devicetree/bindings: display: Document common panel properties

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.