一、input子系统概述
1. 什么是input子系统
input子系统是管理输入的子系统，和pinctrl、gpio子系统一样，都是Linux内核针对某一类设备创建的框架。
2. input子系统的作用
输入设备都是典型的字符设备，比如按键、鼠标、键盘、触摸屏等。
这些设备的工作机理是底层在按键、触摸等动作发生时产生一个中断，或者通过软件定时器去查询时间，然后CPU通过SPI、I2C等总线读取键值、坐标等数据，并将它们放入一个缓冲区，由驱动模块管理这些缓冲区，通过驱动的read接口就可以让用户读取键值、坐标等数据。
显然 ，在这些工作中，只有中断、读取键值/坐标这些操作是与硬件相关的，而输入事件的管理则对输入设备是通用的，因此，Linux内核设计了input子系统，由核心层处理公共的工作。
有了input子系统后，驱动编写时就不需要考虑应用层的事情，只需要按照input子系统的要求，上报输入事件即可。
3. input子系统的框架
input子系统分为input驱动层、input核心层、input事件处理层。
- input驱动层：输入设备的具体驱动程序，比如按键驱动，向内核层报告输入事件
- input核心层：对下提供设备注册和操作接口，对上通知input事件层对输入事件进行处理
- input事件层：和用户空间进行交互
二、input子系统使用方法
1. 输入设备
input子系统的核心层会向Linux内核注册一个字符设备，主设备号为INPUT_MAJOR，
#define INPUT_MAJOR 13
因此，input子系统所有设备主设备号都为13，在使用input子系统时不需要再注册字符设备，只注册一个输入设备即可。
1.1. input_dev结构体
Linux内核用 input_dev 结构体描述一个输入设备，定义在文件 include/linux/input.h 文件中，如下：
/**
* 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 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;
};
1.2. 分配/释放一个输入设备
（1）申请一个输入设备
/**
* input_allocate_device - allocate memory for new input device
*
* Returns prepared struct input_dev or %NULL.
*
* NOTE: Use input_free_device() to free devices that have not been
* registered; input_unregister_device() should be used for already
* registered devices.
*/
struct input_dev *input_allocate_device(void);
（2）释放输入设备
/**
* input_free_device - free memory occupied by input_dev structure
* @dev: input device to free
*
* This function should only be used if input_register_device()
* was not called yet or if it failed. Once device was registered
* use input_unregister_device() and memory will be freed once last
* reference to the device is dropped.
*
* Device should be allocated by input_allocate_device().
*
* NOTE: If there are references to the input device then memory
* will not be freed until last reference is dropped.
*/
void input_free_device(struct input_dev *dev)
1.3. 注册/注销输入设备
（1）向内核注册输入设备
/**
* input_register_device - register device with input core
* @dev: device to be registered
*
* This function registers device with input core. The device must be
* allocated with input_allocate_device() and all it's capabilities
* set up before registering.
* If function fails the device must be freed with input_free_device().
* Once device has been successfully registered it can be unregistered
* with input_unregister_device(); input_free_device() should not be
* called in this case.
*
* Note that this function is also used to register managed input devices
* (ones allocated with devm_input_allocate_device()). Such managed input
* devices need not be explicitly unregistered or freed, their tear down
* is controlled by the devres infrastructure. It is also worth noting
* that tear down of managed input devices is internally a 2-step process:
* registered managed input device is first unregistered, but stays in
* memory and can still handle input_event() calls (although events will
* not be delivered anywhere). The freeing of managed input device will
* happen later, when devres stack is unwound to the point where device
* allocation was made.
*/
int input_register_device(struct input_dev *dev);
（2）从内核注销输入设备
/**
* input_unregister_device - unregister previously registered device
* @dev: device to be unregistered
*
* This function unregisters an input device. Once device is unregistered
* the caller should not try to access it as it may get freed at any moment.
*/
void input_unregister_device(struct input_dev *dev);
2. 上报输入事件
2.1. 上报指定的事件以及对应的值
/**
* input_event() - report new input event
* @dev: device that generated the event
* @type: type of the event
* @code: event code
* @value: value of the event
*
* This function should be used by drivers implementing various input
* devices to report input events. See also input_inject_event().
*
* NOTE: input_event() may be safely used right after input device was
* allocated with input_allocate_device(), even before it is registered
* with input_register_device(), but the event will not reach any of the
* input handlers. Such early invocation of input_event() may be used
* to 'seed' initial state of a switch or initial position of absolute
* axis, etc.
*/
void input_event(struct input_dev *dev,
unsigned int type, unsigned int code, int value)
参数意义如下：
- dev：需要上报的输入设备
- type：上报的事件类型
- code：事件码
- value：事件值
2.2. 针对特定事件的上报函数
（1）上报按键
static inline void input_report_key(struct input_dev *dev, unsigned int code, int value)
{
input_event(dev, EV_KEY, code, !!value);
}
（2）上报相对坐标
static inline void input_report_rel(struct input_dev *dev, unsigned int code, int value)
{
input_event(dev, EV_REL, code, value);
}
（3）上报绝对坐标
static inline void input_report_abs(struct input_dev *dev, unsigned int code, int value)
{
input_event(dev, EV_ABS, code, value);
}
2.3. 告知内核上报结束
static inline void input_sync(struct input_dev *dev)
{
input_event(dev, EV_SYN, SYN_REPORT, 0);
}
三、应用获取事件
Linux内核使用 input_event 结构体来描述所有的输入事件，定义在 include/uapi/linux/input.h 头文件中：
/*
* The event structure itself
*/
struct input_event {
struct timeval time;
__u16 type;
__u16 code;
__s32 value;
};
其中每个成员的意义如下：
- time：事件发生的时间
- type：事件类型
- code：事件码
- value：事件值
所有的输入设备最终都是按照input_event结构体呈现给用户，用户应用程序可以通过 input_event来获取到具体的输入事件或相关的值，比如按键值等。