1. Introduction to Linux Device Drivers
Class 4: Input Device Drivers with Linux Kernel
Aug 7, 2014 Khem Raj

2. Introduction Driver Collection Supporting input devices Keyboards Mice
Touchscreens
Wheels …

3. Introduction Standardize handling of similar devices
E.g. mice can be USB, bluetooth, PS/2
Standard event system to interact with userspace applications
Input core extract common bits e.g. serio
Name one advantage of kernel’s input subsystem ?

4. Input Subsystem Input device driver Input event driver
Talks to the device
Input event driver
Talks to Applications

5. Input Subsystem XWin Qt GUI apps Input Devices /dev/input/*
Userspace Kernel Space h/w space
XWin Qt
GUI apps
Input Devices
/dev/input/*
Input Core subsystem
Input Device Drivers
Input Event Handers
Serio USB HID HIDP
PS/2 Keyboard
USB Mice
Bluetooth Mice

6. Input Events
Understood by different Graphical systems e.g. X11, QT etc.
H/W Independent abstraction
Evdev
Generic Input Event Driver

7. Input Event Handlers
/** * struct input_value - input value representation type of value (EV_KEY, EV_ABS, etc) the value code the value */ struct input_value { __u16 type; __u16 code; __s32 value; };
What is a Input Event Driver ?

8. Input Device Drivers Serio Used to access keyboard controllers
PS/2 mouse and keyboards, trakcpoints
Serial port controllers
Touch pads
Using serio via callback
serio_register_driver() sends cmds to controllers
To Add new driver
Define open, close, start, stop, etc.
Use serio_register_port ()

9. Application/emulates mouse moves
Example
Console/gpm server
Application/emulates mouse moves
/dev/input/event Dispatch coordinates through sysfs (/sys/device/..)
evdev
Virtual Mouse Driver
Input core

10. Application Opens a sysfs node Generate random relative coordinates
open("/sys/devices/platform/vms/coordinates", O_RDWR)
Generate random relative coordinates
Write it down to device file descriptor
Close the file descriptor

11. Mouse Driver Defines a struct input_dev – Input device
Defines platform_device – Device struct
Initialization
Register a platform device and allocate struct input_dev
platform_device_register_simple and input_allocate_device
Create a sysfs node to read simulated coordinates
sysfs_create_group(..)
Define what data it will generate ( coordinates )
Use set_bit () to set members in input_dev struct e.g. evbit
set_bit(REL_X, …) set_bit(REL_Y, …)
Register with the input subsystem
input_register_device ()
Write Function
Gets the buffer with coordinates
Use input_report_rel () to generate events ( REL_X, REL_Y)
input_sync () denotes completions of events

12. Mouse Driver Attach to sysfs write function
DEVICE_ATTR(<devicename>, 0644, NULL, <driver write function>);
Define attribute group and descriptor
Exit/Cleanup
Unregister from input subsystem
input_unregister_device ()
Clean sysfs node
sysfs_remove_group ()
Unregister Driver
platform_device_unregister ()
How does device driver write function gets registered with sysfs ?

13. Input Subsystem Creates evdev structure from events
Send it on /dev/input/eventX
gpm receives event in input_event format
gpm -m /dev/input/eventX -t evdev

14. Evdev Interface This driver uses generic evdev interface
Some devices define own Event drivers
They populate input_handler struct
Register
input_register_handler(&mousedev_handler);
Which structure is initialized to define event driver ?

15. Output Events Keyboard/mice LEDs and Sound beeps
Set by evbits and callback function
See drivers/input/misc/pcspkr.c
Callback - static int pcspkr_event ()
static int __devinit pcspkr_probe ()
Declared evbits
pcspkr_dev->evbit[0] = BIT(EV_SND);
pcspkr_dev->sndbit[0] = BIT(SND_BELL) | BIT(SND_TONE);
/* The Callback routine */
pcspkr_dev->event = pcspkr_event;

16. Debugging Input Drivers
Use evbug module
Dumps type, code and value (input_event struct)
Output looks like
/* Touchpad Movement */
evbug.c Event. Dev: isa0060/serio1/input0: Type: 3, Code: 28, Value: 0
How do you debug input drivers ?

17. Source Event handlers Device drivers reside close to subsystem code
drivers/input directory
But keyboard driver is in drivers/char
Device drivers reside close to subsystem code
net/bluetooth
Documentation/input describes some of input interfaces
Most of interfaces in include/linux/input.h

18. Thanks
Q & A