LINUX HOTPLUGGING

In hotpluggable busses like USB (and Cardbus PCI), end-users plug devices
into the bus with power on.  In most cases, users expect the devices to become
immediately usable.  That means the system must do many things, including:

    - Find a driver that can handle the device.  That may involve
      loading a kernel module; newer drivers can use modutils to
      publish their device (and class) support to user utilities.

    - Bind a driver to that device.  Bus frameworks do that using a
      device driver's probe() routine.
    
    - Tell other subsystems to configure the new device.  Print
      queues may need to be enabled, networks brought up, disk
      partitions mounted, and so on.  In some cases these will
      be driver-specific actions.

This involves a mix of kernel mode and user mode actions.  Making devices
be immediately usable means that any user mode actions can't wait for an
administrator to do them:  the kernel must trigger them, either passively
(triggering some monitoring daemon to invoke a helper program) or
actively (calling such a user mode helper program directly).

Those triggered actions must support a system's administrative policies;
such programs are called "policy agents" here.  Typically they involve
shell scripts that dispatch to more familiar administration tools.

Because some of those actions rely on information about drivers (metadata)
that is currently available only when the drivers are dynamically linked,
you get the best hotplugging when you configure a highly modular system.


KERNEL HOTPLUG HELPER (/sbin/hotplug)

When you compile with CONFIG_HOTPLUG, you get a new kernel parameter:
/proc/sys/kernel/hotplug, which normally holds the pathname "/sbin/hotplug".
That parameter names a program which the kernel may invoke at various times.

The /sbin/hotplug program can be invoked by any subsystem as part of its
reaction to a configuration change, from a thread in that subsystem.
Only one parameter is required: the name of a subsystem being notified of
some kernel event.  That name is used as the first key for further event
dispatch; any other argument and environment parameters are specified by
the subsystem making that invocation.

Hotplug software and other resources is available at:

	http://linux-hotplug.sourceforge.net

Mailing list information is also available at that site.


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USB POLICY AGENT

The USB subsystem currently invokes /sbin/hotplug when USB devices
are added or removed from system.  The invocation is done by the kernel
hub daemon thread [khubd], or else as part of root hub initialization
(done by init, modprobe, kapmd, etc).  Its single command line parameter
is the string "usb", and it passes these environment variables:

    ACTION ... "add", "remove"
    PRODUCT ... USB vendor, product, and version codes (hex)
    TYPE ... device class codes (decimal)
    INTERFACE ... interface 0 class codes (decimal)

If "usbdevfs" is configured, DEVICE and DEVFS are also passed.  DEVICE is
the pathname of the device, and is useful for devices with multiple and/or
alternate interfaces that complicate driver selection.

Currently available policy agent implementations can load drivers for
modules, and can invoke driver-specific setup scripts.  The newest ones
leverage USB modutils support.  Later agents might unload drivers.


USB MODUTILS SUPPORT

Current versions of modutils will create a "modules.usbmap" file which
contains the entries from each driver's MODULE_DEVICE_TABLE.  Such files
can be used by various user mode policy agents to make sure all the right
driver modules get loaded, either at boot time or later. 

See <linux/usb.h> for full information about such table entries; or look
at existing drivers.  Each table entry describes one or more criteria to
be used when matching a driver to a device or class of devices.

A short example, for a driver that supports several specific USB devices
and their quirks, might have a MODULE_DEVICE_TABLE like this:

    static const struct usb_device_id mydriver_id_table = {
	{ idVendor: 0x9999, idProduct 0xaaaa, driver_info: QUIRK_X },
	{ idVendor: 0xbbbb, idProduct 0x8888, driver_info: QUIRK_Y|QUIRK_Z },
	...
	{ } /* end with an all-zeroes entry */
    }
    MODULE_DEVICE_TABLE (usb, mydriver_id_table);

Most USB device drivers should pass these tables to the USB subsystem as
well as to the module management subsystem.  Not all, though: some driver
frameworks connect using interfaces layered over USB, and so they won't
need such a "struct usb_driver".

Drivers that connect directly to the USB subsystem should be declared
something like this:

    static struct usb_driver mydriver = {
	name:		"mydriver",
	id_table:	mydriver_id_table,
	probe:		my_probe,
	disconnect:	my_disconnect,

	/*
	if using the usb chardev framework:
	    minor:		MY_USB_MINOR_START,
	    fops:		my_file_ops,
	if exposing any operations through usbdevfs:
	    ioctl:		my_ioctl,
	*/
    }

When the USB subsystem knows about a driver's device ID table, it's used when
choosing drivers to probe().  The thread doing new device processing checks
drivers' device ID entries from the MODULE_DEVICE_TABLE against interface and
device descriptors for the device.  It will only call probe() if there is a
match, and the third argument to probe() will be the entry that matched.

If you don't provide an id_table for your driver, then your driver may get
probed for each new device; the third parameter to probe() will be null.