/**
 * 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)
{
  struct input_dev *dev;
     /*分配一個input_dev結構體，并初始化為0*/ 
  dev = kzalloc(sizeof(struct input_dev), GFP_KERNEL);
  if (dev) {
    dev->dev.type = &input_dev_type;/*初始化設備的類型*/ 
    dev->dev.class = &input_class; /*設置為輸入設備類*/ 
    device_initialize(&dev->dev);/*初始化device結構*/ 
    mutex_init(&dev->mutex); /*初始化互斥鎖*/ 
    spin_lock_init(&dev->event_lock); /*初始化事件自旋鎖*/ 
    INIT_LIST_HEAD(&dev->h_list);/*初始化鏈表*/ 
    INIT_LIST_HEAD(&dev->node); /*初始化鏈表*/ 

    __module_get(THIS_MODULE);/*模塊引用技術加1*/ 
  }

  return dev;
}

/**
 * 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.
 */
int input_register_device(struct input_dev *dev)
{
    //定義一些函數中將用到的局部變量
  static atomic_t input_no = ATOMIC_INIT(0);
  struct input_handler *handler;
  const char *path;
  int error;
  //設置 input_dev 所支持的事件類型，由 evbit 成員來表示。具體類型在后面歸納。
  /* Every input device generates EV_SYN/SYN_REPORT events. */
  __set_bit(EV_SYN, dev->evbit);

  /* KEY_RESERVED is not supposed to be transmitted to userspace. */
  __clear_bit(KEY_RESERVED, dev->keybit);

  /* Make sure that bitmasks not mentioned in dev->evbit are clean. */
  input_cleanse_bitmasks(dev);

   //初始化 timer 定時器，用來處理重復點擊按鍵。(去抖)
  /*
   * If delay and period are pre-set by the driver, then autorepeating
   * is handled by the driver itself and we don't do it in input.c.
   */
  init_timer(&dev->timer);
    //如果 rep[REP_DELAY] 和 [REP_PERIOD] 沒有設值，則賦默認值。為了去抖。
  if (!dev->rep[REP_DELAY] && !dev->rep[REP_PERIOD]) {
    dev->timer.data = (long) dev;
    dev->timer.function = input_repeat_key;
    dev->rep[REP_DELAY] = 250;
    dev->rep[REP_PERIOD] = 33;
  }
   //檢查下列兩個函數是否被定義，沒有被定義則賦默認值。
  if (!dev->getkeycode)
    dev->getkeycode = input_default_getkeycode;//得到指定位置鍵值

  if (!dev->setkeycode)
    dev->setkeycode = input_default_setkeycode;//設置指定位置鍵值
    //設置 input_dev 中 device 的名字為 inputN
    //將如 input0 input1 input2 出現在 sysfs 文件系統中
  dev_set_name(&dev->dev, "input%ld",
       (unsigned long) atomic_inc_return(&input_no) - 1);
    //將 input->dev 包含的 device 結構注冊到 Linux 設備模型中。
  error = device_add(&dev->dev);
  if (error)
    return error;
  //打印設備的路徑并輸出調試信息
  path = kobject_get_path(&dev->dev.kobj, GFP_KERNEL);
  printk(KERN_INFO "input: %s as %s\n",
    dev->name ? dev->name : "Unspecified device", path ? path : "N/A");
  kfree(path);

  error = mutex_lock_interruptible(&input_mutex);
  if (error) {
    device_del(&dev->dev);
    return error;
  }
    //將 input_dev 加入 input_dev_list 鏈表中（這個鏈表中包含有所有 input 設備）
  list_add_tail(&dev->node, &input_dev_list);

  list_for_each_entry(handler, &input_handler_list, node)
   //調用 input_attatch_handler()函數匹配 handler 和 input_dev。
    //這個函數很重要，在后面單獨分析。
    input_attach_handler(dev, handler);

  input_wakeup_procfs_readers();

  mutex_unlock(&input_mutex);

  return 0;
}

/*
 * The event structure itself
 */

struct input_event {
  struct timeval time; //<輸入事件發生的時間
  __u16 type;     //<輸入事件的類型
  __u16 code;     //<在輸入事件類型下的編碼
  __s32 value;     //<code的值
};

