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Fork of AMDESE/linux with modifications for CachePC side-channel attack
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multi-touch-protocol.rst (17894B)


      1.. include:: <isonum.txt>
      2
      3=========================
      4Multi-touch (MT) Protocol
      5=========================
      6
      7:Copyright: |copy| 2009-2010	Henrik Rydberg <rydberg@euromail.se>
      8
      9
     10Introduction
     11------------
     12
     13In order to utilize the full power of the new multi-touch and multi-user
     14devices, a way to report detailed data from multiple contacts, i.e.,
     15objects in direct contact with the device surface, is needed.  This
     16document describes the multi-touch (MT) protocol which allows kernel
     17drivers to report details for an arbitrary number of contacts.
     18
     19The protocol is divided into two types, depending on the capabilities of the
     20hardware. For devices handling anonymous contacts (type A), the protocol
     21describes how to send the raw data for all contacts to the receiver. For
     22devices capable of tracking identifiable contacts (type B), the protocol
     23describes how to send updates for individual contacts via event slots.
     24
     25.. note::
     26   MT protocol type A is obsolete, all kernel drivers have been
     27   converted to use type B.
     28
     29Protocol Usage
     30--------------
     31
     32Contact details are sent sequentially as separate packets of ABS_MT
     33events. Only the ABS_MT events are recognized as part of a contact
     34packet. Since these events are ignored by current single-touch (ST)
     35applications, the MT protocol can be implemented on top of the ST protocol
     36in an existing driver.
     37
     38Drivers for type A devices separate contact packets by calling
     39input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
     40event, which instructs the receiver to accept the data for the current
     41contact and prepare to receive another.
     42
     43Drivers for type B devices separate contact packets by calling
     44input_mt_slot(), with a slot as argument, at the beginning of each packet.
     45This generates an ABS_MT_SLOT event, which instructs the receiver to
     46prepare for updates of the given slot.
     47
     48All drivers mark the end of a multi-touch transfer by calling the usual
     49input_sync() function. This instructs the receiver to act upon events
     50accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
     51of events/packets.
     52
     53The main difference between the stateless type A protocol and the stateful
     54type B slot protocol lies in the usage of identifiable contacts to reduce
     55the amount of data sent to userspace. The slot protocol requires the use of
     56the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
     57the raw data [#f5]_.
     58
     59For type A devices, the kernel driver should generate an arbitrary
     60enumeration of the full set of anonymous contacts currently on the
     61surface. The order in which the packets appear in the event stream is not
     62important.  Event filtering and finger tracking is left to user space [#f3]_.
     63
     64For type B devices, the kernel driver should associate a slot with each
     65identified contact, and use that slot to propagate changes for the contact.
     66Creation, replacement and destruction of contacts is achieved by modifying
     67the ABS_MT_TRACKING_ID of the associated slot.  A non-negative tracking id
     68is interpreted as a contact, and the value -1 denotes an unused slot.  A
     69tracking id not previously present is considered new, and a tracking id no
     70longer present is considered removed.  Since only changes are propagated,
     71the full state of each initiated contact has to reside in the receiving
     72end.  Upon receiving an MT event, one simply updates the appropriate
     73attribute of the current slot.
     74
     75Some devices identify and/or track more contacts than they can report to the
     76driver.  A driver for such a device should associate one type B slot with each
     77contact that is reported by the hardware.  Whenever the identity of the
     78contact associated with a slot changes, the driver should invalidate that
     79slot by changing its ABS_MT_TRACKING_ID.  If the hardware signals that it is
     80tracking more contacts than it is currently reporting, the driver should use
     81a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
     82being tracked by the hardware at that moment.  The driver should do this by
     83explicitly sending the corresponding BTN_TOOL_*TAP event and setting
     84use_count to false when calling input_mt_report_pointer_emulation().
     85The driver should only advertise as many slots as the hardware can report.
     86Userspace can detect that a driver can report more total contacts than slots
     87by noting that the largest supported BTN_TOOL_*TAP event is larger than the
     88total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
     89
     90The minimum value of the ABS_MT_SLOT axis must be 0.
