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Documentation / usb / hotplug.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1	LINUX HOTPLUGGING
2	
3	In hotpluggable busses like USB (and Cardbus PCI), end-users plug devices
4	into the bus with power on.  In most cases, users expect the devices to become
5	immediately usable.  That means the system must do many things, including:
6	
7	    - Find a driver that can handle the device.  That may involve
8	      loading a kernel module; newer drivers can use module-init-tools
9	      to publish their device (and class) support to user utilities.
10	
11	    - Bind a driver to that device.  Bus frameworks do that using a
12	      device driver's probe() routine.
13	
14	    - Tell other subsystems to configure the new device.  Print
15	      queues may need to be enabled, networks brought up, disk
16	      partitions mounted, and so on.  In some cases these will
17	      be driver-specific actions.
18	
19	This involves a mix of kernel mode and user mode actions.  Making devices
20	be immediately usable means that any user mode actions can't wait for an
21	administrator to do them:  the kernel must trigger them, either passively
22	(triggering some monitoring daemon to invoke a helper program) or
23	actively (calling such a user mode helper program directly).
24	
25	Those triggered actions must support a system's administrative policies;
26	such programs are called "policy agents" here.  Typically they involve
27	shell scripts that dispatch to more familiar administration tools.
28	
29	Because some of those actions rely on information about drivers (metadata)
30	that is currently available only when the drivers are dynamically linked,
31	you get the best hotplugging when you configure a highly modular system.
32	
33	
34	KERNEL HOTPLUG HELPER (/sbin/hotplug)
35	
36	There is a kernel parameter: /proc/sys/kernel/hotplug, which normally
37	holds the pathname "/sbin/hotplug".  That parameter names a program
38	which the kernel may invoke at various times.
39	
40	The /sbin/hotplug program can be invoked by any subsystem as part of its
41	reaction to a configuration change, from a thread in that subsystem.
42	Only one parameter is required: the name of a subsystem being notified of
43	some kernel event.  That name is used as the first key for further event
44	dispatch; any other argument and environment parameters are specified by
45	the subsystem making that invocation.
46	
47	Hotplug software and other resources is available at:
48	
49		http://linux-hotplug.sourceforge.net
50	
51	Mailing list information is also available at that site.
52	
53	
54	--------------------------------------------------------------------------
55	
56	
57	USB POLICY AGENT
58	
59	The USB subsystem currently invokes /sbin/hotplug when USB devices
60	are added or removed from system.  The invocation is done by the kernel
61	hub daemon thread [khubd], or else as part of root hub initialization
62	(done by init, modprobe, kapmd, etc).  Its single command line parameter
63	is the string "usb", and it passes these environment variables:
64	
65	    ACTION ... "add", "remove"
66	    PRODUCT ... USB vendor, product, and version codes (hex)
67	    TYPE ... device class codes (decimal)
68	    INTERFACE ... interface 0 class codes (decimal)
69	
70	If "usbdevfs" is configured, DEVICE and DEVFS are also passed.  DEVICE is
71	the pathname of the device, and is useful for devices with multiple and/or
72	alternate interfaces that complicate driver selection.  By design, USB
73	hotplugging is independent of "usbdevfs":  you can do most essential parts
74	of USB device setup without using that filesystem, and without running a
75	user mode daemon to detect changes in system configuration.
76	
77	Currently available policy agent implementations can load drivers for
78	modules, and can invoke driver-specific setup scripts.  The newest ones
79	leverage USB module-init-tools support.  Later agents might unload drivers.
80	
81	
82	USB MODUTILS SUPPORT
83	
84	Current versions of module-init-tools will create a "modules.usbmap" file
85	which contains the entries from each driver's MODULE_DEVICE_TABLE.  Such
86	files can be used by various user mode policy agents to make sure all the
87	right driver modules get loaded, either at boot time or later.
88	
89	See <linux/usb.h> for full information about such table entries; or look
90	at existing drivers.  Each table entry describes one or more criteria to
91	be used when matching a driver to a device or class of devices.  The
92	specific criteria are identified by bits set in "match_flags", paired
93	with field values.  You can construct the criteria directly, or with
94	macros such as these, and use driver_info to store more information.
95	
96	    USB_DEVICE (vendorId, productId)
97		... matching devices with specified vendor and product ids
98	    USB_DEVICE_VER (vendorId, productId, lo, hi)
99		... like USB_DEVICE with lo <= productversion <= hi
100	    USB_INTERFACE_INFO (class, subclass, protocol)
101		... matching specified interface class info
102	    USB_DEVICE_INFO (class, subclass, protocol)
103		... matching specified device class info
104	
105	A short example, for a driver that supports several specific USB devices
106	and their quirks, might have a MODULE_DEVICE_TABLE like this:
107	
108	    static const struct usb_device_id mydriver_id_table = {
109		{ USB_DEVICE (0x9999, 0xaaaa), driver_info: QUIRK_X },
110		{ USB_DEVICE (0xbbbb, 0x8888), driver_info: QUIRK_Y|QUIRK_Z },
111		...
112		{ } /* end with an all-zeroes entry */
113	    }
114	    MODULE_DEVICE_TABLE (usb, mydriver_id_table);
115	
116	Most USB device drivers should pass these tables to the USB subsystem as
117	well as to the module management subsystem.  Not all, though: some driver
118	frameworks connect using interfaces layered over USB, and so they won't
119	need such a "struct usb_driver".
120	
121	Drivers that connect directly to the USB subsystem should be declared
122	something like this:
123	
124	    static struct usb_driver mydriver = {
125		.name		= "mydriver",
126		.id_table	= mydriver_id_table,
127		.probe		= my_probe,
128		.disconnect	= my_disconnect,
129	
130		/*
131		if using the usb chardev framework:
132		    .minor		= MY_USB_MINOR_START,
133		    .fops		= my_file_ops,
134		if exposing any operations through usbdevfs:
135		    .ioctl		= my_ioctl,
136		*/
137	    }
138	
139	When the USB subsystem knows about a driver's device ID table, it's used when
140	choosing drivers to probe().  The thread doing new device processing checks
141	drivers' device ID entries from the MODULE_DEVICE_TABLE against interface and
142	device descriptors for the device.  It will only call probe() if there is a
143	match, and the third argument to probe() will be the entry that matched.
144	
145	If you don't provide an id_table for your driver, then your driver may get
146	probed for each new device; the third parameter to probe() will be null.
147	
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