/*
 * Event types
 */

#define EV_SYN      0x00 //< 同步事件
#define EV_KEY      0x01 //< 按鍵事件
#define EV_REL      0x02 //<相對坐標(如：鼠標移動，報告相對最后一次位置的偏移) 
#define EV_ABS      0x03 //< 絕對坐標(如：觸摸屏或操作桿，報告絕對的坐標位置) 
#define EV_MSC      0x04 //< 其它 
#define EV_SW       0x05 //<開關 
#define EV_LED      0x11 //<按鍵/設備燈 
#define EV_SND      0x12 //<聲音/警報 
#define EV_REP      0x14 //<重復 
#define EV_FF       0x15 //<力反饋 
#define EV_PWR      0x16 //<電源 
#define EV_FF_STATUS   0x17 //<力反饋狀態 
#define EV_MAX      0x1f //< 事件類型最大個數和提供位掩碼支持 
#define EV_CNT      (EV_MAX+1)

/* 報告指定type、code的輸入事件 */
void input_event(struct input_dev *dev, unsigned int type, unsigned int code, int value);
/* 報告鍵值 */
static inline void input_report_key(struct input_dev *dev, unsigned int code, int value)
{
  input_event(dev, EV_KEY, code, !!value);
}
/* 報告相對坐標 */
static inline void input_report_rel(struct input_dev *dev, unsigned int code, int value)
{
  input_event(dev, EV_REL, code, value);
}
/* 報告絕對坐標 */
static inline void input_report_abs(struct input_dev *dev, unsigned int code, int value)
{
  input_event(dev, EV_ABS, code, value);
}
...

input_reprot_abs(input_dev,ABS_X,x); //x坐標
input_reprot_abs(input_dev,ABS_Y,y); // y坐標
input_reprot_abs(input_dev,ABS_PRESSURE,1);
input_sync(input_dev);//同步結束

//按鍵初始化
static int __init button_init(void)
{//申請中斷
  if(request_irq(BUTTON_IRQ,button_interrupt,0,”button”,NUll))
    return –EBUSY;
  set_bit(EV_KEY,button_dev.evbit); //支持EV_KEY事件
  set_bit(BTN_0,button_dev.keybit); //支持設備兩個鍵
  set_bit(BTN_1,button_dev.keybit); //
  input_register_device(&button_dev);//注冊input設備
}

/*在按鍵中斷中報告事件*/
Static void button_interrupt(int irq,void *dummy,struct pt_regs *fp)
{
  input_report_key(&button_dev,BTN_0,inb(BUTTON_PORT0));//讀取寄存器BUTTON_PORT0的值
  input_report_key(&button_dev,BTN_1,inb(BUTTON_PORT1));
  input_sync(&button_dev);
}

struct input_handle;

/**
 * struct input_handler - implements one of interfaces for input devices
 * @private: driver-specific data
 * @event: event handler. This method is being called by input core with
 *  interrupts disabled and dev->event_lock spinlock held and so
 *  it may not sleep
 * @filter: similar to @event; separates normal event handlers from
 *  "filters".
 * @match: called after comparing device's id with handler's id_table
 *  to perform fine-grained matching between device and handler
 * @connect: called when attaching a handler to an input device
 * @disconnect: disconnects a handler from input device
 * @start: starts handler for given handle. This function is called by
 *  input core right after connect() method and also when a process
 *  that "grabbed" a device releases it
 * @fops: file operations this driver implements
 * @minor: beginning of range of 32 minors for devices this driver
 *  can provide
 * @name: name of the handler, to be shown in /proc/bus/input/handlers
 * @id_table: pointer to a table of input_device_ids this driver can
 *  handle
 * @h_list: list of input handles associated with the handler
 * @node: for placing the driver onto input_handler_list
 *
 * Input handlers attach to input devices and create input handles. There
 * are likely several handlers attached to any given input device at the
 * same time. All of them will get their copy of input event generated by
 * the device.
 *
 * The very same structure is used to implement input filters. Input core
 * allows filters to run first and will not pass event to regular handlers
 * if any of the filters indicate that the event should be filtered (by
 * returning %true from their filter() method).
 *
 * Note that input core serializes calls to connect() and disconnect()
 * methods.
 */
struct input_handler {

