devicetree/bindings: display: Document common panel properties

Document properties common to several display panels in a central
location that can be referenced by the panel device tree bindings.
Signed-off-by: NLaurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/display/panel/panel-common.txt

+Common Properties for Display Panel
+===================================
+
+This document defines device tree properties common to several classes of
+display panels. It doesn't constitue a device tree binding specification by
+itself but is meant to be referenced by device tree bindings.
+
+When referenced from panel device tree bindings the properties defined in this
+document are defined as follows. The panel device tree bindings are
+responsible for defining whether each property is required or optional.
+
+
+Descriptive Properties
+----------------------
+
+- width-mm,
+- height-mm: The width-mm and height-mm specify the width and height of the
+  physical area where images are displayed. These properties are expressed in
+  millimeters and rounded to the closest unit.
+
+- label: The label property specifies a symbolic name for the panel as a
+  string suitable for use by humans. It typically contains a name inscribed on
+  the system (e.g. as an affixed label) or specified in the system's
+  documentation (e.g. in the user's manual).
+
+  If no such name exists, and unless the property is mandatory according to
+  device tree bindings, it shall rather be omitted than constructed of
+  non-descriptive information. For instance an LCD panel in a system that
+  contains a single panel shall not be labelled "LCD" if that name is not
+  inscribed on the system or used in a descriptive fashion in system
+  documentation.
+
+
+Display Timings
+---------------
+
+- panel-timing: Most display panels are restricted to a single resolution and
+  require specific display timings. The panel-timing subnode expresses those
+  timings as specified in the timing subnode section of the display timing
+  bindings defined in
+  Documentation/devicetree/bindings/display/display-timing.txt.
+
+
+Connectivity
+------------
+
+- ports: Panels receive video data through one or multiple connections. While
+  the nature of those connections is specific to the panel type, the
+  connectivity is expressed in a standard fashion using ports as specified in
+  the device graph bindings defined in
+  Documentation/devicetree/bindings/graph.txt.
+
+- ddc-i2c-bus: Some panels expose EDID information through an I2C-compatible
+  bus such as DDC2 or E-DDC. For such panels the ddc-i2c-bus contains a
+  phandle to the system I2C controller connected to that bus.
+
+
+Control I/Os
+------------
+
+Many display panels can be controlled through pins driven by GPIOs. The nature
+and timing of those control signals are device-specific and left for panel
+device tree bindings to specify. The following GPIO specifiers can however be
+used for panels that implement compatible control signals.
+
+- enable-gpios: Specifier for a GPIO connected to the panel enable control
+  signal. The enable signal is active high and enables operation of the panel.
+  This property can also be used for panels implementing an active low power
+  down signal, which is a negated version of the enable signal. Active low
+  enable signals (or active high power down signals) can be supported by
+  inverting the GPIO specifier polarity flag.
+
+  Note that the enable signal control panel operation only and must not be
+  confused with a backlight enable signal.
+
+- reset-gpios: Specifier for a GPIO coonnected to the panel reset control
+  signal. The reset signal is active low and resets the panel internal logic
+  while active. Active high reset signals can be supported by inverting the
+  GPIO specifier polarity flag.
+
+
+Backlight
+---------
+
+Most display panels include a backlight. Some of them also include a backlight
+controller exposed through a control bus such as I2C or DSI. Others expose
+backlight control through GPIO, PWM or other signals connected to an external
+backlight controller.
+
+- backlight: For panels whose backlight is controlled by an external backlight
+  controller, this property contains a phandle that references the controller.