Message-ID: <20110325004240.GA13453@barra.bne.redhat.com>
Date:	Fri, 25 Mar 2011 10:42:40 +1000
From:	Peter Hutterer <peter.hutterer@...-t.net>
To:	Chase Douglas <chase.douglas@...onical.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Henrik Rydberg <rydberg@...omail.se>,
	Chris Bagwell <chris@...bagwell.com>,
	Nikolai Kondrashov <spbnick@...il.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] Documentation: Add evdev type and code definitions

On Thu, Mar 24, 2011 at 01:44:12PM -0400, Chase Douglas wrote:
> This commit adds the file Documentation/input/evdev-codes.txt.
> 
> Cc: Henrik Rydberg <rydberg@...omail.se>
> Cc: Chris Bagwell <chris@...bagwell.com>
> Cc: Peter Hutterer <peter.hutterer@...-t.net>
> Cc: Nikolai Kondrashov <spbnick@...il.com>
> Cc: linux-input@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Acked-by: Henrik Rydberg <rydberg@...omail.se>
> Signed-off-by: Chase Douglas <chase.douglas@...onical.com>
> ---
> This revision has been a long time coming. Sorry for the delay! Changes from the
> previous revision:
> 
> * Use EVIOCG* to retrieve evdev state and note sysfs capabilities info
> * Clarify wording of BTN_TOOL_<name> and BTN_TOUCH usage for multi-finger modes
> * Remove KEY_POWER, KEY_SUSPEND documentation
> * State that EV_PWR is not well defined and will be addressed later
> * Add text about reporting switch state when binding or resuming
> * Add a "Guidelines" section describing basic mouse, touchscreen, and trackpad
>   protocol usage
> 
> This is perhaps more important than ever before. We're seeing a lot of new
> drivers that were obviously only written to support Android and do not conform
> to the evdev protocol that X.org's input modules need and expect. I've had
> multiple people come to me asking why their android derived touchscreen driver
> isn't working properly in Ubuntu, only to find that the driver is only reporting
> ABS_MT_POSITION_{X,Y} and ABS_MT_TOUCH_MAJOR. Hopefully this will help others
> understand what is required for a full window management system based on
> historical X.org usage.
> 
>  Documentation/input/evdev-codes.txt |  238 +++++++++++++++++++++++++++++++++++
>  1 files changed, 238 insertions(+), 0 deletions(-)
>  create mode 100644 Documentation/input/evdev-codes.txt
> 
> diff --git a/Documentation/input/evdev-codes.txt b/Documentation/input/evdev-codes.txt
> new file mode 100644
> index 0000000..473ce9d
> --- /dev/null
> +++ b/Documentation/input/evdev-codes.txt
> @@ -0,0 +1,238 @@
> +The evdev protocol uses a map of types and codes to express input device values
> +to userspace. This document describes the types and codes and how and when they
> +may be used.

the term "event" is overloaded. You use EV_SYN to "separate events" but
EV_REL to "describe relative input events". some explanation before you
start with the Types would be useful. e.g.

    A single hardware event may result in multiple struct input_event. Each
    such event contains the new value of a single data item. EV_SYN is used
    to separate between hardware events. In the following, the term "event"
    refers to a single struct input_event and the term "hardware event" to
    an a series of struct input_events that reflect the hardware state
    change.

and then use some other word instead of events for the remainder.

also missing something like this:
"A (struct input_event) event is a triplet of type, code and value. The type
classifies the logical category of the event. The code specifies the data
item affected.  The value is the new value for this data item."

> +
> +The evdev protocol is a stateful protocol. Events are emitted only when values
> +of evdev codes have changed. However, the state is maintained within the Linux
> +input subsystem; drivers do not need to maintain the state and may attempt to
> +emit unchanged values without harm. Userspace may obtain the current state of
> +evdev code values using the EVIOCG* ioctls defined in linux/input.h. The evdev
> +reports supported by a device are also provided by sysfs in
> +class/input/event*/device/capabilities/, and the properties of a device are
> +provided in class/input/event*/device/properties.
> +
> +Types:
> +==========
> +Types are groupings of codes under a logical input construct. Each type has a
> +set of applicable codes to be used in generating events. See the Codes section
> +for details on valid codes for each type.
> +
> +* EV_SYN:
> +  - Used as markers to separate events. Events may be separated in time or in
> +    space, such as with the multitouch protocol.
> +
> +* EV_KEY:
> +  - Used to describe keyboard and other key-like input events.

