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-Purpose of rules file:
-
-This rules file provides nonvolatile, unique names (in the form of symlinks)
-for input devices that cooperate.
-
-
-Description of rules:
-
-This file starts off with a few rules that make Udev skip the entire file if
-the current uevent is not input related. If ACTION is not "add", or SUBSYSTEM
-is not "input", or KERNEL (the device node) matches "input[0-9]*", then Udev
-will GOTO the LABEL named "persistent_input_end", which is the last rule in
-this file. (input[0-9]* uevents are skipped because they do not create device
-nodes.)
-
-This type of "skip this list of rules if X" operation is done in both the
-persistent input and persistent storage rules files. The reason is efficiency
--- if Udev had to go run the usb_id and/or path_id programs for non-input and
-non-storage rules, those rules would take much longer to process for no good
-reason.
-
-
-First in this file is a set of rules for by-ID style symlinks. These attempt
-to uniquely identify a device based on its serial number, but there are some
-issues with this. Many USB manufacturers do not provide a unique serial number
-for each device -- for instance, my Microsoft Intellimouse Optical has a USB
-serial number of "Microsoft_Microsoft_IntelliMouse_Optical". This kind of
-nonsensical "serial number" means that if you plug in two Intellimouse Optical
-devices, they will both get the same by-id symlink, and the device that the
-symlink points to will be random. This defeats the purpose of by-ID symlinks.
-(However, I believe this behavior is technically valid according to the USB
-standard. I believe it is not recommended, though.)
-
-Anyway, first in the by-ID rules, we have a rule that runs for any (input)
-device hanging anywhere off a USB bus. It uses the IMPORT{program} option to
-run the "/lib/udev/usb_id -x" program. usb_id looks at the environment to find
-out which device to look at, generates a list of environment-variable VAR=value
-pairs, and prints them. Udev stores this output away while the process is
-running. After the process exits, Udev modifies the current environment to
-include the VARs that usb_id printed. (It assigns the "value"s that usb_id
-printed to each of those VARs.) Specifically, usb_id prints ID_VENDOR,
-ID_MODEL, ID_REVISION, ID_SERIAL, ID_TYPE, and ID_BUS (at least in the case of
-the aforementioned USB optical mouse). These variable names will all be set in
-the environment.
-
-Then, we have a set of rules to set ID_CLASS for various types of devices. The
-rules first check for a "usb"-bus device that has a "bInterfaceClass" of 03 and
-a "bInterfaceProtocol" of 01. If the interface class is 03, this is an HID
-device. If the protocol is 01, it's a keyboard device. So we set ID_CLASS to
-"kbd". The next rule checks whether the interface protocol is 02, and if so,
-sets ID_CLASS to "mouse" (HID devices with a protocol of 02 are mice).
-
-Any input device that the "pcspkr" driver claims must be a speaker. Any input
-device that the "atkbd" driver claims must be a keyboard. Any input device
-that the "psmouse" driver claims must be a mouse. If there's a sysfs attribute
-named "name", whose contents contain "dvb", "DVB", or " IR ", then we set
-ID_CLASS to "ir".
-
-Then, we have a rule to search the tree and find the first parent that has a
-modalias. If that modalias matches the big long ugly string in the rules file,
-we assume this is a joystick device, and set ID_CLASS appropriately. (This
-parent should be the kobject for the joystick device itself. The reason we
-search the tree is that the current uevent is for a device node, not the
-physical joystick device.)
-
-Once the ID_CLASS variable is set properly, we have one more modification to
-perform: if the ID_SERIAL variable was not set at all by the usb_id program, we
-set it to "noserial".
-
-Now that all the environment variables are set up properly, we start generating
-the by-ID symlinks in /dev/input/by-id/. If the current device node's name
-starts with "event", we add "event" into the symlink name. Otherwise, we don't
-add anything for mice. (Other device types don't get a persistent by-ID
-symlink.)
-
-
-Next, we create by-path symlinks. The /lib/udev/path_id program takes the path
-of the device as an argument, and prints out "ID_PATH=string", where "string"
-is the "shortest physical path" to the device. We import this value into the
-environment.
-
-If the path is non-empty, and the device node name starts with "mouse" or
-"event", we add a by-path symlink based on the path and the device class (and
-we also add "event" if it's an event device). This symlink should be stable as
-long as the device never moves to a different port.
-