xref: /freebsd/sys/contrib/device-tree/Bindings/common-properties.txt (revision c66ec88fed842fbaad62c30d510644ceb7bd2d71)

Common properties
=================

Endianness
----------

The Devicetree Specification does not define any properties related to hardware
byte swapping, but endianness issues show up frequently in porting drivers to
different machine types.  This document attempts to provide a consistent
way of handling byte swapping across drivers.

Optional properties:
 - big-endian: Boolean; force big endian register accesses
   unconditionally (e.g. ioread32be/iowrite32be).  Use this if you
   know the peripheral always needs to be accessed in big endian (BE) mode.
 - little-endian: Boolean; force little endian register accesses
   unconditionally (e.g. readl/writel).  Use this if you know the
   peripheral always needs to be accessed in little endian (LE) mode.
 - native-endian: Boolean; always use register accesses matched to the
   endianness of the kernel binary (e.g. LE vmlinux -> readl/writel,
   BE vmlinux -> ioread32be/iowrite32be).  In this case no byte swaps
   will ever be performed.  Use this if the hardware "self-adjusts"
   register endianness based on the CPU's configured endianness.

If a binding supports these properties, then the binding should also
specify the default behavior if none of these properties are present.
In such cases, little-endian is the preferred default, but it is not
a requirement.  Some implementations assume that little-endian is
the default, because most existing (PCI-based) drivers implicitly
default to LE for their MMIO accesses.

Examples:
Scenario 1 : CPU in LE mode & device in LE mode.
dev: dev@40031000 {
	      compatible = "name";
	      reg = <0x40031000 0x1000>;
	      ...
	      native-endian;
};

Scenario 2 : CPU in LE mode & device in BE mode.
dev: dev@40031000 {
	      compatible = "name";
	      reg = <0x40031000 0x1000>;
	      ...
	      big-endian;
};

Scenario 3 : CPU in BE mode & device in BE mode.
dev: dev@40031000 {
	      compatible = "name";
	      reg = <0x40031000 0x1000>;
	      ...
	      native-endian;
};

Scenario 4 : CPU in BE mode & device in LE mode.
dev: dev@40031000 {
	      compatible = "name";
	      reg = <0x40031000 0x1000>;
	      ...
	      little-endian;
};

Daisy-chained devices
---------------------

Many serially-attached GPIO and IIO devices are daisy-chainable.  To the
host controller, a daisy-chain appears as a single device, but the number
of inputs and outputs it provides is the sum of inputs and outputs provided
by all of its devices.  The driver needs to know how many devices the
daisy-chain comprises to determine the amount of data exchanged, how many
inputs and outputs to register and so on.

Optional properties:
 - #daisy-chained-devices: Number of devices in the daisy-chain (default is 1).

Example:
gpio@0 {
	      compatible = "name";
	      reg = <0>;
	      gpio-controller;
	      #gpio-cells = <2>;
	      #daisy-chained-devices = <3>;
};