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Diffstat (limited to 'Documentation/usb/hotplug.txt')
| -rw-r--r-- | Documentation/usb/hotplug.txt | 148 |
1 files changed, 148 insertions, 0 deletions
diff --git a/Documentation/usb/hotplug.txt b/Documentation/usb/hotplug.txt new file mode 100644 index 00000000000..f53170665f3 --- /dev/null +++ b/Documentation/usb/hotplug.txt @@ -0,0 +1,148 @@ +LINUX HOTPLUGGING + +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) + +When you compile with CONFIG_HOTPLUG, you get a new 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: + + http://linux-hotplug.sourceforge.net + +Mailing list information is also available at that site. + + +-------------------------------------------------------------------------- + + +USB POLICY AGENT + +The USB subsystem currently invokes /sbin/hotplug when USB devices +are added or removed from system. The invocation is done by the kernel +hub daemon thread [khubd], 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. + + +USB MODUTILS SUPPORT + +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. + + |