Documentation/devicetree/bindings/common-properties.txt

   1 Common properties
   2
   3 The Devicetree Specification does not define any properties related to hardware
   4 byteswapping, but endianness issues show up frequently in porting Linux to
   5 different machine types.  This document attempts to provide a consistent
   6 way of handling byteswapping across drivers.
   7
   8 Optional properties:
   9  - big-endian: Boolean; force big endian register accesses
  10    unconditionally (e.g. ioread32be/iowrite32be).  Use this if you
  11    know the peripheral always needs to be accessed in BE mode.
  12  - little-endian: Boolean; force little endian register accesses
  13    unconditionally (e.g. readl/writel).  Use this if you know the
  14    peripheral always needs to be accessed in LE mode.
  15  - native-endian: Boolean; always use register accesses matched to the
  16    endianness of the kernel binary (e.g. LE vmlinux -> readl/writel,
  17    BE vmlinux -> ioread32be/iowrite32be).  In this case no byteswaps
  18    will ever be performed.  Use this if the hardware "self-adjusts"
  19    register endianness based on the CPU's configured endianness.
  20
  21 If a binding supports these properties, then the binding should also
  22 specify the default behavior if none of these properties are present.
  23 In such cases, little-endian is the preferred default, but it is not
  24 a requirement.  The of_device_is_big_endian() and of_fdt_is_big_endian()
  25 helper functions do assume that little-endian is the default, because
  26 most existing (PCI-based) drivers implicitly default to LE by using
  27 readl/writel for MMIO accesses.
  28
  29 Examples:
  30 Scenario 1 : CPU in LE mode & device in LE mode.
  31 dev: dev@40031000 {
  32               compatible = "name";
  33               reg = <0x40031000 0x1000>;
  34               ...
  35               native-endian;
  36 };
  37
  38 Scenario 2 : CPU in LE mode & device in BE mode.
  39 dev: dev@40031000 {
  40               compatible = "name";
  41               reg = <0x40031000 0x1000>;
  42               ...
  43               big-endian;
  44 };
  45
  46 Scenario 3 : CPU in BE mode & device in BE mode.
  47 dev: dev@40031000 {
  48               compatible = "name";
  49               reg = <0x40031000 0x1000>;
  50               ...
  51               native-endian;
  52 };
  53
  54 Scenario 4 : CPU in BE mode & device in LE mode.
  55 dev: dev@40031000 {
  56               compatible = "name";
  57               reg = <0x40031000 0x1000>;
  58               ...
  59               little-endian;
  60 };