61 lines
2.1 KiB
Plaintext
61 lines
2.1 KiB
Plaintext
|
Common properties
|
||
|
|
|
||
|
|
The ePAPR specification does not define any properties related to hardware
|
||
|
|
byteswapping, but endianness issues show up frequently in porting Linux to
|
||
|
|
different machine types. This document attempts to provide a consistent
|
||
|
|
way of handling byteswapping 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 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 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 byteswaps
|
||
|
|
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. The of_device_is_big_endian() and of_fdt_is_big_endian()
|
||
|
|
helper functions do assume that little-endian is the default, because
|
||
|
|
most existing (PCI-based) drivers implicitly default to LE by using
|
||
|
|
readl/writel for 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;
|
||
|
|
};
|