hotplug
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NAME
]
/etc/init.d/hotplug
[start|stop|status|restart]
DESCRIPTION
hotplug
is a program which is used by the kernel to notify user mode software
when some significant (usually hardware-related) events take place.
An example is when a USB or Cardbus device has just been plugged in.
This is useful for automatically loading and setting up drivers,
packaged either as kernel modules or as user mode programs.
The hotplug program path name is registered in
/proc/sys/kernel/hotplug,
and normally holds the value
/sbin/hotplug.
Except for specialized configurations such as
initrd(4)
configurations,
that identifies a shell script which delegates to specific "policy agents",
/etc/hotplug/NAME.agent.
The name of the agent is given by the kernel as the first command-line
parameter. The name corresponds to a specific kernel subsystem whose
events it processes (e.g. "usb", "pci", "net" etc.).
Information about an event is passed in the form of environment variables.
Most agents accept a "ACTION" variable that describes the type of the event.
It is set to "add" or "remove" to signify that some device has just been
connected to or disconnected from the system.
The location of the device is provided in the variable "DEVPATH".
With kernel version 2.5 and later it is set to the corresponding sysfs device
path without the mountpoint but a leading slash.
hotplug
agents aren't necessarily called only by the kernel though. While the OS
is initializing, the "cold-plugging" case, the hotplug script in
/etc/init.d/hotplug
is invoked to make sure that all device connected at boot are properly
configured, applying the same rules as if they were connected later.
Hotplug agents should be used to dynamically adapt to system
configuration changes.
It helps to think of them as just delivering events from
different layers of the Linux software.
So low level bus framework code will deliver events when new physical
devices are plugged in, and bus agents make sure the
device's driver is loaded, configured, and activated.
(They use
depmod(8)
style device descriptions, used among other things to publish
MODULE_DEVICE_TABLE entries found in
the source to kernel mode device drivers.)
Then other layers in the Linux stack will take over, reporting
higher level events like "there is a new network interface".
Agents at those levels tend to have more complex tasks.
When used during system initialization, hotplugging may be
called "coldplugging" to highlight certain problems.
For example, program files and data needed to run hotplug
agents might be on filesystems that aren't yet available.
FILES
/lib/modules/*/modules.*map depmod output
/proc/sys/kernel/hotplug specifies hotplug program path
/sbin/hotplug hotplug program (default path name)
/etc/hotplug/* hotplug files
/etc/hotplug/NAME.agent hotplug subsystem-specific agents
/etc/hotplug/NAME* subsystem-specific files, for agents
/etc/hotplug/NAME/DRIVER driver setup scripts, invoked by agents
/etc/hotplug/usb/DRIVER.usermap depmod data for user-mode drivers
/etc/init.d/hotplug hotplug system service script
This manual page was originally
written by Fumitoshi UKAI <[email protected]>,
for the Debian GNU/Linux system (but may be used by others), based
on original Japanese version of manpage.