  void *private;

  void (*event)(struct input_handle *handle, unsigned int type, unsigned int code, int value);
  bool (*filter)(struct input_handle *handle, unsigned int type, unsigned int code, int value);
  bool (*match)(struct input_handler *handler, struct input_dev *dev);
  int (*connect)(struct input_handler *handler, struct input_dev *dev, const struct input_device_id *id);
  void (*disconnect)(struct input_handle *handle);
  void (*start)(struct input_handle *handle);

  const struct file_operations *fops;
  int minor;
  const char *name;

  const struct input_device_id *id_table;

  struct list_head  h_list;
  struct list_head  node;
};

static struct input_handler evdev_handler = {
        .event = evdev_event, //<向系統報告input事件，系統通過read方法讀取
        .connect = evdev_connect, //<和input_dev匹配后調用connect構建
        .disconnect = evdev_disconnect,
        .fops = &evdev_fops, //<event設備文件的操作方法
        .minor = EVDEV_MINOR_BASE, //<次設備號基準值
        .name = "evdev",
        .id_table = evdev_ids, //<匹配規則
    };

Programming input drivers
~~~~~~~~~~~~~~~~~~~~~~~~~

1. Creating an input device driver
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1.0 The simplest example
~~~~~~~~~~~~~~~~~~~~~~~~

Here comes a very simple example of an input device driver. The device has
just one button and the button is accessible at i/o port BUTTON_PORT. When
pressed or released a BUTTON_IRQ happens. The driver could look like:

#include <linux/input.h>
#include <linux/module.h>
#include <linux/init.h>

#include <asm/irq.h>
#include <asm/io.h>

static struct input_dev *button_dev;

static irqreturn_t button_interrupt(int irq, void *dummy)
{
  input_report_key(button_dev, BTN_0, inb(BUTTON_PORT) & 1);
  input_sync(button_dev);
  return IRQ_HANDLED;
}

static int __init button_init(void)
{
  int error;

  if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
        printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
        return -EBUSY;
    }

  button_dev = input_allocate_device();
  if (!button_dev) {
    printk(KERN_ERR "button.c: Not enough memory\n");
    error = -ENOMEM;
    goto err_free_irq;
  }

  button_dev->evbit[0] = BIT_MASK(EV_KEY);
  button_dev->keybit[BIT_WORD(BTN_0)] = BIT_MASK(BTN_0);

  error = input_register_device(button_dev);
  if (error) {
    printk(KERN_ERR "button.c: Failed to register device\n");
    goto err_free_dev;
  }

  return 0;

 err_free_dev:
  input_free_device(button_dev);
 err_free_irq:
  free_irq(BUTTON_IRQ, button_interrupt);
  return error;
}

static void __exit button_exit(void)
{
    input_unregister_device(button_dev);
  free_irq(BUTTON_IRQ, button_interrupt);
}

module_init(button_init);
module_exit(button_exit);

