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torvalds
GitHub Repository: torvalds/linux
 Path: blob/master/Documentation/devicetree/bindings/common-properties.txt
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Common properties

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=================

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Endianness

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----------

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The Devicetree Specification does not define any properties related to hardware

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byte swapping, but endianness issues show up frequently in porting drivers to

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different machine types.  This document attempts to provide a consistent

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way of handling byte swapping across drivers.

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Optional properties:

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 - big-endian: Boolean; force big endian register accesses

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   unconditionally (e.g. ioread32be/iowrite32be).  Use this if you

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   know the peripheral always needs to be accessed in big endian (BE) mode.

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 - little-endian: Boolean; force little endian register accesses

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   unconditionally (e.g. readl/writel).  Use this if you know the

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   peripheral always needs to be accessed in little endian (LE) mode.

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 - native-endian: Boolean; always use register accesses matched to the

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   endianness of the kernel binary (e.g. LE vmlinux -> readl/writel,

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   BE vmlinux -> ioread32be/iowrite32be).  In this case no byte swaps

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   will ever be performed.  Use this if the hardware "self-adjusts"

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   register endianness based on the CPU's configured endianness.

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If a binding supports these properties, then the binding should also

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specify the default behavior if none of these properties are present.

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In such cases, little-endian is the preferred default, but it is not

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a requirement.  Some implementations assume that little-endian is

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the default, because most existing (PCI-based) drivers implicitly

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default to LE for their MMIO accesses.

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Examples:

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Scenario 1 : CPU in LE mode & device in LE mode.

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dev: dev@40031000 {

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	      compatible = "name";

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	      reg = <0x40031000 0x1000>;

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	      ...

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	      native-endian;

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};

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Scenario 2 : CPU in LE mode & device in BE mode.

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dev: dev@40031000 {

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	      compatible = "name";

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	      reg = <0x40031000 0x1000>;

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	      ...

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	      big-endian;

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};

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Scenario 3 : CPU in BE mode & device in BE mode.

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dev: dev@40031000 {

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	      compatible = "name";

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	      reg = <0x40031000 0x1000>;

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	      ...

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	      native-endian;

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};

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Scenario 4 : CPU in BE mode & device in LE mode.

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dev: dev@40031000 {

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	      compatible = "name";

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	      reg = <0x40031000 0x1000>;

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	      ...

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	      little-endian;

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};

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Daisy-chained devices

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---------------------

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Many serially-attached GPIO and IIO devices are daisy-chainable.  To the

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host controller, a daisy-chain appears as a single device, but the number

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of inputs and outputs it provides is the sum of inputs and outputs provided

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by all of its devices.  The driver needs to know how many devices the

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daisy-chain comprises to determine the amount of data exchanged, how many

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inputs and outputs to register and so on.

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Optional properties:

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

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Example:

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gpio@0 {

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	      compatible = "name";

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	      reg = <0>;

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	      gpio-controller;

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	      #gpio-cells = <2>;

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	      #daisy-chained-devices = <3>;

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};

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