     91
     92Protocol Example A
     93------------------
     94
     95Here is what a minimal event sequence for a two-contact touch would look
     96like for a type A device::
     97
     98   ABS_MT_POSITION_X x[0]
     99   ABS_MT_POSITION_Y y[0]
    100   SYN_MT_REPORT
    101   ABS_MT_POSITION_X x[1]
    102   ABS_MT_POSITION_Y y[1]
    103   SYN_MT_REPORT
    104   SYN_REPORT
    105
    106The sequence after moving one of the contacts looks exactly the same; the
    107raw data for all present contacts are sent between every synchronization
    108with SYN_REPORT.
    109
    110Here is the sequence after lifting the first contact::
    111
    112   ABS_MT_POSITION_X x[1]
    113   ABS_MT_POSITION_Y y[1]
    114   SYN_MT_REPORT
    115   SYN_REPORT
    116
    117And here is the sequence after lifting the second contact::
    118
    119   SYN_MT_REPORT
    120   SYN_REPORT
    121
    122If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
    123ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
    124last SYN_REPORT will be dropped by the input core, resulting in no
    125zero-contact event reaching userland.
    126
    127
    128Protocol Example B
    129------------------
    130
    131Here is what a minimal event sequence for a two-contact touch would look
    132like for a type B device::
    133
    134   ABS_MT_SLOT 0
    135   ABS_MT_TRACKING_ID 45
    136   ABS_MT_POSITION_X x[0]
    137   ABS_MT_POSITION_Y y[0]
    138   ABS_MT_SLOT 1
    139   ABS_MT_TRACKING_ID 46
    140   ABS_MT_POSITION_X x[1]
    141   ABS_MT_POSITION_Y y[1]
    142   SYN_REPORT
    143
    144Here is the sequence after moving contact 45 in the x direction::
    145
    146   ABS_MT_SLOT 0
    147   ABS_MT_POSITION_X x[0]
    148   SYN_REPORT
    149
    150Here is the sequence after lifting the contact in slot 0::
    151
    152   ABS_MT_TRACKING_ID -1
    153   SYN_REPORT
    154
    155The slot being modified is already 0, so the ABS_MT_SLOT is omitted.  The
    156message removes the association of slot 0 with contact 45, thereby
    157destroying contact 45 and freeing slot 0 to be reused for another contact.
    158
    159Finally, here is the sequence after lifting the second contact::
    160
    161   ABS_MT_SLOT 1
    162   ABS_MT_TRACKING_ID -1
    163   SYN_REPORT
    164
    165
    166Event Usage
    167-----------
    168
    169A set of ABS_MT events with the desired properties is defined. The events
    170are divided into categories, to allow for partial implementation.  The
    171minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
    172allows for multiple contacts to be tracked.  If the device supports it, the
    173ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
    174of the contact area and approaching tool, respectively.
    175
    176The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
    177looking through a window at someone gently holding a finger against the
    178glass.  You will see two regions, one inner region consisting of the part
    179of the finger actually touching the glass, and one outer region formed by
    180the perimeter of the finger. The center of the touching region (a) is
    181ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
    182ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
    183diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
    184harder against the glass. The touch region will increase, and in general,
    185the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
    186than unity, is related to the contact pressure. For pressure-based devices,
    187ABS_MT_PRESSURE may be used to provide the pressure on the contact area
    188instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
    189indicate the distance between the contact and the surface.
    190
    191::
    192
    193
    194	  Linux MT                               Win8
    195         __________                     _______________________
    196        /          \                   |                       |
    197       /            \                  |                       |
    198      /     ____     \                 |                       |
    199     /     /    \     \                |                       |
    200     \     \  a  \     \               |       a               |
    201      \     \____/      \              |                       |
    202       \                 \             |                       |
    203        \        b        \            |           b           |
    204         \                 \           |                       |
    205          \                 \          |                       |
    206           \                 \         |                       |
    207            \                /         |                       |
    208             \              /          |                       |
    209              \            /           |                       |
    210               \__________/            |_______________________|
    211
    212
    213In addition to the MAJOR parameters, the oval shape of the touch and finger
    214regions can be described by adding the MINOR parameters, such that MAJOR
    215and MINOR are the major and minor axis of an ellipse. The orientation of
    216the touch ellipse can be described with the ORIENTATION parameter, and the
    217direction of the finger ellipse is given by the vector (a - b).
