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    Subject[PATCH] input: Document the multi-touch (MT) protocol
    This patchs adds documentation for the multi-touch protocol to

    Signed-off-by: Henrik Rydberg <>
    Documentation/input/multi-touch-protocol.txt | 140 ++++++++++++++++++++++++++
    1 files changed, 140 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/input/multi-touch-protocol.txt

    diff --git a/Documentation/input/multi-touch-protocol.txt b/Documentation/input/multi-touch-protocol.txt
    new file mode 100644
    index 0000000..2e99cd6
    --- /dev/null
    +++ b/Documentation/input/multi-touch-protocol.txt
    @@ -0,0 +1,140 @@
    +Multi-touch (MT) Protocol
    + Copyright (C) 2009 Henrik Rydberg <>
    +In order to utilize the full power of the new multi-touch devices, a way to
    +report detailed finger data to user space is needed. This document
    +describes the multi-touch (MT) protocol which allows kernel drivers to
    +report details for an arbitrary number of fingers.
    +Anonymous finger details are sent sequentially as separate packets of ABS
    +events. Only the ABS_MT events are recognized as part of a finger
    +packet. The end of a packet is marked by calling the input_mt_sync()
    +function, which generates a SYN_MT_REPORT event. The end of multi-touch
    +transfer is marked by calling the usual input_sync() function.
    +A set of ABS_MT events with the desired properties are defined. The events
    +are divided into categories, to allow for partial implementation. The
    +minimum set consists of ABS_MT_TOUCH_MAJOR, ABS_MT_POSITION_X and
    +ABS_MT_POSITION_Y, which allows for multiple fingers to be tracked. If the
    +device supports it, the ABS_MT_WIDTH_MAJOR may be used to provide the size
    +of the approaching finger. Anisotropy and direction may be specified with
    +more granular information may specify general shapes as blobs, i.e., as a
    +sequence of rectangular shapes grouped together by a
    +ABS_MT_BLOB_ID. Finally, the ABS_MT_TOOL_TYPE may be used to specify
    +whether the touching tool is a finger or a pen or something else.
    +Event Semantics
    +The word "contact" is used to describe a tool which is in direct contact
    +with the surface. A finger, a pen or a rubber all classify as contacts.
    +The length of the major axis of the contact. The length should be given in
    +surface units. If the surface has a X time Y resolution, the largest
    +possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diameter.
    +The length, in surface units, of the minor axis of the contact. If the
    +contact is circular, this event can be omitted.
    +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
    +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 [1].
    +The orientation of the ellipse. The value should describe half a revolution
    +clockwise around the touch center. The scale of the value is arbitrary, but
    +zero should be returned for an ellipse aligned along the Y axis of the
    +surface. As an example, an index finger placed straight onto the axis could
    +return zero orientation, something negative when twisted to the left, and
    +something positive when twisted to the right. This value can be omitted if
    +the touching object is circular, or if the information is not available in
    +the kernel driver.
    +The surface X coordinate of the center of the touching ellipse.
    +The surface Y coordinate of the center of the touching ellipse.
    +The type of approaching tool. A lot of kernel drivers cannot distinguish
    +between different tool types, such as a finger or a pen. I such cases, the
    +event should be omitted. The protocol currently supports MT_TOOL_FINGER and
    +MT_TOOL_PEN [2].
    +The BLOB_ID groups several packets together into one arbitrarily shaped
    +contact. This is a low-level anonymous grouping, and should not be confused
    +with the high-level contactID, explained below. Most kernel drivers will
    +not have this capability, and can safely omit the event.
    +Finger Tracking
    +The kernel driver should generate an arbitrary enumeration of the set of
    +anonymous contacts currently on the surface. The order in which the packets
    +appear in the event stream is not important.
    +The process of finger tracking, i.e., to assign a unique contactID to each
    +initiated contact on the surface, is left to user space; preferably the
    +multi-touch X driver [3]. In that driver, the contactID stays the same and
    +unique until the contact vanishes (when the finger leaves the surface). The
    +problem of assigning a set of anonymous fingers to a set of identified
    +fingers is a euclidian bipartite matching problem at each event update, and
    +relies on a sufficiently rapid update rate.
    +In order to stay compatible with existing applications, the data
    +reported in a finger packet must not be recognized as single-touch
    +events. In addition, all finger data must bypass input filtering,
    +since subsequent events of the same type refer to different fingers.
    +The first kernel driver to utilize the MT protocol is the bcm5974 driver,
    +where examples can be found.
    +[1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the
    +difference between the contact position and the approaching tool position
    +could be used to derive tilt.
    +[2] The list can of course be extended.
    +[3] The multi-touch X driver is currently in the prototyping stage. At the
    +time of writing (April 2009), the MT protocol is not yet merged, and the
    +prototype implements finger matching, basic mouse support and two-finger
    +scrolling. The project aims at improving the quality of current multi-touch
    +functionality available in the synaptics X driver, and in addition
    +implement more advanced gestures.

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