"Used to describe state changes of keyboards, buttons or other key-like
devices"

> +
> +* EV_REL:
> +  - Used to describe relative input events, e.g. moving the mouse 5 units to the
> +    left.
> +
> +* EV_ABS:
> +  - Used to describe absolute input events, e.g. describing the coordinates of a
> +    touch on a touchscreen.

I'd use "relative axis value changes" to avoid the "event" term overloading.

> +
> +* EV_MSC:
> +  - Used to describe miscellaneous input events that do not fit into other
> +    types.

s/input events/input data/

> +
> +* EV_SW:
> +  - Used to describe binary state input switches.
> +
> +* EV_LED:
> +  - Used to turn LEDs on devices on and off.
> +
> +* EV_SND:
> +  - Used to output sound to devices.
> +
> +* EV_REP:
> +  - Used for autorepeating devices.
> +
> +* EV_FF:
> +  - Used to send force feedback commands to an input device.
> +
> +* EV_PWR:
> +  - A special type for power button and switch input.
> +
> +* EV_FF_STATUS:
> +  - Used to receive force feedback device status.
> +
> +Codes:
> +==========
> +Codes define the precise type of event.

s/event/<something less ambiguous> :)

> +
> +EV_SYN:
> +----------
> +EV_SYN event values are undefined. Their usage is

> +defined only by when they are sent in the evdev event stream.
> +
> +* SYN_REPORT:
> +  - Used to synchronize and separate events into frames of time. For example,
> +    motion of a mouse may set the REL_X and REL_Y values for one motion, then
> +    emit a SYN_REPORT. The next motion will emit more REL_X and REL_Y values and
> +    send another SYN_REPORT.
> +
> +* SYN_CONFIG:
> +  - TBD
> +
> +* SYN_MT_REPORT:
> +  - Used to synchronize and separate touch events. See the
> +    multi-touch-protocol.txt document for more information.
> +
> +EV_KEY:
> +----------
> +EV_KEY events take the form KEY_<name> or BTN_<name>. For example, KEY_A is used
> +to represent the 'A' key on a keyboard. When a key is depressed, an event with
> +the key's code is emitted with value 1. When the key is released, an event is
> +emitted with value 0. Some hardware send events when a key is repeated. These
> +events have a value of 2. In general, KEY_<name> is used for keyboard keys, and
> +BTN_<name> is used for other types of momentary switch events.
> +
> +A few EV_KEY codes have special meanings:
> +
> +* BTN_TOOL_<name>:
> +  - These codes are used in conjunction with input trackpads, tablets, and
> +    touchscreens. These devices may be used with fingers, pens, or other tools.
> +    When an event occurs and a tool is used, the corresponding BTN_TOOL_<name>
> +    code should be set to a value of 1. When the tool is no longer interacting
> +    with the input device, the BTN_TOOL_<name> code should be reset to 0. All
> +    trackpads, tablets, and touchscreens should use at least one BTN_TOOL_<name>
> +    code when events are generated.
> +
> +* BTN_TOUCH:
> +    BTN_TOUCH is used for touch contact. While an input tool is determined to be
> +    within meaningful physical contact, the value of this property must be set
> +    to 1. Meaningful physical contact may mean any contact, or it may mean
> +    contact conditioned by an implementation defined property. For example, a
> +    touchpad may set the value to 1 only when the touch pressure rises above a
> +    certain value. BTN_TOUCH may be combined with BTN_TOOL_<name> codes. For
> +    example, a pen tablet may set BTN_TOOL_PEN to 1 and BTN_TOUCH to 0 while the
> +    pen is hovering over but not touching the tablet surface.
> +
> +Note: For appropriate function of the legacy mousedev emulation driver,
> +BTN_TOUCH must be the first evdev code emitted in a synchronization frame.
> +
> +Note: Historically a touch device with BTN_TOOL_FINGER and BTN_TOUCH was
> +interpreted as a touchpad by userspace, while a similar device without
> +BTN_TOOL_FINGER was interpreted as a touchscreen. For backwards compatibility
> +with current userspace it is recommended to follow this distinction. In the
> +future, this distinction will be deprecated and the device properties ioctl
> +EVIOCGPROP, defined in linux/input.h, will be used to convey the device type.
> +
> +* BTN_TOOL_FINGER, BTN_TOOL_DOUBLETAP, BTN_TOOL_TRIPLETAP, BTN_TOOL_QUADTAP:
> +  - These codes denote one, two, three, and four finger interaction on a
> +    trackpad or touchscreen. For example, if the user uses two fingers and moves
> +    them on the touchpad in an effort to scroll content on screen,
> +    BTN_TOOL_DOUBLETAP should be set to value 1 for the duration of the motion.
> +    Note that all BTN_TOOL_<name> codes and the BTN_TOUCH code are orthogonal in
> +    purpose. A trackpad event generated by finger touches should generate events
> +    for one code from each group. At most only one of these BTN_TOOL_<name>
> +    codes should have a value of 1 during any synchronization frame.
> +
> +Note: Historically some drivers emitted multiple of the finger count codes with
> +a value of 1 in the same synchronization frame. This usage is deprecated.
> +
> +Note: In multitouch drivers, the input_mt_report_finger_count() function should
> +be used to emit these codes. Please see multi-touch-protocol.txt for details.
> +
> +EV_REL:
> +----------
> +EV_REL events describe relative changes in a property. For example, a mouse may
> +move to the left by a certain number of units, but its absolute position in
> +space is unknown. If the absolute position is known, EV_ABS codes should be used
> +instead of EV_REL codes.
> +
> +A few EV_REL codes have special meanings:
> +
> +* REL_WHEEL, REL_HWHEEL:
> +  - These codes are used for vertical and horizontal scroll wheels,
> +    respectively.
> +
> +EV_ABS:
> +----------
> +EV_ABS events describe absolute changes in a property. For example, a touchpad
> +may emit coordinates for a touch location.
> +
> +A few EV_ABS codes have special meanings:
> +
> +* ABS_DISTANCE:
> +  - Used to describe the distance of a tool from an interaction surface. This
> +    should only be used while the tool is hovering, meaning in close proximity
> +    of the device. If the input device may be used freely in three dimensions,
> +    consider ABS_Z instead.