    218
    219For type A devices, further specification of the touch shape is possible
    220via ABS_MT_BLOB_ID.
    221
    222The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
    223finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
    224may be used to track identified contacts over time [#f5]_.
    225
    226In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
    227implicitly handled by input core; drivers should instead call
    228input_mt_report_slot_state().
    229
    230
    231Event Semantics
    232---------------
    233
    234ABS_MT_TOUCH_MAJOR
    235    The length of the major axis of the contact. The length should be given in
    236    surface units. If the surface has an X times Y resolution, the largest
    237    possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_.
    238
    239ABS_MT_TOUCH_MINOR
    240    The length, in surface units, of the minor axis of the contact. If the
    241    contact is circular, this event can be omitted [#f4]_.
    242
    243ABS_MT_WIDTH_MAJOR
    244    The length, in surface units, of the major axis of the approaching
    245    tool. This should be understood as the size of the tool itself. The
    246    orientation of the contact and the approaching tool are assumed to be the
    247    same [#f4]_.
    248
    249ABS_MT_WIDTH_MINOR
    250    The length, in surface units, of the minor axis of the approaching
    251    tool. Omit if circular [#f4]_.
    252
    253    The above four values can be used to derive additional information about
    254    the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
    255    the notion of pressure. The fingers of the hand and the palm all have
    256    different characteristic widths.
    257
    258ABS_MT_PRESSURE
    259    The pressure, in arbitrary units, on the contact area. May be used instead
    260    of TOUCH and WIDTH for pressure-based devices or any device with a spatial
    261    signal intensity distribution.
    262
    263    If the resolution is zero, the pressure data is in arbitrary units.
    264    If the resolution is non-zero, the pressure data is in units/gram. See
    265    :ref:`input-event-codes` for details.
    266
    267ABS_MT_DISTANCE
    268    The distance, in surface units, between the contact and the surface. Zero
    269    distance means the contact is touching the surface. A positive number means
    270    the contact is hovering above the surface.
    271
    272ABS_MT_ORIENTATION
    273    The orientation of the touching ellipse. The value should describe a signed
    274    quarter of a revolution clockwise around the touch center. The signed value
    275    range is arbitrary, but zero should be returned for an ellipse aligned with
    276    the Y axis (north) of the surface, a negative value when the ellipse is
    277    turned to the left, and a positive value when the ellipse is turned to the
    278    right. When aligned with the X axis in the positive direction, the range
    279    max should be returned; when aligned with the X axis in the negative
    280    direction, the range -max should be returned.
    281
    282    Touch ellipses are symmetrical by default. For devices capable of true 360
    283    degree orientation, the reported orientation must exceed the range max to
    284    indicate more than a quarter of a revolution. For an upside-down finger,
    285    range max * 2 should be returned.
    286
    287    Orientation can be omitted if the touch area is circular, or if the
    288    information is not available in the kernel driver. Partial orientation
    289    support is possible if the device can distinguish between the two axes, but
    290    not (uniquely) any values in between. In such cases, the range of
    291    ABS_MT_ORIENTATION should be [0, 1] [#f4]_.
    292
    293ABS_MT_POSITION_X
    294    The surface X coordinate of the center of the touching ellipse.
    295
    296ABS_MT_POSITION_Y
    297    The surface Y coordinate of the center of the touching ellipse.
    298
    299ABS_MT_TOOL_X
    300    The surface X coordinate of the center of the approaching tool. Omit if
    301    the device cannot distinguish between the intended touch point and the
    302    tool itself.
    303
    304ABS_MT_TOOL_Y
    305    The surface Y coordinate of the center of the approaching tool. Omit if the
    306    device cannot distinguish between the intended touch point and the tool
    307    itself.
    308
    309    The four position values can be used to separate the position of the touch
    310    from the position of the tool. If both positions are present, the major
    311    tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are
    312    aligned with the touch axes.
