Analysis of the Linux input subsystem framework principle

/**
 * input_allocate_device - allocates 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;
     /*Allocate an input_dev structure and initialize it to 0*/ 
  dev = kzalloc(sizeof(struct input_dev), GFP_KERNEL);
  if (dev) {
    dev->dev.type = &input_dev_type;/*Initialize the device type*/ 
    dev->dev.class = &input_class; /*Set to input device class*/ 
    device_initialize(&dev->dev);/*Initialize device structure*/ 
    mutex_init(&dev->mutex); /*Initialize mutex*/ 
    spin_lock_init(&dev->event_lock); /*Initialize event spin lock*/ 
    INIT_LIST_HEAD(&dev->h_list);/*Initialize the linked list*/ 
    INIT_LIST_HEAD(&dev->node); /*Initialize the linked list*/ 

    __module_get(THIS_MODULE);/*Module reference technology plus 1*/ 
  }

  return dev;
}

/**
 * input_register_device - register device with input core
 * @dev: device to be registered
 *
 * This function registers the 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 the 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)
{
    //Define some local variables that will be used in some functions static atomic_t input_no = ATOMIC_INIT(0);
  struct input_handler *handler;
  const char *path;
  int error;
  //Set the event type supported by input_dev, represented by the evbit member. The specific types are summarized later.
  /* 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);

   // Initialize the timer to handle repeated button clicks. (Debounce)
  /*
   * 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);
    //If rep[REP_DELAY] and [REP_PERIOD] are not set, assign default values. To debounce.
  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;
  }
   // Check whether the following two functions are defined. If not, assign default values.
  if (!dev->getkeycode)
    dev->getkeycode = input_default_getkeycode; //Get the key value at the specified position if (!dev->setkeycode)
    dev->setkeycode = input_default_setkeycode; //Set the key value of the specified position //Set the name of the device in input_dev to inputN
    //Will input0 input1 input2 appear in the sysfs file system dev_set_name(&dev->dev, "input%ld",
       (unsigned long) atomic_inc_return(&input_no) - 1);
    //Register the device structure contained in input->dev into the Linux device model.
  error = device_add(&dev->dev);
  if (error)
    return error;
  //Print the device path and output debugging information 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;
  }
    //Add input_dev to the input_dev_list linked list (this linked list contains all input devices)
  list_add_tail(&dev->node, &input_dev_list);

  list_for_each_entry(handler, &input_handler_list, node)
   //Call input_attatch_handler() function to match handler and input_dev.
    //This function is very important and will be analyzed separately later.
    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; //<The time when the input event occurs__u16 type; //<The type of the input event__u16 code; //<The code under the input event type__s32 value; //<The value of code};

/*
 * Event types
 */

#define EV_SYN 0x00 //< Synchronous event #define EV_KEY 0x01 //< Key event #define EV_REL 0x02 //< Relative coordinates (e.g., mouse movement, report the offset relative to the last position) 
#define EV_ABS 0x03 //< Absolute coordinates (such as touch screen or joystick, reporting absolute coordinate positions) 
#define EV_MSC 0x04 //< Others#define EV_SW 0x05 //< Switch#define EV_LED 0x11 //< Button/Device Light#define EV_SND 0x12 //< Sound/Alarm#define EV_REP 0x14 //< Repeat#define EV_FF 0x15 //< Force Feedback#define EV_PWR 0x16 //< Power#define EV_FF_STATUS 0x17 //< Force Feedback Status#define EV_MAX 0x1f //< Maximum number of event types and bit mask support#define EV_CNT (EV_MAX+1)

/* Report input events of specified type and code*/
void input_event(struct input_dev *dev, unsigned int type, unsigned int code, int value);
/* Report key value */
static inline void input_report_key(struct input_dev *dev, unsigned int code, int value)
{
  input_event(dev, EV_KEY, code, !!value);
}
/* Report relative coordinates */
static inline void input_report_rel(struct input_dev *dev, unsigned int code, int value)
{
  input_event(dev, EV_REL, code, value);
}
/* Report absolute coordinates */
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 coordinateinput_reprot_abs(input_dev,ABS_Y,y); //y coordinateinput_reprot_abs(input_dev,ABS_PRESSURE,1);
input_sync(input_dev); //Synchronization ends

//Button initialization static int __init button_init(void)
{//Request interrupt if(request_irq(BUTTON_IRQ,button_interrupt,0,"button",NUll))
    return –EBUSY;
  set_bit(EV_KEY,button_dev.evbit); //Support EV_KEY event set_bit(BTN_0,button_dev.keybit); //Support two keys of the device set_bit(BTN_1,button_dev.keybit); //
  input_register_device(&button_dev); //Register input device}

/*Report event in key interrupt*/
Static void button_interrupt(int irq,void *dummy,struct pt_regs *fp)
{
  input_report_key(&button_dev,BTN_0,inb(BUTTON_PORT0));//Read the value of register BUTTON_PORT0 input_report_key(&button_dev,BTN_1,inb(BUTTON_PORT1));
  input_sync(&button_dev);
}

struct input_handle;

/**
 * struct input_handler - implements one of the 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, //<Report input events to the system, which is read by the read method.connect = evdev_connect, //<Call connect to build after matching with input_dev.disconnect = evdev_disconnect,
        .fops = &evdev_fops, //<Event device file operation method.minor = EVDEV_MINOR_BASE, //<Minor device number base value.name = "evdev",
        .id_table = evdev_ids, //<matching rule};

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 it is quite important
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 means
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
The 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; /*input device structure*/ 
/*Interrupt handling function*/ 
static irqreturn_t button_interrupt(int irq, void *dummy)
{
  /*Report key events to the input subsystem*/ 
  input_report_key(button_dev, BTN_0, inb(BUTTON_PORT) & 1);
  /*Notify the receiver that a report has been sent*/ 
  input_sync(button_dev);
  return IRQ_HANDLED;
}
/*Load function*/ 
static int __init button_init(void)
{
  int error;
  /*Request interrupt processing function*/ //Return 0 for success, return -INVAL for invalid if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
        /*If the application fails, print the error message*/ 
        printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
        return -EBUSY;
    }
  /* Allocate a device structure */ 
  //Will create device attribute file under sys/class/input/input-n button_dev = input_allocate_device();
  if (!button_dev) { /*Judge whether the allocation is successful*/ 
    printk(KERN_ERR "button.c: Not enough memory\n");
    error = -ENOMEM;
    goto err_free_irq;
  }

  button_dev->evbit[0] = BIT_MASK(EV_KEY); /*Set key information*/ 
  button_dev->keybit[BIT_WORD(BTN_0)] = BIT_MASK(BTN_0);

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

  return 0;
 /*The following is error handling*/ 
 err_free_dev:
  input_free_device(button_dev);
 err_free_irq:
  free_irq(BUTTON_IRQ, button_interrupt);
  return error;
}
 /*Uninstall function*/ 
static void __exit button_exit(void)
{
  input_unregister_device(button_dev); /*Unregister button device*/ 
  free_irq(BUTTON_IRQ, button_interrupt);/*Release the interrupt line occupied by the button*/ 
}

module_init(button_init);
module_exit(button_exit);