Linux Input Subsystem详解：驱动开发与API指南

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Linux Input Subsystem是Linux操作系统中的一个关键组件，它负责管理设备输入事件，如键盘、鼠标、触摸屏、游戏控制器、触摸板等，并提供用户空间和内核空间的API接口供应用程序和驱动程序开发者使用。本文档详细介绍了Input Subsystem在Linux 4.13.0-rc4及以后版本中的设计和功能，涵盖了多个特定驱动协议和设备支持。 1. **Linux Input Subsystem 用户空间API** - 该部分首先介绍了Input Subsystem的整体概念，包括输入事件的处理流程。用户空间API允许应用程序通过标准库函数接收和解析来自硬件设备的输入事件，如按键、触摸和移动等。这对于编写跨平台的应用程序至关重要，确保了对不同设备的支持。 2. **Input事件代码（Event Codes）** - 详细解释了Linux系统中用于标识各种输入事件的标准编码，这有助于理解和处理不同类型的输入数据，如键值、触摸点坐标、压力等级等。 3. **多点触摸（Multi-Touch）与游戏手柄** - 文档探讨了Linux对多点触摸输入的支持，以及如何处理游戏手柄输入，这对于游戏开发者来说尤为重要，能实现对多种游戏控制器的兼容性。 4. **力反馈（Force Feedback）、Linux Gamepad规范和Joystick支持** - Linux提供了力反馈技术，使得设备能够模拟物理触感，同时定义了游戏手柄和传统机械式joystick的驱动接口，方便开发者集成这些特性。 5. **uinput模块** - uinput是一个核心模块，允许用户空间创建虚拟输入设备，对于需要模拟或调试输入设备的应用场景非常有用。 6. **用户IO协议** - 描述了操作系统如何与用户空间进行交互，以便于传递和处理输入事件。 2. **Linux Input Subsystem 内核API** - 这一部分深入到内核层面，讲解了驱动开发者如何创建和编程输入设备驱动，涉及游戏端口驱动、键盘通知机制等内容，强调了驱动程序与系统之间的接口设计。 3. **驱动特定文档** - 提供了针对各种具体硬件设备的驱动文档，如ALPS触摸板协议、Amiga joystick扩展、Apple TachoPad驱动等，帮助开发者了解和定制针对特定设备的驱动。 Linux Input Subsystem文档是Linux开发者的宝贵资源，它不仅涵盖了基础的输入事件处理和用户空间编程接口，还提供了针对各种硬件设备的驱动开发指南。无论你是应用程序开发者还是驱动程序开发者，理解并利用好这个子系统，都能提升你的Linux程序和硬件设备的兼容性和性能。

The Linux input driver subsystem, Release 4.13.0-rc4+

INPUT_PROP_ACCELEROMETER

Directional axes on this device (absolute and/or relative x, y, z) represent accelerometer data. Some

devices also report gyroscope data, which devices can report through the rotational axes (absolute and/or

relative rx, ry, rz).

All other axes retain their meaning. A device must not mix regular directional axes and accelerometer

axes on the same event node.

1.2.4 Guidelines

The guidelines below ensure proper single-touch and multi-ﬁnger functionality. For multi-touch function-

ality, see the multi-touch-protocol.txt document for more information.

Mice

REL_{X,Y} must be reported when the mouse moves. BTN_LEFT must be used to report the primary button

press. BTN_{MIDDLE,RIGHT,4,5,etc.} should be used to report further buttons of the device. REL_WHEEL

and REL_HWHEEL should be used to report scroll wheel events where available.

Touchscreens

ABS_{X,Y} must be reported with the location of the touch. BTN_TOUCH must be used to report when a

touch is active on the screen. BTN_{MOUSE,LEFT,MIDDLE,RIGHT} must not be reported as the result of

touch contact. BTN_TOOL_<name> events should be reported where possible.

For new hardware, INPUT_PROP_DIRECT should be set.

Trackpads

Legacy trackpads that only provide relative position information must report events like mice described

above.

Trackpads that provide absolute touch position must report ABS_{X,Y} for the location of the touch.

BTN_TOUCH should be used to report when a touch is active on the trackpad. Where multi-ﬁnger support

is available, BTN_TOOL_<name> should be used to report the number of touches active on the trackpad.

For new hardware, INPUT_PROP_POINTER should be set.

Tablets

BTN_TOOL_<name> events must be reported when a stylus or other tool is active on the tablet. ABS_{X,Y}

must be reported with the location of the tool. BTN_TOUCH should be used to report when the tool is

in contact with the tablet. BTN_{STYLUS,STYLUS2} should be used to report buttons on the tool itself.