    313
    314ABS_MT_TOOL_TYPE
    315    The type of approaching tool. A lot of kernel drivers cannot distinguish
    316    between different tool types, such as a finger or a pen. In such cases, the
    317    event should be omitted. The protocol currently mainly supports
    318    MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_.
    319    For type B devices, this event is handled by input core; drivers should
    320    instead use input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may
    321    change over time while still touching the device, because the firmware may
    322    not be able to determine which tool is being used when it first appears.
    323
    324ABS_MT_BLOB_ID
    325    The BLOB_ID groups several packets together into one arbitrarily shaped
    326    contact. The sequence of points forms a polygon which defines the shape of
    327    the contact. This is a low-level anonymous grouping for type A devices, and
    328    should not be confused with the high-level trackingID [#f5]_. Most type A
    329    devices do not have blob capability, so drivers can safely omit this event.
    330
    331ABS_MT_TRACKING_ID
    332    The TRACKING_ID identifies an initiated contact throughout its life cycle
    333    [#f5]_. The value range of the TRACKING_ID should be large enough to ensure
    334    unique identification of a contact maintained over an extended period of
    335    time. For type B devices, this event is handled by input core; drivers
    336    should instead use input_mt_report_slot_state().
    337
    338
    339Event Computation
    340-----------------
    341
    342The flora of different hardware unavoidably leads to some devices fitting
    343better to the MT protocol than others. To simplify and unify the mapping,
    344this section gives recipes for how to compute certain events.
    345
    346For devices reporting contacts as rectangular shapes, signed orientation
    347cannot be obtained. Assuming X and Y are the lengths of the sides of the
    348touching rectangle, here is a simple formula that retains the most
    349information possible::
    350
    351   ABS_MT_TOUCH_MAJOR := max(X, Y)
    352   ABS_MT_TOUCH_MINOR := min(X, Y)
    353   ABS_MT_ORIENTATION := bool(X > Y)
    354
    355The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
    356the device can distinguish between a finger along the Y axis (0) and a
    357finger along the X axis (1).
    358
    359For Win8 devices with both T and C coordinates, the position mapping is::
    360
    361   ABS_MT_POSITION_X := T_X
    362   ABS_MT_POSITION_Y := T_Y
    363   ABS_MT_TOOL_X := C_X
    364   ABS_MT_TOOL_Y := C_Y
    365
    366Unfortunately, there is not enough information to specify both the touching
    367ellipse and the tool ellipse, so one has to resort to approximations.  One
    368simple scheme, which is compatible with earlier usage, is::
    369
    370   ABS_MT_TOUCH_MAJOR := min(X, Y)
    371   ABS_MT_TOUCH_MINOR := <not used>
    372   ABS_MT_ORIENTATION := <not used>
    373   ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
    374   ABS_MT_WIDTH_MINOR := min(X, Y)
    375
    376Rationale: We have no information about the orientation of the touching
    377ellipse, so approximate it with an inscribed circle instead. The tool
    378ellipse should align with the vector (T - C), so the diameter must
    379increase with distance(T, C). Finally, assume that the touch diameter is
    380equal to the tool thickness, and we arrive at the formulas above.
    381
    382Finger Tracking
    383---------------
    384
    385The process of finger tracking, i.e., to assign a unique trackingID to each
    386initiated contact on the surface, is a Euclidian Bipartite Matching
    387problem.  At each event synchronization, the set of actual contacts is
    388matched to the set of contacts from the previous synchronization. A full
    389implementation can be found in [#f3]_.
    390
    391
    392Gestures
    393--------
    394
    395In the specific application of creating gesture events, the TOUCH and WIDTH
    396parameters can be used to, e.g., approximate finger pressure or distinguish
    397between index finger and thumb. With the addition of the MINOR parameters,
    398one can also distinguish between a sweeping finger and a pointing finger,
    399and with ORIENTATION, one can detect twisting of fingers.
    400
    401
    402Notes
    403-----
    404
    405In order to stay compatible with existing applications, the data reported
    406in a finger packet must not be recognized as single-touch events.
    407
    408For type A devices, all finger data bypasses input filtering, since
    409subsequent events of the same type refer to different fingers.
    410
    411.. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
    412.. [#f2] The list can of course be extended.
    413.. [#f3] The mtdev project: http://bitmath.org/code/mtdev/.
    414.. [#f4] See the section on event computation.
    415.. [#f5] See the section on finger tracking.