Udev
udev deals with hotplugging (and coldplugging) devices such as USB key drives, usb cameras and firewire drives. It takes messages from the kernel (2.6 onwards) and processes them in userland. The structure of the config file is very simple. It defines how you will bind kernel messages to actions by way of pattern matching.
USB
This document may be useful Mounting and unmounting removable devices in linux
USB devices live at /proc/bus/usb. When a device is plugged this file is updated. Here is an example of the file:
T: Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 B: Alloc= 0/900 us ( 0%), #Int= 0, #Iso= 0 D: Ver= 1.10 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 P: Vendor=0000 ProdID=0000 Rev= 2.06 S: Manufacturer=Linux 2.6.8-2-386 uhci_hcd S: Product=Intel Corp. 82801AA USB S: SerialNumber=0000:00:1f.2 C:* #Ifs= 1 Cfg#= 1 Atr=c0 MxPwr= 0mA I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub E: Ad=81(I) Atr=03(Int.) MxPS= 2 Ivl=255ms T: Bus=01 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 6 Spd=12 MxCh= 0 D: Ver= 2.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS=64 #Cfgs= 1 P: Vendor=0d7d ProdID=1300 Rev= 0.50 S: Product=USB2.0 FlashDisk S: SerialNumber=1D4402000054 C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=200mA I: If#= 0 Alt= 0 #EPs= 2 Cls=08(stor.) Sub=06 Prot=50 Driver=usb-storage E: Ad=81(I) Atr=02(Bulk) MxPS= 64 Ivl=0ms E: Ad=02(O) Atr=02(Bulk) MxPS= 64 Ivl=0ms
This computer has only one external USB device plugged in (USB2.0 FlashDisk). The other entry is the host controler (Intel Corp. 82801AA USB).
Firewire and others
These should work in a similar way. The default scripts should detect and initialise the device, it is up to the user to mount the device on the file system. This is fine because it gives us control over how it is mounted and the path used.
Hotplugging and the Graphical User Interface
It is important to provide feedback to the user when they plug or unplug a device. It is suggested that both visual and audible feedback be provided where possible. The latency for plugging is about 1 second. For unplugging the response is almost immediate. The use of perl rather than shell scripts for the hotplug mechanisim would improve the performance further. The sounds used should be customisable and a small, well chosen selection will be provided by default.