reference to BTN_TOUCH == 0 maybe?

Cheers,
  Peter
> +
> +* ABS_MT_<name>:
> +  - Used to describe multitouch input events. Please see
> +    multi-touch-protocol.txt for details.
> +
> +EV_SW:
> +----------
> +EV_SW events describe stateful binary switches. For example, the SW_LID code is
> +used to denote when a laptop lid is closed.
> +
> +Upon binding to a device or resuming from suspend, a driver must report the
> +the current switch state. This ensures that the device, kernel, and userspace
> +state is in sync.
> +
> +Upon resume, if the switch state is the same as before suspend, then the input
> +subsystem will filter out the duplicate switch state reports. The driver does
> +not need to keep the state of the switch at any time.
> +
> +EV_MSC:
> +----------
> +EV_MSC events are used for input and output events that do not fall under other
> +categories.
> +
> +EV_LED:
> +----------
> +EV_LED events are used for input and output to set and query the state of
> +various LEDs on devices.
> +
> +EV_REP:
> +----------
> +EV_REP events are used for specifying autorepeating events.
> +
> +EV_SND:
> +----------
> +EV_SND events are used for sending sound commands to simple sound output
> +devices.
> +
> +EV_FF:
> +----------
> +EV_FF events are used to initialize a force feedback capable device and to cause
> +such device to feedback.
> +
> +EV_PWR:
> +----------
> +EV_PWR events are a special type of event used specifically for power
> +mangement. Its usage is not well defined. To be addressed later.
> +
> +Guidelines:
> +==========
> +The guidelines below ensure proper single-touch and multi-finger functionality.
> +For multi-touch functionality, 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. BTN_TOOL_<name> events
> +should be reported where possible.
> +
> +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.
> -- 
> 1.7.1
> 
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