Documentation/input/multi-touch-protocol.rst

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=========================
Multi-touch (MT) Protocol
=========================

:Copyright: |copy| 2009-2010	Henrik Rydberg <rydberg@euromail.se>


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   MT protocol type A is obsolete, all kernel drivers have been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

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 [#f5]_.

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 [#f3]_.

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,

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