1USB hotplugging 2~~~~~~~~~~~~~~~ 3 4Linux Hotplugging 5================= 6 7 8In hotpluggable busses like USB (and Cardbus PCI), end-users plug devices 9into the bus with power on. In most cases, users expect the devices to become 10immediately usable. That means the system must do many things, including: 11 12 - Find a driver that can handle the device. That may involve 13 loading a kernel module; newer drivers can use module-init-tools 14 to publish their device (and class) support to user utilities. 15 16 - Bind a driver to that device. Bus frameworks do that using a 17 device driver's probe() routine. 18 19 - Tell other subsystems to configure the new device. Print 20 queues may need to be enabled, networks brought up, disk 21 partitions mounted, and so on. In some cases these will 22 be driver-specific actions. 23 24This involves a mix of kernel mode and user mode actions. Making devices 25be immediately usable means that any user mode actions can't wait for an 26administrator to do them: the kernel must trigger them, either passively 27(triggering some monitoring daemon to invoke a helper program) or 28actively (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 involve 32shell 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 normally 42holds the pathname ``/sbin/hotplug``. That parameter names a program 43which the kernel may invoke at various times. 44 45The /sbin/hotplug program can be invoked by any subsystem as part of its 46reaction to a configuration change, from a thread in that subsystem. 47Only one parameter is required: the name of a subsystem being notified of 48some kernel event. That name is used as the first key for further event 49dispatch; any other argument and environment parameters are specified by 50the subsystem making that invocation. 51 52Hotplug software and other resources is available at: 53 54 http://linux-hotplug.sourceforge.net 55 56Mailing list information is also available at that site. 57 58 59USB Policy Agent 60================ 61 62The USB subsystem currently invokes ``/sbin/hotplug`` when USB devices 63are added or removed from system. The invocation is done by the kernel 64hub workqueue [hub_wq], or else as part of root hub initialization 65(done by init, modprobe, kapmd, etc). Its single command line parameter 66is 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 is 76the pathname of the device, and is useful for devices with multiple and/or 77alternate interfaces that complicate driver selection. By design, USB 78hotplugging is independent of ``usbdevfs``: you can do most essential parts 79of USB device setup without using that filesystem, and without running a 80user mode daemon to detect changes in system configuration. 81 82Currently available policy agent implementations can load drivers for 83modules, and can invoke driver-specific setup scripts. The newest ones 84leverage USB module-init-tools support. Later agents might unload drivers. 85 86 87USB Modutils Support 88==================== 89 90Current versions of module-init-tools will create a ``modules.usbmap`` file 91which contains the entries from each driver's ``MODULE_DEVICE_TABLE``. Such 92files can be used by various user mode policy agents to make sure all the 93right driver modules get loaded, either at boot time or later. 94 95See ``linux/usb.h`` for full information about such table entries; or look 96at existing drivers. Each table entry describes one or more criteria to 97be used when matching a driver to a device or class of devices. The 98specific criteria are identified by bits set in "match_flags", paired 99with field values. You can construct the criteria directly, or with 100macros 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 ids 104 USB_DEVICE_VER (vendorId, productId, lo, hi) 105 ... like USB_DEVICE with lo <= productversion <= hi 106 USB_INTERFACE_INFO (class, subclass, protocol) 107 ... matching specified interface class info 108 USB_DEVICE_INFO (class, subclass, protocol) 109 ... matching specified device class info 110 111A short example, for a driver that supports several specific USB devices 112and 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 as 123well as to the module management subsystem. Not all, though: some driver 124frameworks connect using interfaces layered over USB, and so they won't 125need such a struct :c:type:`usb_driver`. 126 127Drivers that connect directly to the USB subsystem should be declared 128something 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 when 146choosing drivers to probe(). The thread doing new device processing checks 147drivers' device ID entries from the ``MODULE_DEVICE_TABLE`` against interface 148and device descriptors for the device. It will only call ``probe()`` if there 149is a match, and the third argument to ``probe()`` will be the entry that 150matched. 151 152If you don't provide an ``id_table`` for your driver, then your driver may get 153probed for each new device; the third parameter to ``probe()`` will be 154``NULL``. 155