From 65a71007a20cfe7ebd456d72c0bb155fe42de963 Mon Sep 17 00:00:00 2001 From: Kevin Cernekee Date: Thu, 9 Apr 2015 13:05:16 -0700 Subject: of: Document {little,big,native}-endian bindings These apply to newly converted drivers, like serial8250/libahci/... The examples were adapted from the regmap bindings document. Signed-off-by: Kevin Cernekee Reviewed-by: Peter Hurley Acked-by: Greg Kroah-Hartman Signed-off-by: Rob Herring --- .../devicetree/bindings/common-properties.txt | 60 ++++++++++++++++++++++ 1 file changed, 60 insertions(+) create mode 100644 Documentation/devicetree/bindings/common-properties.txt (limited to 'Documentation/devicetree/bindings/common-properties.txt') diff --git a/Documentation/devicetree/bindings/common-properties.txt b/Documentation/devicetree/bindings/common-properties.txt new file mode 100644 index 000000000000..3193979b1d05 --- /dev/null +++ b/Documentation/devicetree/bindings/common-properties.txt @@ -0,0 +1,60 @@ +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; +}; -- cgit v1.2.3