1.1 What the example does
~~~~~~~~~~~~~~~~~~~~~~~~~

First it has to include the <linux/input.h> file, which interfaces to the
input subsystem. This provides all the definitions needed.

In the _init function, which is called either upon module load or when
booting the kernel, it grabs the required resources (it should also check
for the presence of the device).

Then it allocates a new input device structure with input_allocate_device()
and sets up input bitfields. This way the device driver tells the other
parts of the input systems what it is - what events can be generated or
accepted by this input device. Our example device can only generate EV_KEY
type events, and from those only BTN_0 event code. Thus we only set these
two bits. We could have used

  set_bit(EV_KEY, button_dev.evbit);
  set_bit(BTN_0, button_dev.keybit);

as well, but with more than single bits the first approach tends to be
shorter.

Then the example driver registers the input device structure by calling

  input_register_device(&button_dev);

This adds the button_dev structure to linked lists of the input driver and
calls device handler modules _connect functions to tell them a new input
device has appeared. input_register_device() may sleep and therefore must
not be called from an interrupt or with a spinlock held.

While in use, the only used function of the driver is

  button_interrupt()

which upon every interrupt from the button checks its state and reports it
via the

  input_report_key()

call to the input system. There is no need to check whether the interrupt
routine isn't reporting two same value events (press, press for example) to
the input system, because the input_report_* functions check that
themselves.

Then there is the

  input_sync()

call to tell those who receive the events that we've sent a complete report.
This doesn't seem important in the one button case, but is quite important
for for example mouse movement, where you don't want the X and Y values
to be interpreted separately, because that'd result in a different movement.

1.2 dev->open() and dev->close()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In case the driver has to repeatedly poll the device, because it doesn't
have an interrupt coming from it and the polling is too expensive to be done
all the time, or if the device uses a valuable resource (eg. interrupt), it
can use the open and close callback to know when it can stop polling or
release the interrupt and when it must resume polling or grab the interrupt
again. To do that, we would add this to our example driver:

static int button_open(struct input_dev *dev)
{
  if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
        printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
        return -EBUSY;
    }

    return 0;
}

static void button_close(struct input_dev *dev)
{
    free_irq(IRQ_AMIGA_VERTB, button_interrupt);
}

static int __init button_init(void)
{
  ...
  button_dev->open = button_open;
  button_dev->close = button_close;
  ...
}

Note that input core keeps track of number of users for the device and
makes sure that dev->open() is called only when the first user connects
to the device and that dev->close() is called when the very last user
disconnects. Calls to both callbacks are serialized.

The open() callback should return a 0 in case of success or any nonzero value
in case of failure. The close() callback (which is void) must always succeed.

1.3 Basic event types
~~~~~~~~~~~~~~~~~~~~~

The most simple event type is EV_KEY, which is used for keys and buttons.
It's reported to the input system via:

  input_report_key(struct input_dev *dev, int code, int value)

See linux/input.h for the allowable values of code (from 0 to KEY_MAX).
Value is interpreted as a truth value, ie any nonzero value means key
pressed, zero value means key released. The input code generates events only
in case the value is different from before.

In addition to EV_KEY, there are two more basic event types: EV_REL and
EV_ABS. They are used for relative and absolute values supplied by the
device. A relative value may be for example a mouse movement in the X axis.
The mouse reports it as a relative difference from the last position,
because it doesn't have any absolute coordinate system to work in. Absolute
events are namely for joysticks and digitizers - devices that do work in an
absolute coordinate systems.

Having the device report EV_REL buttons is as simple as with EV_KEY, simply
set the corresponding bits and call the

  input_report_rel(struct input_dev *dev, int code, int value)

function. Events are generated only for nonzero value.

However EV_ABS requires a little special care. Before calling
input_register_device, you have to fill additional fields in the input_dev
struct for each absolute axis your device has. If our button device had also
the ABS_X axis:

  button_dev.absmin[ABS_X] = 0;
  button_dev.absmax[ABS_X] = 255;
  button_dev.absfuzz[ABS_X] = 4;
  button_dev.absflat[ABS_X] = 8;

Or, you can just say:

  input_set_abs_params(button_dev, ABS_X, 0, 255, 4, 8);

This setting would be appropriate for a joystick X axis, with the minimum of
0, maximum of 255 (which the joystick *must* be able to reach, no problem if
it sometimes reports more, but it must be able to always reach the min and
max values), with noise in the data up to +- 4, and with a center flat
position of size 8.

If you don't need absfuzz and absflat, you can set them to zero, which mean
that the thing is precise and always returns to exactly the center position
(if it has any).

