Based on kernel version 4.10.8. Page generated on 2017-04-01 14:44 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 workqueue [hub_wq], 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