Chapter 17: Leveraging the Linux Kernel Input Subsystem

Input devices are devices that you can use to interact with the system. Such devices include buttons, keyboards, touchscreens, mice, and more. They work by sending events that are caught and broadcast over the system by the input core. This chapter will explain each structure that's used by the input core to handle input devices, as well as how to manage events from the user space.

In this chapter, we will cover the following topics:

• Introduction to the Linux kernel input subsystem – its data structures and APIs
• Allocating and registering an input device
• Using polled input devices

• Generating and reporting input events
• Handling input devices from the user space

Introduction to the Linux kernel input subsystem – its data structures and APIs

The main data structures and APIs of this subsystem can be found in the include/linux/input.h files. The following line is required in any input device driver:

#include <linux/input.h>

Whatever type of input device it is, whatever type of event it sends, an input device is represented in the kernel as an instance of the struct input_dev:

struct input_dev {
  const char *name;
  const char *phys;
  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 int repeat_key;
  int rep[REP_CNT];
  struct input_absinfo *absinfo;
  unsigned long key[BITS_TO_LONGS(KEY_CNT)];
  int (*open...

Allocating and registering an input device

Before the events that are supported by an input device can be seen by the system, memory needs to be allocated for this device first using the devm_input_allocate_device() API. Then, the device needs to be registered with the system using input_device_register(). The former API will take care of freeing up the memory and unregistering the device when it leaves the system. However, non-managed allocation is still available but not recommended, input_allocate_device(). By using non-managed allocation, the driver becomes responsible for making sure that input_unregister_device() and input_free_device() are called to unregister the device and free its memory when they're on the unloading path of the driver, respectively. The following are the respective prototypes of these APIs:

struct input_dev *input_allocate_device(void)
struct input_dev *devm_input_allocate_device(
           ...

Using polled input devices

Polled input devices are special input devices that rely on polling to sense device state changes; the generic input device type relies on IRQ to sense changes and send events to the input core.

A polled input device is described in the kernel as an instance of struct input_polled_dev structure, which is a wrapper around the generic struct input_dev structure. The following is its declaration:

struct input_polled_dev {
    void *private;
    void (*open)(struct input_polled_dev *dev);
    void (*close)(struct input_polled_dev *dev);
    void (*poll)(struct input_polled_dev *dev);
    unsigned int poll_interval; /* msec */
    unsigned int poll_interval_max; /* msec */
    unsigned int poll_interval_min; /* msec */
 
    struct input_dev *input;
    bool devres_managed;
};

...

Generating and reporting input events

Device allocation and registration are essential, but they are useless if the device is unable to report events to the input core, which is what input devices are designed to do. Depending on the type of event our device can support, the kernel provides the appropriate APIs to report them to the core.

Given an EV_XXX capable device, the corresponding report function would be input_report_xxx(). The following table shows the mappings between the most important event types and their report functions:

Table 17.1 – Mapping the input device's capabilities and the report APIs

The prototypes for these report APIs are as follows:

void input_report_abs(struct input_dev *dev,
                      unsigned int code, int value)
void input_report_key(struct input_dev *dev,
      ...

Handling input devices from the user space

A node will be created in the /dev/input/ directory for each input device (polled or not) that has been successfully registered with the system. In my case, the node corresponds to event0 because it is the first and only input device on my target board. You can use the udevadm tool to display information about the device:

# udevadm info /dev/input/event0 
P: /devices/platform/input-button.0/input/input0/event0
N: input/event0
S: input/by-path/platform-input-button.0-event
E: DEVLINKS=/dev/input/by-path/platform-input-button.0-event
E: DEVNAME=/dev/input/event0
E: DEVPATH=/devices/platform/input-button.0/input/input0/event0
E: ID_INPUT=1
E: ID_PATH=platform-input-button.0
E: ID_PATH_TAG=platform-input-button_0
E: MAJOR=13
E: MINOR=64
E: SUBSYSTEM=input
E: USEC_INITIALIZED=74842430

Another tool that you can use, which allows you to print the keys that are supported by the device, is evetest. It can also catch and print events when they...

Summary

This chapter described the input framework and highlighted the difference between polled and interrupt-driven input devices. At this point, you should have the necessary knowledge to write a driver for any input driver, whatever its type, and whatever input event it supports. The user space interface was also discussed, and an example was provided.

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