path: root/Documentation/usb
diff options
authorMauro Carvalho Chehab <>2017-04-05 10:23:08 -0300
committerJonathan Corbet <>2017-04-11 14:40:01 -0600
commit76f650f077f3edd7001c89da44eade2449e8f495 (patch)
tree18c281772f83fc61f8bbb92bc4b57586bde7dc81 /Documentation/usb
parent360a7b5f57e7734d34bc37574822ce74c5ba5d25 (diff)
usb/hotplug.txt: convert to ReST and add to driver-api book
This document describe some USB core features. Add it to the driver-api book. Signed-off-by: Mauro Carvalho Chehab <> Acked-by: Greg Kroah-Hartman <> Signed-off-by: Jonathan Corbet <>
Diffstat (limited to 'Documentation/usb')
1 files changed, 0 insertions, 148 deletions
diff --git a/Documentation/usb/hotplug.txt b/Documentation/usb/hotplug.txt
deleted file mode 100644
index 5b243f315b2c..000000000000
--- a/Documentation/usb/hotplug.txt
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-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 module-init-tools
- 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)
-There is a 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:
-Mailing list information is also available at that site.
-The USB subsystem currently invokes /sbin/hotplug when USB devices
-are added or removed from system. The invocation is done by the kernel
-hub workqueue [hub_wq], 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. By design, USB
-hotplugging is independent of "usbdevfs": you can do most essential parts
-of USB device setup without using that filesystem, and without running a
-user mode daemon to detect changes in system configuration.
-Currently available policy agent implementations can load drivers for
-modules, and can invoke driver-specific setup scripts. The newest ones
-leverage USB module-init-tools support. Later agents might unload drivers.
-Current versions of module-init-tools 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. The
-specific criteria are identified by bits set in "match_flags", paired
-with field values. You can construct the criteria directly, or with
-macros such as these, and use driver_info to store more information.
- USB_DEVICE (vendorId, productId)
- ... matching devices with specified vendor and product ids
- USB_DEVICE_VER (vendorId, productId, lo, hi)
- ... like USB_DEVICE with lo <= productversion <= hi
- USB_INTERFACE_INFO (class, subclass, protocol)
- ... matching specified interface class info
- USB_DEVICE_INFO (class, subclass, protocol)
- ... matching specified device class info
-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[] = {
- { USB_DEVICE (0x9999, 0xaaaa), driver_info: QUIRK_X },
- { USB_DEVICE (0xbbbb, 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.