155 lines
6.4 KiB
ReStructuredText
155 lines
6.4 KiB
ReStructuredText
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USB hotplugging
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~~~~~~~~~~~~~~~
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Linux Hotplugging
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=================
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In hotpluggable busses like USB (and Cardbus PCI), end-users plug devices
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into the bus with power on. In most cases, users expect the devices to become
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immediately usable. That means the system must do many things, including:
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- Find a driver that can handle the device. That may involve
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loading a kernel module; newer drivers can use module-init-tools
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to publish their device (and class) support to user utilities.
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- Bind a driver to that device. Bus frameworks do that using a
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device driver's probe() routine.
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- Tell other subsystems to configure the new device. Print
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queues may need to be enabled, networks brought up, disk
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partitions mounted, and so on. In some cases these will
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be driver-specific actions.
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This involves a mix of kernel mode and user mode actions. Making devices
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be immediately usable means that any user mode actions can't wait for an
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administrator to do them: the kernel must trigger them, either passively
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(triggering some monitoring daemon to invoke a helper program) or
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actively (calling such a user mode helper program directly).
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Those triggered actions must support a system's administrative policies;
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such programs are called "policy agents" here. Typically they involve
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shell scripts that dispatch to more familiar administration tools.
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Because some of those actions rely on information about drivers (metadata)
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that is currently available only when the drivers are dynamically linked,
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you get the best hotplugging when you configure a highly modular system.
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Kernel Hotplug Helper (``/sbin/hotplug``)
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=========================================
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There is a kernel parameter: ``/proc/sys/kernel/hotplug``, which normally
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holds the pathname ``/sbin/hotplug``. That parameter names a program
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which the kernel may invoke at various times.
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The /sbin/hotplug program can be invoked by any subsystem as part of its
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reaction to a configuration change, from a thread in that subsystem.
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Only one parameter is required: the name of a subsystem being notified of
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some kernel event. That name is used as the first key for further event
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dispatch; any other argument and environment parameters are specified by
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the subsystem making that invocation.
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Hotplug software and other resources is available at:
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http://linux-hotplug.sourceforge.net
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Mailing list information is also available at that site.
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USB Policy Agent
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================
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The USB subsystem currently invokes ``/sbin/hotplug`` when USB devices
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are added or removed from system. The invocation is done by the kernel
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hub workqueue [hub_wq], or else as part of root hub initialization
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(done by init, modprobe, kapmd, etc). Its single command line parameter
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is the string "usb", and it passes these environment variables:
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========== ============================================
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ACTION ``add``, ``remove``
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PRODUCT USB vendor, product, and version codes (hex)
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TYPE device class codes (decimal)
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INTERFACE interface 0 class codes (decimal)
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========== ============================================
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If "usbdevfs" is configured, DEVICE and DEVFS are also passed. DEVICE is
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the pathname of the device, and is useful for devices with multiple and/or
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alternate interfaces that complicate driver selection. By design, USB
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hotplugging is independent of ``usbdevfs``: you can do most essential parts
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of USB device setup without using that filesystem, and without running a
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user mode daemon to detect changes in system configuration.
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Currently available policy agent implementations can load drivers for
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modules, and can invoke driver-specific setup scripts. The newest ones
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leverage USB module-init-tools support. Later agents might unload drivers.
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USB Modutils Support
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====================
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Current versions of module-init-tools will create a ``modules.usbmap`` file
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which contains the entries from each driver's ``MODULE_DEVICE_TABLE``. Such
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files can be used by various user mode policy agents to make sure all the
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right driver modules get loaded, either at boot time or later.
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See ``linux/usb.h`` for full information about such table entries; or look
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at existing drivers. Each table entry describes one or more criteria to
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be used when matching a driver to a device or class of devices. The
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specific criteria are identified by bits set in "match_flags", paired
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with field values. You can construct the criteria directly, or with
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macros such as these, and use driver_info to store more information::
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USB_DEVICE (vendorId, productId)
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... matching devices with specified vendor and product ids
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USB_DEVICE_VER (vendorId, productId, lo, hi)
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... like USB_DEVICE with lo <= productversion <= hi
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USB_INTERFACE_INFO (class, subclass, protocol)
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... matching specified interface class info
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USB_DEVICE_INFO (class, subclass, protocol)
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... matching specified device class info
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A short example, for a driver that supports several specific USB devices
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and their quirks, might have a MODULE_DEVICE_TABLE like this::
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static const struct usb_device_id mydriver_id_table[] = {
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{ USB_DEVICE (0x9999, 0xaaaa), driver_info: QUIRK_X },
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{ USB_DEVICE (0xbbbb, 0x8888), driver_info: QUIRK_Y|QUIRK_Z },
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...
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{ } /* end with an all-zeroes entry */
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};
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MODULE_DEVICE_TABLE(usb, mydriver_id_table);
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Most USB device drivers should pass these tables to the USB subsystem as
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well as to the module management subsystem. Not all, though: some driver
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frameworks connect using interfaces layered over USB, and so they won't
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need such a struct usb_driver.
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Drivers that connect directly to the USB subsystem should be declared
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something like this::
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static struct usb_driver mydriver = {
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.name = "mydriver",
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.id_table = mydriver_id_table,
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.probe = my_probe,
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.disconnect = my_disconnect,
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/*
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if using the usb chardev framework:
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.minor = MY_USB_MINOR_START,
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.fops = my_file_ops,
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if exposing any operations through usbdevfs:
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.ioctl = my_ioctl,
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*/
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};
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When the USB subsystem knows about a driver's device ID table, it's used when
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choosing drivers to probe(). The thread doing new device processing checks
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drivers' device ID entries from the ``MODULE_DEVICE_TABLE`` against interface
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and device descriptors for the device. It will only call ``probe()`` if there
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is a match, and the third argument to ``probe()`` will be the entry that
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matched.
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If you don't provide an ``id_table`` for your driver, then your driver may get
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probed for each new device; the third parameter to ``probe()`` will be
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``NULL``.
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