Re: [PATCH v3] Documentation: Add evdev type and code definitions

[Chris Bagwell]

Looking good.  Minor comments at end.

On Thu, Mar 24, 2011 at 12:44 PM, Chase Douglas
<chase.douglas@xxxxxxxxxxxxx> wrote:
> This commit adds the file Documentation/input/evdev-codes.txt.
>
> Cc: Henrik Rydberg <rydberg@xxxxxxxxxxx>
> Cc: Chris Bagwell <chris@xxxxxxxxxxxxxx>
> Cc: Peter Hutterer <peter.hutterer@xxxxxxxxx>
> Cc: Nikolai Kondrashov <spbnick@xxxxxxxxx>
> Cc: linux-input@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Acked-by: Henrik Rydberg <rydberg@xxxxxxxxxxx>
> Signed-off-by: Chase Douglas <chase.douglas@xxxxxxxxxxxxx>
> ---
> 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 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.
> +
> +* 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.
> +
> +* EV_MSC:
> +  - Used to describe miscellaneous input events that do not fit into other
> +    types.
> +
> +* 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.
> +
> +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.
> +
> +* 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.

Does the real button event restriction solve something?  I know
majority of touchscreens are not sending a button today but usually
there is a button on the touchscreen frame that I'd image at least
some devices hooking it up to touchscreen interface.

stylus+touchscreen combo devices are surely going to want to do this.

> +
> +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.

How about a Tablet?

Something like:

Tablets:
--------

BTN_TOOL_<name> events must be reported when a stylus or other tool is
active on 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,STLUS2} should be used to report buttons
on tool itself and BTN_{LEFT,MIDDLE,RIGHT,1,2,etc} should be used to
report buttons on tablet.

Some tablets send ABS_PRESSURE with no BTN_TOUCH (why I said should
instead of must).  Probably this is not worth mentioning here?

Chris
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