Any button may be used for buttons on the tablet except BTN_{MOUSE,LEFT}. BTN_{0,1,2,etc} are good

generic codes for unlabeled buttons. Do not use meaningful buttons, like BTN_FORWARD, unless the

button is labeled for that purpose on the device.

For new hardware, both INPUT_PROP_DIRECT and INPUT_PROP_POINTER should be set.

1.3 Multi-touch (MT) Protocol

12 Chapter 1. Linux Input Subsystem userspace API

The Linux input driver subsystem, Release 4.13.0-rc4+

1.3.1 Introduction

In order to utilize the full power of the new multi-touch and multi-user devices, a way to report detailed data

from multiple contacts, i.e., objects in direct contact with the device surface, is needed. This document

describes the multi-touch (MT) protocol which allows kernel drivers to report details for an arbitrary number

of contacts.

The protocol is divided into two types, depending on the capabilities of the hardware. For devices handling

anonymous contacts (type A), the protocol describes how to send the raw data for all contacts to the

receiver. For devices capable of tracking identiﬁable contacts (type B), the protocol describes how to

send updates for individual contacts via event slots.

Note:

MT potocol type A is obsolete, all kernel drivers have been converted to use type B.

1.3.2 Protocol Usage

Contact details are sent sequentially as separate packets of ABS_MT events. Only the ABS_MT events

are recognized as part of a contact packet. Since these events are ignored by current single-touch (ST)

applications, the MT protocol can be implemented on top of the ST protocol in an existing driver.

Drivers for type A devices separate contact packets by calling input_mt_sync() at the end of each packet.

This generates a SYN_MT_REPORT event, which instructs the receiver to accept the data for the current

contact and prepare to receive another.

Drivers for type B devices separate contact packets by calling input_mt_slot(), with a slot as argument,

at the beginning of each packet. This generates an ABS_MT_SLOT event, which instructs the receiver to

prepare for updates of the given slot.

All drivers mark the end of a multi-touch transfer by calling the usual input_sync() function. This instructs

the receiver to act upon events accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new

set of events/packets.

The main diﬀerence between the stateless type A protocol and the stateful type B slot protocol lies in the

usage of identiﬁable contacts to reduce the amount of data sent to userspace. The slot protocol requires

the use of the ABS_MT_TRACKING_ID, either provided by the hardware or computed from the raw data

For type A devices, the kernel driver should generate an arbitrary enumeration of the full set of anonymous

contacts currently on the surface. The order in which the packets appear in the event stream is not

important. Event ﬁltering and ﬁnger tracking is left to user space

For type B devices, the kernel driver should associate a slot with each identiﬁed contact, and use that slot

to propagate changes for the contact. Creation, replacement and destruction of contacts is achieved by

modifying the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id is interpreted as a

contact, and the value -1 denotes an unused slot. A tracking id not previously present is considered new,

and a tracking id no longer present is considered removed. Since only changes are propagated, the full

state of each initiated contact has to reside in the receiving end. Upon receiving an MT event, one simply

updates the appropriate attribute of the current slot.

Some devices identify and/or track more contacts than they can report to the driver. A driver for such

a device should associate one type B slot with each contact that is reported by the hardware. When-

ever the identity of the contact associated with a slot changes, the driver should invalidate that slot

by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is tracking more contacts than it

is currently reporting, the driver should use a BTN_TOOL_*TAP event to inform userspace of the total

See the section on ﬁnger tracking.

The mtdev project: http://bitmath.org/code/mtdev/.

1.3. Multi-touch (MT) Protocol 13

The Linux input driver subsystem, Release 4.13.0-rc4+

Here is the sequence after lifting the contact in slot 0:

ABS_MT_TRACKING_ID -1

SYN_REPORT

The slot being modiﬁed is already 0, so the ABS_MT_SLOT is omitted. The message removes the associ-

ation of slot 0 with contact 45, thereby destroying contact 45 and freeing slot 0 to be reused for another

contact.

Finally, here is the sequence after lifting the second contact:

ABS_MT_SLOT 1

ABS_MT_TRACKING_ID -1

SYN_REPORT

1.3.5 Event Usage

A set of ABS_MT events with the desired properties is deﬁned. The events are divided into cate-

gories, to allow for partial implementation. The minimum set consists of ABS_MT_POSITION_X and

ABS_MT_POSITION_Y, which allows for multiple contacts to be tracked. If the device supports it, the

ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size of the contact area

and approaching tool, respectively.