1.4 BITS_TO_LONGS(), BIT_WORD(), BIT_MASK()
~~~~~~~~~~~~~~~~~~~~~~~~~~

These three macros from bitops.h help some bitfield computations:

  BITS_TO_LONGS(x) - returns the length of a bitfield array in longs for
        x bits
  BIT_WORD(x)   - returns the index in the array in longs for bit x
  BIT_MASK(x)   - returns the index in a long for bit x

1.5 The id* and name fields
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The dev->name should be set before registering the input device by the input
device driver. It's a string like 'Generic button device' containing a
user friendly name of the device.

The id* fields contain the bus ID (PCI, USB, ...), vendor ID and device ID
of the device. The bus IDs are defined in input.h. The vendor and device ids
are defined in pci_ids.h, usb_ids.h and similar include files. These fields
should be set by the input device driver before registering it.

The idtype field can be used for specific information for the input device
driver.

The id and name fields can be passed to userland via the evdev interface.

1.6 The keycode, keycodemax, keycodesize fields
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

These three fields should be used by input devices that have dense keymaps.
The keycode is an array used to map from scancodes to input system keycodes.
The keycode max should contain the size of the array and keycodesize the
size of each entry in it (in bytes).

Userspace can query and alter current scancode to keycode mappings using
EVIOCGKEYCODE and EVIOCSKEYCODE ioctls on corresponding evdev interface.
When a device has all 3 aforementioned fields filled in, the driver may
rely on kernel's default implementation of setting and querying keycode
mappings.

1.7 dev->getkeycode() and dev->setkeycode()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
getkeycode() and setkeycode() callbacks allow drivers to override default
keycode/keycodesize/keycodemax mapping mechanism provided by input core
and implement sparse keycode maps.

1.8 Key autorepeat
~~~~~~~~~~~~~~~~~~

... is simple. It is handled by the input.c module. Hardware autorepeat is
not used, because it's not present in many devices and even where it is
present, it is broken sometimes (at keyboards: Toshiba notebooks). To enable
autorepeat for your device, just set EV_REP in dev->evbit. All will be
handled by the input system.

1.9 Other event types, handling output events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The other event types up to now are:

EV_LED - used for the keyboard LEDs.
EV_SND - used for keyboard beeps.

They are very similar to for example key events, but they go in the other
direction - from the system to the input device driver. If your input device
driver can handle these events, it has to set the respective bits in evbit,
*and* also the callback routine:

  button_dev->event = button_event;

int button_event(struct input_dev *dev, unsigned int type, unsigned int code, int value);
{
  if (type == EV_SND && code == SND_BELL) {
    outb(value, BUTTON_BELL);
    return 0;
  }
  return -1;
}

This callback routine can be called from an interrupt or a BH (although that
isn't a rule), and thus must not sleep, and must not take too long to finish.

input-programming.txt

#include <linux/input.h>
#include <linux/module.h>
#include <linux/init.h>

#include <asm/irq.h>
#include <asm/io.h>

static struct input_dev *button_dev; /*輸入設備結構體*/ 
/*中斷處理函數*/ 
static irqreturn_t button_interrupt(int irq, void *dummy)
{
  /*向輸入子系統報告產生按鍵事件*/ 
  input_report_key(button_dev, BTN_0, inb(BUTTON_PORT) & 1);
  /*通知接收者，一個報告發送完畢*/ 
  input_sync(button_dev);
  return IRQ_HANDLED;
}
/*加載函數*/ 
static int __init button_init(void)
{
  int error;
  /*申請中斷處理函數*/ //返回0表示成功，返回-INVAL表示無效
  if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
        /*申請失敗，則打印出錯信息*/ 
        printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
        return -EBUSY;
    }
  /*分配一個設備結構體*/ 
  //將在 sys/class/input/input-n 下面創建設備屬性文件
  button_dev = input_allocate_device();
  if (!button_dev) {   /*判斷分配是否成功*/ 
    printk(KERN_ERR "button.c: Not enough memory\n");
    error = -ENOMEM;
    goto err_free_irq;
  }

  button_dev->evbit[0] = BIT_MASK(EV_KEY); /*設置按鍵信息*/ 
  button_dev->keybit[BIT_WORD(BTN_0)] = BIT_MASK(BTN_0);

  error = input_register_device(button_dev); /*注冊一個輸入設備*/ 
  if (error) {
    printk(KERN_ERR "button.c: Failed to register device\n");
    goto err_free_dev;
  }

  return 0;
 /*以下是錯誤處理*/ 
 err_free_dev:
  input_free_device(button_dev);
 err_free_irq:
  free_irq(BUTTON_IRQ, button_interrupt);
  return error;
}
 /*卸載函數*/ 
static void __exit button_exit(void)
{
  input_unregister_device(button_dev); /*注銷按鍵設備*/ 
  free_irq(BUTTON_IRQ, button_interrupt);/*釋放按鍵占用的中斷線*/ 
}

module_init(button_init);
module_exit(button_exit);