1、Linux Input子系统概述
Linux系统的Input子系统处理输入事务，输入设备的驱动程序通过Input输入子系统提供的接口注册到内核，利用子系统提供的功能与用户空间或系统中的其它程序进行交互。整个Input子系统的实现可以看作三层，如下：
- 输入驱动层(input driver)：针对各个具体输入设备的驱动功能程序；
- 输入核心层(input core)：Linux内核统一管理各输入设备及其驱动程序；
- 事件处理层(event handler)：将底层输入硬件事件翻译成用户空间所认知的软事件。
Linux内核Input子系统的框架如下所示：
从上图可以知道，Linux系统中一般的输入设备都是字符类设备，Input子系统的所有输入设备的主设备号都是13。
2、Input数据结构
（1）input_dev结构体
在Linux Input子系统框架中可以看到，input_dev结构体是Input子系统中最重要的数据结构，它代表了一类输入设备，每个具体的input驱动程序，都必须分配并初始化这样的一个结构体，该结构体的定义如下所示：
/** * struct input_dev - represents an input device * @name: name of the device * @phys: physical path to the device in the system hierarchy * @uniq: unique identification code for the device (if device has it) * @id: id of the device (struct input_id) * @propbit: bitmap of device properties and quirks * @evbit: bitmap of types of events supported by the device (EV_KEY, * EV_REL, etc.) * @keybit: bitmap of keys/buttons this device has * @relbit: bitmap of relative axes for the device * @absbit: bitmap of absolute axes for the device * @mscbit: bitmap of miscellaneous events supported by the device * @ledbit: bitmap of leds present on the device * @sndbit: bitmap of sound effects supported by the device * @ffbit: bitmap of force feedback effects supported by the device * @swbit: bitmap of switches present on the device * @hint_events_per_packet: average number of events generated by the * device in a packet (between EV_SYN/SYN_REPORT events). Used by * event handlers to estimate size of the buffer needed to hold * events. * @keycodemax: size of keycode table * @keycodesize: size of elements in keycode table * @keycode: map of scancodes to keycodes for this device * @getkeycode: optional legacy method to retrieve current keymap. * @setkeycode: optional method to alter current keymap, used to implement * sparse keymaps. If not supplied default mechanism will be used. * The method is being called while holding event_lock and thus must * not sleep * @ff: force feedback structure associated with the device if device * supports force feedback effects * @repeat_key: stores key code of the last key pressed; used to implement * software autorepeat * @timer: timer for software autorepeat * @rep: current values for autorepeat parameters (delay, rate) * @mt: pointer to multitouch state * @absinfo: array of &struct input_absinfo elements holding information * about absolute axes (current value, min, max, flat, fuzz, * resolution) * @key: reflects current state of device's keys/buttons * @led: reflects current state of device's LEDs * @snd: reflects current state of sound effects * @sw: reflects current state of device's switches * @open: this method is called when the very first user calls * input_open_device(). The driver must prepare the device * to start generating events (start polling thread, * request an IRQ, submit URB, etc.) * @close: this method is called when the very last user calls * input_close_device(). * @flush: purges the device. Most commonly used to get rid of force * feedback effects loaded into the device when disconnecting * from it * @event: event handler for events sent _to_ the device, like EV_LED * or EV_SND. The device is expected to carry out the requested * action (turn on a LED, play sound, etc.) The call is protected * by @event_lock and must not sleep * @grab: input handle that currently has the device grabbed (via * EVIOCGRAB ioctl). When a handle grabs a device it becomes sole * recipient for all input events coming from the device * @event_lock: this spinlock is taken when input core receives * and processes a new event for the device (in input_event()). * Code that accesses and/or modifies parameters of a device * (such as keymap or absmin, absmax, absfuzz, etc.) after device * has been registered with input core must take this lock. * @mutex: serializes calls to open(), close() and flush() methods * @users: stores number of users (input handlers) that opened this * device. It is used by input_open_device() and input_close_device() * to make sure that dev->open() is only called when the first * user opens device and dev->close() is called when the very * last user closes the device * @going_away: marks devices that are in a middle of unregistering and * causes input_open_device*() fail with -ENODEV. * @dev: driver model's view of this device * @h_list: list of input handles associated with the device. When * accessing the list dev->mutex must be held * @node: used to place the device onto input_dev_list * @num_vals: number of values queued in the current frame * @max_vals: maximum number of values queued in a frame * @vals: array of values queued in the current frame * @devres_managed: indicates that devices is managed with devres framework * and needs not be explicitly unregistered or freed. */ struct input_dev { const char *name; const char *phys; const char *uniq; struct input_id id; unsigned long propbit[BITS_TO_LONGS(INPUT_PROP_CNT)]; unsigned long evbit[BITS_TO_LONGS(EV_CNT)]; unsigned long keybit[BITS_TO_LONGS(KEY_CNT)]; unsigned long relbit[BITS_TO_LONGS(REL_CNT)]; unsigned long absbit[BITS_TO_LONGS(ABS_CNT)]; unsigned long mscbit[BITS_TO_LONGS(MSC_CNT)]; unsigned long ledbit[BITS_TO_LONGS(LED_CNT)]; unsigned long sndbit[BITS_TO_LONGS(SND_CNT)]; unsigned long ffbit[BITS_TO_LONGS(FF_CNT)]; unsigned long swbit[BITS_TO_LONGS(SW_CNT)]; unsigned int hint_events_per_packet; unsigned int keycodemax; unsigned int keycodesize; void *keycode; int (*setkeycode)(struct input_dev *dev, const struct input_keymap_entry *ke, unsigned int *old_keycode); int (*getkeycode)(struct input_dev *dev, struct input_keymap_entry *ke); struct ff_device *ff; unsigned int repeat_key; struct timer_list timer; int rep[REP_CNT]; struct input_mt *mt; struct input_absinfo *absinfo; unsigned long key[BITS_TO_LONGS(KEY_CNT)]; unsigned long led[BITS_TO_LONGS(LED_CNT)]; unsigned long snd[BITS_TO_LONGS(SND_CNT)]; unsigned long sw[BITS_TO_LONGS(SW_CNT)]; int (*open)(struct input_dev *dev); void (*close)(struct input_dev *dev); int (*flush)(struct input_dev *dev, struct file *file); int (*event)(struct input_dev *dev, unsigned int type, unsigned int code, int value); struct input_handle __rcu *grab; spinlock_t event_lock; struct mutex mutex; unsigned int users; bool going_away; struct device dev; struct list_head h_list; struct list_head node; unsigned int num_vals; unsigned int max_vals; struct input_value *vals; bool devres_managed; };