The TOUCH and WIDTH parameters have a geometrical interpretation; imagine looking through a window

at someone gently holding a ﬁnger against the glass. You will see two regions, one inner region consist-

ing of the part of the ﬁnger actually touching the glass, and one outer region formed by the perimeter

of the ﬁnger. The center of the touching region (a) is ABS_MT_POSITION_X/Y and the center of the ap-

proaching ﬁnger (b) is ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the ﬁnger

diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the ﬁnger harder against the glass.

The touch region will increase, and in general, the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR,

which is always smaller than unity, is related to the contact pressure. For pressure-based devices,

ABS_MT_PRESSURE may be used to provide the pressure on the contact area instead. Devices capa-

ble of contact hovering can use ABS_MT_DISTANCE to indicate the distance between the contact and the

surface.

Linux MT Win8

__________ _______________________

/ \ | |

/ ____ \ | |

/ / \ \ | |

\ \ a \ \ | a |

\ \____/ \ | |

\ \ | |

\ b \ | b |

\ \ | |

\ / | |

\__________/ |_______________________|

In addition to the MAJOR parameters, the oval shape of the touch and ﬁnger regions can be described by

adding the MINOR parameters, such that MAJOR and MINOR are the major and minor axis of an ellipse.

The orientation of the touch ellipse can be described with the ORIENTATION parameter, and the direction

of the ﬁnger ellipse is given by the vector (a - b).

For type A devices, further speciﬁcation of the touch shape is possible via ABS_MT_BLOB_ID.

1.3. Multi-touch (MT) Protocol 15

The Linux input driver subsystem, Release 4.13.0-rc4+

The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a ﬁnger or a pen or something

else. Finally, the ABS_MT_TRACKING_ID event may be used to track identiﬁed contacts over time

In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are implicitly handled by input core;

drivers should instead call input_mt_report_slot_state().

1.3.6 Event Semantics

ABS_MT_TOUCH_MAJOR The length of the major axis of the contact. The length should be given

in surface units. If the surface has an X times Y resolution, the largest possible value of

ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal

ABS_MT_TOUCH_MINOR The length, in surface units, of the minor axis of the contact. If the contact is

circular, this event can be omitted

ABS_MT_WIDTH_MAJOR The length, in surface units, of the major axis of the approaching tool. This

should be understood as the size of the tool itself. The orientation of the contact and the approaching

tool are assumed to be the same

ABS_MT_WIDTH_MINOR The length, in surface units, of the minor axis of the approaching tool. Omit if

circular

The above four values can be used to derive additional information about the contact. The ratio

ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates the notion of pressure. The ﬁngers

of the hand and the palm all have diﬀerent characteristic widths.

ABS_MT_PRESSURE The pressure, in arbitrary units, on the contact area. May be used instead of TOUCH

and WIDTH for pressure-based devices or any device with a spatial signal intensity distribution.

ABS_MT_DISTANCE The distance, in surface units, between the contact and the surface. Zero distance

means the contact is touching the surface. A positive number means the contact is hovering above

the surface.

ABS_MT_ORIENTATION The orientation of the touching ellipse. The value should describe a signed

quarter of a revolution clockwise around the touch center. The signed value range is arbitrary, but

zero should be returned for an ellipse aligned with the Y axis of the surface, a negative value when

the ellipse is turned to the left, and a positive value when the ellipse is turned to the right. When

completely aligned with the X axis, the range max should be returned.

Touch ellipsis are symmetrical by default. For devices capable of true 360 degree orientation, the

reported orientation must exceed the range max to indicate more than a quarter of a revolution. For

an upside-down ﬁnger, range max * 2 should be returned.

Orientation can be omitted if the touch area is circular, or if the information is not available in the

kernel driver. Partial orientation support is possible if the device can distinguish between the two

axis, but not (uniquely) any values in between. In such cases, the range of ABS_MT_ORIENTATION

should be [0, 1]

ABS_MT_POSITION_X The surface X coordinate of the center of the touching ellipse.

ABS_MT_POSITION_Y The surface Y coordinate of the center of the touching ellipse.

ABS_MT_TOOL_X The surface X coordinate of the center of the approaching tool. Omit if the device

cannot distinguish between the intended touch point and the tool itself.

ABS_MT_TOOL_Y The surface Y coordinate of the center of the approaching tool. Omit if the device

cannot distinguish between the intended touch point and the tool itself.

The four position values can be used to separate the position of the touch from the position of the

tool. If both positions are present, the major tool axis points towards the touch point

. Otherwise,

the tool axes are aligned with the touch axes.

See the section on event computation.

Also, the diﬀerence (TOOL_X - POSITION_X) can be used to model tilt.

16 Chapter 1. Linux Input Subsystem userspace API

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