[media] Documentation: add support for V4L touch devices

Document the new touch API.

Signed-off-by: Nick Dyer <nick@shmanahar.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: fix up videodev2.h.rst.exceptions]

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

1 files changed, 56 insertions, 0 deletions

diff --git a/Documentation/media/uapi/v4l/dev-touch.rst b/Documentation/media/uapi/v4l/dev-touch.rst
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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _touch:
+
+*************
+Touch Devices
+*************
+
+Touch devices are accessed through character device special files named
+``/dev/v4l-touch0`` to ``/dev/v4l-touch255`` with major number 81 and
+dynamically allocated minor numbers 0 to 255.
+
+Overview
+========
+
+Sensors may be Optical, or Projected Capacitive touch (PCT).
+
+Processing is required to analyse the raw data and produce input events. In
+some systems, this may be performed on the ASIC and the raw data is purely a
+side-channel for diagnostics or tuning. In other systems, the ASIC is a simple
+analogue front end device which delivers touch data at high rate, and any touch
+processing must be done on the host.
+
+For capacitive touch sensing, the touchscreen is composed of an array of
+horizontal and vertical conductors (alternatively called rows/columns, X/Y
+lines, or tx/rx). Mutual Capacitance measured is at the nodes where the
+conductors cross. Alternatively, Self Capacitance measures the signal from each
+column and row independently.
+
+A touch input may be determined by comparing the raw capacitance measurement to
+a no-touch reference (or "baseline") measurement:
+
+Delta = Raw - Reference
+
+The reference measurement takes account of variations in the capacitance across
+the touch sensor matrix, for example manufacturing irregularities,
+environmental or edge effects.
+
+Querying Capabilities
+=====================
+
+Devices supporting the touch interface set the ``V4L2_CAP_VIDEO_CAPTURE`` flag
+and the ``V4L2_CAP_TOUCH`` flag in the ``capabilities`` field of
+:ref:`v4l2_capability <v4l2-capability>` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl.
+
+At least one of the read/write or streaming I/O methods must be
+supported.
+
+The formats supported by touch devices are documented in
+:ref:`Touch Formats <tch-formats>`.
+
+Data Format Negotiation
+=======================
+
+A touch device may support any I/O method.