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1USB hotplugging2~~~~~~~~~~~~~~~3 4Linux Hotplugging5=================6 7 8In hotpluggable busses like USB (and Cardbus PCI), end-users plug devices9into the bus with power on. In most cases, users expect the devices to become10immediately usable. That means the system must do many things, including:11 12 - Find a driver that can handle the device. That may involve13 loading a kernel module; newer drivers can use module-init-tools14 to publish their device (and class) support to user utilities.15 16 - Bind a driver to that device. Bus frameworks do that using a17 device driver's probe() routine.18 19 - Tell other subsystems to configure the new device. Print20 queues may need to be enabled, networks brought up, disk21 partitions mounted, and so on. In some cases these will22 be driver-specific actions.23 24This involves a mix of kernel mode and user mode actions. Making devices25be immediately usable means that any user mode actions can't wait for an26administrator to do them: the kernel must trigger them, either passively27(triggering some monitoring daemon to invoke a helper program) or28actively (calling such a user mode helper program directly).29 30Those triggered actions must support a system's administrative policies;31such programs are called "policy agents" here. Typically they involve32shell scripts that dispatch to more familiar administration tools.33 34Because some of those actions rely on information about drivers (metadata)35that is currently available only when the drivers are dynamically linked,36you get the best hotplugging when you configure a highly modular system.37 38Kernel Hotplug Helper (``/sbin/hotplug``)39=========================================40 41There is a kernel parameter: ``/proc/sys/kernel/hotplug``, which normally42holds the pathname ``/sbin/hotplug``. That parameter names a program43which the kernel may invoke at various times.44 45The /sbin/hotplug program can be invoked by any subsystem as part of its46reaction to a configuration change, from a thread in that subsystem.47Only one parameter is required: the name of a subsystem being notified of48some kernel event. That name is used as the first key for further event49dispatch; any other argument and environment parameters are specified by50the subsystem making that invocation.51 52Hotplug software and other resources is available at:53 54 http://linux-hotplug.sourceforge.net55 56Mailing list information is also available at that site.57 58 59USB Policy Agent60================61 62The USB subsystem currently invokes ``/sbin/hotplug`` when USB devices63are added or removed from system. The invocation is done by the kernel64hub workqueue [hub_wq], or else as part of root hub initialization65(done by init, modprobe, kapmd, etc). Its single command line parameter66is the string "usb", and it passes these environment variables:67 68========== ============================================69ACTION ``add``, ``remove``70PRODUCT USB vendor, product, and version codes (hex)71TYPE device class codes (decimal)72INTERFACE interface 0 class codes (decimal)73========== ============================================74 75If "usbdevfs" is configured, DEVICE and DEVFS are also passed. DEVICE is76the pathname of the device, and is useful for devices with multiple and/or77alternate interfaces that complicate driver selection. By design, USB78hotplugging is independent of ``usbdevfs``: you can do most essential parts79of USB device setup without using that filesystem, and without running a80user mode daemon to detect changes in system configuration.81 82Currently available policy agent implementations can load drivers for83modules, and can invoke driver-specific setup scripts. The newest ones84leverage USB module-init-tools support. Later agents might unload drivers.85 86 87USB Modutils Support88====================89 90Current versions of module-init-tools will create a ``modules.usbmap`` file91which contains the entries from each driver's ``MODULE_DEVICE_TABLE``. Such92files can be used by various user mode policy agents to make sure all the93right driver modules get loaded, either at boot time or later.94 95See ``linux/usb.h`` for full information about such table entries; or look96at existing drivers. Each table entry describes one or more criteria to97be used when matching a driver to a device or class of devices. The98specific criteria are identified by bits set in "match_flags", paired99with field values. You can construct the criteria directly, or with100macros such as these, and use driver_info to store more information::101 102 USB_DEVICE (vendorId, productId)103 ... matching devices with specified vendor and product ids104 USB_DEVICE_VER (vendorId, productId, lo, hi)105 ... like USB_DEVICE with lo <= productversion <= hi106 USB_INTERFACE_INFO (class, subclass, protocol)107 ... matching specified interface class info108 USB_DEVICE_INFO (class, subclass, protocol)109 ... matching specified device class info110 111A short example, for a driver that supports several specific USB devices112and their quirks, might have a MODULE_DEVICE_TABLE like this::113 114 static const struct usb_device_id mydriver_id_table[] = {115 { USB_DEVICE (0x9999, 0xaaaa), driver_info: QUIRK_X },116 { USB_DEVICE (0xbbbb, 0x8888), driver_info: QUIRK_Y|QUIRK_Z },117 ...118 { } /* end with an all-zeroes entry */119 };120 MODULE_DEVICE_TABLE(usb, mydriver_id_table);121 122Most USB device drivers should pass these tables to the USB subsystem as123well as to the module management subsystem. Not all, though: some driver124frameworks connect using interfaces layered over USB, and so they won't125need such a struct usb_driver.126 127Drivers that connect directly to the USB subsystem should be declared128something like this::129 130 static struct usb_driver mydriver = {131 .name = "mydriver",132 .id_table = mydriver_id_table,133 .probe = my_probe,134 .disconnect = my_disconnect,135 136 /*137 if using the usb chardev framework:138 .minor = MY_USB_MINOR_START,139 .fops = my_file_ops,140 if exposing any operations through usbdevfs:141 .ioctl = my_ioctl,142 */143 };144 145When the USB subsystem knows about a driver's device ID table, it's used when146choosing drivers to probe(). The thread doing new device processing checks147drivers' device ID entries from the ``MODULE_DEVICE_TABLE`` against interface148and device descriptors for the device. It will only call ``probe()`` if there149is a match, and the third argument to ``probe()`` will be the entry that150matched.151 152If you don't provide an ``id_table`` for your driver, then your driver may get153probed for each new device; the third parameter to ``probe()`` will be154``NULL``.155