Commit 571552c4 authored by Luca Ceresoli's avatar Luca Ceresoli Committed by Thomas Petazzoni
Browse files

system: clarify /dev management using devtmpfs + {mdev, eudev} 



The "Dynamic using mdev" and "Dynamic using eudev" /dev management
methods are using devtmpfs, just like "Dynamic using devtmpfs only",
on top of which they add either mdev or eudev. But their names might
suggest they use mdev only or eudev only.

Clarify their descriptions by adding "devtmpfs +" to make it
unambiguous.

[Thomas: rewrap the paragraphs in the manual. It makes the diff a lot
less readable, but we actually want the result to be wrapped.]

Signed-off-by: default avatarLuca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: default avatarArnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: default avatarThomas Petazzoni <thomas.petazzoni@free-electrons.com>
parent 844f2401

docs/manual/configure.txt

+30 −27
Original line number Diff line number Diff line
@@ -295,39 +295,42 @@ different solutions to handle the +/dev+ directory : 
   responsibility 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

 * The third solution is *Dynamic using devtmpfs + 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 fourth solution is *Dynamic using devtmpfs + 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.

needed, in which case *Dynamic using devtmpfs + 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+.

system/Config.in

+2 −2
Original line number Diff line number Diff line
@@ -110,11 +110,11 @@ config BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_DEVTMPFS 
	bool "Dynamic using devtmpfs only"



config BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_MDEV

	bool "Dynamic using mdev"

	bool "Dynamic using devtmpfs + mdev"

	select BR2_PACKAGE_BUSYBOX



config BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_EUDEV

	bool "Dynamic using eudev"

	bool "Dynamic using devtmpfs + eudev"

	depends on BR2_USE_WCHAR

	depends on !BR2_STATIC_LIBS

	depends on BR2_USE_MMU # eudev