1 Common properties
2 =================
3
4 Endianness
5 ----------
6
7 The Devicetree Specification does not define any properties related to hardware
8 byte swapping, but endianness issues show up frequently in porting drivers to
9 different machine types. This document attempts to provide a consistent
10 way of handling byte swapping across drivers.
11
12 Optional properties:
13 - big-endian: Boolean; force big endian register accesses
14 unconditionally (e.g. ioread32be/iowrite32be). Use this if you
15 know the peripheral always needs to be accessed in big endian (BE) mode.
16 - little-endian: Boolean; force little endian register accesses
17 unconditionally (e.g. readl/writel). Use this if you know the
18 peripheral always needs to be accessed in little endian (LE) mode.
19 - native-endian: Boolean; always use register accesses matched to the
20 endianness of the kernel binary (e.g. LE vmlinux -> readl/writel,
21 BE vmlinux -> ioread32be/iowrite32be). In this case no byte swaps
22 will ever be performed. Use this if the hardware "self-adjusts"
23 register endianness based on the CPU's configured endianness.
24
25 If a binding supports these properties, then the binding should also
26 specify the default behavior if none of these properties are present.
27 In such cases, little-endian is the preferred default, but it is not
28 a requirement. Some implementations assume that little-endian is
29 the default, because most existing (PCI-based) drivers implicitly
30 default to LE for their MMIO accesses.
31
32 Examples:
33 Scenario 1 : CPU in LE mode & device in LE mode.
34 dev: dev@40031000 {
35 compatible = "name";
36 reg = <0x40031000 0x1000>;
37 ...
38 native-endian;
39 };
40
41 Scenario 2 : CPU in LE mode & device in BE mode.
42 dev: dev@40031000 {
43 compatible = "name";
44 reg = <0x40031000 0x1000>;
45 ...
46 big-endian;
47 };
48
49 Scenario 3 : CPU in BE mode & device in BE mode.
50 dev: dev@40031000 {
51 compatible = "name";
52 reg = <0x40031000 0x1000>;
53 ...
54 native-endian;
55 };
56
57 Scenario 4 : CPU in BE mode & device in LE mode.
58 dev: dev@40031000 {
59 compatible = "name";
60 reg = <0x40031000 0x1000>;
61 ...
62 little-endian;
63 };
64
65 Daisy-chained devices
66 ---------------------
67
68 Many serially-attached GPIO and IIO devices are daisy-chainable. To the
69 host controller, a daisy-chain appears as a single device, but the number
70 of inputs and outputs it provides is the sum of inputs and outputs provided
71 by all of its devices. The driver needs to know how many devices the
72 daisy-chain comprises to determine the amount of data exchanged, how many
73 inputs and outputs to register and so on.
74
75 Optional properties:
76 - #daisy-chained-devices: Number of devices in the daisy-chain (default is 1).
77
78 Example:
79 gpio@0 {
80 compatible = "name";
81 reg = <0>;
82 gpio-controller;
83 #gpio-cells = <2>;
84 #daisy-chained-devices = <3>;
85 };
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