildroot
  215. generates the final root filesystem image, and the _device files_
  216. are therefore not visible in the +output/target+ directory. The
  217. +BR2_ROOTFS_STATIC_DEVICE_TABLE+ option allows to change the
  218. default device table used by Buildroot, or to add an additional
  219. device table, so that additional _device files_ are created by
  220. Buildroot during the build. So, if you use this method, and a
  221. _device file_ is missing in your system, you can for example create
  222. a +board/<yourcompany>/<yourproject>/device_table_dev.txt+ file
  223. that contains the description of your additional _device files_,
  224. and then you can set +BR2_ROOTFS_STATIC_DEVICE_TABLE+ to
  225. +system/device_table_dev.txt
  226. board/<yourcompany>/<yourproject>/device_table_dev.txt+. For more
  227. details about the format of the device table file, see
  228. xref:makedev-syntax[].
  229. * The second solution is *Dynamic using devtmpfs only*. _devtmpfs_ is
  230. a virtual filesystem inside the Linux kernel that has been
  231. introduced in kernel 2.6.32 (if you use an older kernel, it is not
  232. possible to use this option). When mounted in +/dev+, this virtual
  233. filesystem will automatically make _device files_ appear and
  234. disappear as hardware devices are added and removed from the
  235. system. This filesystem is not persistent across reboots: it is
  236. filled dynamically by the kernel. Using _devtmpfs_ requires the
  237. following kernel configuration options to be enabled:
  238. +CONFIG_DEVTMPFS+ and +CONFIG_DEVTMPFS_MOUNT+. When Buildroot is in
  239. charge of building the Linux kernel for your embedded device, it
  240. makes sure that those two options are enabled. However, if you
  241. build your Linux kernel outside of Buildroot, then it is your
  242. responsibility to enable those two options (if you fail to do so,
  243. your Buildroot system will not boot).
  244. * The third solution is *Dynamic using devtmpfs + mdev*. This method
  245. also relies on the _devtmpfs_ virtual filesystem detailed above (so
  246. the requirement to have +CONFIG_DEVTMPFS+ and
  247. +CONFIG_DEVTMPFS_MOUNT+ enabled in the kernel configuration still
  248. apply), but adds the +mdev+ userspace utility on top of it. +mdev+
  249. is a program part of BusyBox that the kernel will call every time a
  250. device is added or removed. Thanks to the +/etc/mdev.conf+
  251. configuration file, +mdev+ can be configured to for example, set
  252. specific permissions or ownership on a device file, call a script
  253. or application whenever a device appears or disappear,
  254. etc. Basically, it allows _userspace_ to react on device addition
  255. and removal events. +mdev+ can for example be used to automatically
  256. load kernel modules when devices appear on the system. +mdev+ is
  257. also important if you have devices that require a firmware, as it
  258. will be responsible for pushing the firmware contents to the
  259. kernel. +mdev+ is a lightweight implementation (with fewer
  260. features) of +udev+. For more details about +mdev+ and the syntax
  261. of its configuration file, see
  262. http://git.busybox.net/busybox/tree/docs/mdev.txt.
  263. * The fourth solution is *Dynamic using devtmpfs + eudev*. This
  264. method also relies on the _devtmpfs_ virtual filesystem detailed
  265. above, but adds the +eudev+ userspace daemon on top of it. +eudev+
  266. is a daemon that runs in the background, and gets called by the
  267. kernel when a device gets added or removed from the system. It is a
  268. more heavyweight solution than +mdev+, but provides higher
  269. flexibility. +eudev+ is a standalone version of +udev+, the
  270. original userspace daemon used in most desktop Linux distributions,
  271. which is now part of Systemd. For more details, see
  272. http://en.wikipedia.org/wiki/Udev.
  273. The Buildroot developers recommendation is to start with the *Dynamic
  274. using devtmpfs only* solution, until you have the need for userspace
  275. to be notified when devices are added/removed, or if firmwares are
  276. needed, in which case *Dynamic using devtmpfs + mdev* is usually a
  277. good solution.
  278. Note that if +systemd+ is chosen as init system, /dev management will
  279. be performed by the +udev+ program provided by +systemd+.
  280. === init system
  281. The _init_ program is the first userspace program started by the
  282. kernel (it carries the PID number 1), and is responsible for starting
  283. the userspace services and programs (for example: web server,
  284. graphical applications, other network servers, etc.).
  285. Buildroot allows to use three different types of init systems, which
  286. can be chosen from +System configuration+, +Init system+:
  287. * The first solution is *BusyBox*. Amongst many programs, BusyBox has
  288. an implementation of a basic +init+ program, which is sufficient
  289. for most embedded systems. Enabling the +BR2_INIT_BUSYBOX+ will
  290. ensure BusyBox will build and install its +init+ program. This is
  291. the default solution in Buildroot. The BusyBox +init+ program will
  292. read the +/etc/inittab+ file at boot to know what to do. The syntax
  293. of this file can be found in
  294. http://git.busybox.net/busybox/tree/examples/inittab (note that
  295. BusyBox +inittab+ syntax is special: do not use a random +inittab+
  296. documentation from the Internet to learn about BusyBox
  297. +inittab+). The default +inittab+ in Buildroot is stored in
  298. +system/skeleton/etc/inittab+. Apart from mounting a few important
  299. filesystems, the main job the default inittab does is to start the
  300. +/etc/init.d/rcS+ shell script, and start a +getty+ program (which
  301. provides a login prompt).
  302. * The second solution is *systemV*. This solution uses the old
  303. traditional _sysvinit_ program, packed in Buildroot in
  304. +package/sysvinit+. This was the solution used in most desktop
  305. Linux distributions, until they switched to more recent
  306. alternatives such as Upstart or Systemd. +sysvinit+ also works with
  307. an +inittab+ file (which has a slightly different syntax than the
  308. one from BusyBox). The default +inittab+ installed with this init
  309. solution is located in +package/sysvinit/inittab+.
  310. * The third solution is *systemd*. +systemd+ is the new generation
  311. init system for Linux. It does far more than traditional _init_
  312. programs: aggressive parallelization capabilities, uses socket and
  313. D-Bus activation for starting services, offers on-demand starting
  314. of daemons, keeps track of processes using Linux control groups,
  315. supports snapshotting and restoring of the system state,
  316. etc. +systemd+ will be useful on relatively complex embedded
  317. systems, for example the ones requiring D-Bus and services
  318. communicating between each other. It is worth noting that +systemd+
  319. brings a fairly big number of large dependencies: +dbus+, +udev+
  320. and more. For more details about +systemd+, see
  321. http://www.freedesktop.org/wiki/Software/systemd.
  322. The solution recommended by Buildroot developers is to use the
  323. *BusyBox init* as it is sufficient for most embedded
  324. systems. *systemd* can be used for more complex situations.