Linux device management
This text seemed like it might be useful information, even if it contained some inaccuracies. Original document was published under GNU General Public License, Version 2, and those with appropriate knowledge should feel free to edit or expand the text.
- 2005-10-05 Copied from http://www.reactivated.net/writing_udev_rules.html, with minor editing for clarity; the below was originally part of the udev article but seemed to belong here instead. The rest of the article should be checked to make sure it still makes sense without the introduction.
A basic introduction only, might not be totally accurate.
On typical linux-based systems, the /dev directory is used to store file-like device nodes which refer to certain devices in the system. Each node points to a part of the system (a device), which might or might not exist. Userspace applications can use these device nodes to interface with the systems hardware, for example, XFree86 will "listen to" /dev/input/mice so that it can relate the users mouse movements to moving the visual mouse pointer.
The original /dev directories were just populated with every device that might possibly appear in the system. /dev directories were typically very large because of this. devfs came along to provide a more managable approach (noticably, it only populated /dev with hardware that is plugged into the system), as well as some other functionality, but the system proved to have problems which could not be easily fixed.
udev is the "new" way of managing /dev directories, designed to clear up some issues with previous /dev implementations, and provide a robust path forward. In order to create and name /dev device nodes corresponding to devices that are present in the system, udev relies on matching information provided by sysfs with rules provided by the user.
sysfs is a new filesystem to the 2.6 kernels. It is managed by the kernel, and exports basic information about the devices currently plugged into your system. udev can use this information to create device nodes corresponding to your hardware. sysfs is mounted at /sys and is browsable. You may wish to investigate some of the files stored there before getting to grips with udev. Throughout this document, I will use the terms /sys and SYSFS interchangeably.
Mysterious message where digiKam (or other digital camera app) used to be able to talk to a camera (in our case, the Kodak EasyShare C360 and suddenly can't: the problem seems to be with permissions on the device; you need to be able to write as well as read to it. This is probably a change in the udev configuration or something related, and I'll figure that out later, but for now the way to get it working is:
- Run lsusb (on some distros, you need to type /sbin/lsusb) – sample output:
woozle@gonzo:~$ lsusb Bus 002 Device 001: ID 0000:0000 Bus 001 Device 013: ID 040a:0589 Kodak Co. Bus 001 Device 007: ID 0923:010f IC Media Corp. SIIG MobileCam Bus 001 Device 002: ID 05e3:0604 Genesys Logic, Inc. USB 1.1 Hub Bus 001 Device 001: ID 0000:0000
- Look inside /dev/bus/usb. In the case of the above results, you would see the folder /dev/bus/usb/001, and inside that a device called 013. Change the permissions of 013 so it is readable and writable by your user account.
Various things may reset this permission before you can actually view it for files, so if you get a message that the camera can't be read, try resetting the permissions again (without closing anything) and retry accessing the camera.