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Diffstat (limited to 'Documentation/devicetree/bindings/common-properties.txt')
| -rw-r--r-- | Documentation/devicetree/bindings/common-properties.txt | 43 |
1 files changed, 34 insertions, 9 deletions
diff --git a/Documentation/devicetree/bindings/common-properties.txt b/Documentation/devicetree/bindings/common-properties.txt index 697714f8d75c..98a28130e100 100644 --- a/Documentation/devicetree/bindings/common-properties.txt +++ b/Documentation/devicetree/bindings/common-properties.txt @@ -1,30 +1,33 @@ Common properties +================= + +Endianness +---------- The Devicetree Specification does not define any properties related to hardware -byteswapping, but endianness issues show up frequently in porting Linux to +byte swapping, but endianness issues show up frequently in porting drivers to different machine types. This document attempts to provide a consistent -way of handling byteswapping across drivers. +way of handling byte swapping 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. + know the peripheral always needs to be accessed in big endian (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. + peripheral always needs to be accessed in little endian (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 + BE vmlinux -> ioread32be/iowrite32be). In this case no byte swaps 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. +a requirement. Some implementations assume that little-endian is +the default, because most existing (PCI-based) drivers implicitly +default to LE for their MMIO accesses. Examples: Scenario 1 : CPU in LE mode & device in LE mode. @@ -58,3 +61,25 @@ dev: dev@40031000 { ... little-endian; }; + +Daisy-chained devices +--------------------- + +Many serially-attached GPIO and IIO devices are daisy-chainable. To the +host controller, a daisy-chain appears as a single device, but the number +of inputs and outputs it provides is the sum of inputs and outputs provided +by all of its devices. The driver needs to know how many devices the +daisy-chain comprises to determine the amount of data exchanged, how many +inputs and outputs to register and so on. + +Optional properties: + - #daisy-chained-devices: Number of devices in the daisy-chain (default is 1). + +Example: +gpio@0 { + compatible = "name"; + reg = <0>; + gpio-controller; + #gpio-cells = <2>; + #daisy-chained-devices = <3>; +}; |