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253a41c6fb
We shouldn't reference Linux kernel functions or Linux itself in proper bindings. It's OK to reference functions in the kernel when explaining examples, but otherwise we shouldn't reference functions to describe what the binding means. Cc: Hsin-Yi Wang <hsinyi@chromium.org> Signed-off-by: Stephen Boyd <swboyd@chromium.org> Signed-off-by: Rob Herring <robh@kernel.org>
86 lines
2.8 KiB
Plaintext
86 lines
2.8 KiB
Plaintext
Common properties
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=================
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Endianness
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----------
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The Devicetree Specification does not define any properties related to hardware
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byte swapping, but endianness issues show up frequently in porting drivers to
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different machine types. This document attempts to provide a consistent
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way of handling byte swapping across drivers.
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Optional properties:
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- big-endian: Boolean; force big endian register accesses
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unconditionally (e.g. ioread32be/iowrite32be). Use this if you
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know the peripheral always needs to be accessed in big endian (BE) mode.
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- little-endian: Boolean; force little endian register accesses
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unconditionally (e.g. readl/writel). Use this if you know the
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peripheral always needs to be accessed in little endian (LE) mode.
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- native-endian: Boolean; always use register accesses matched to the
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endianness of the kernel binary (e.g. LE vmlinux -> readl/writel,
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BE vmlinux -> ioread32be/iowrite32be). In this case no byte swaps
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will ever be performed. Use this if the hardware "self-adjusts"
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register endianness based on the CPU's configured endianness.
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If a binding supports these properties, then the binding should also
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specify the default behavior if none of these properties are present.
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In such cases, little-endian is the preferred default, but it is not
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a requirement. Some implementations assume that little-endian is
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the default, because most existing (PCI-based) drivers implicitly
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default to LE for their MMIO accesses.
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Examples:
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Scenario 1 : CPU in LE mode & device in LE mode.
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dev: dev@40031000 {
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compatible = "name";
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reg = <0x40031000 0x1000>;
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...
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native-endian;
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};
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Scenario 2 : CPU in LE mode & device in BE mode.
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dev: dev@40031000 {
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compatible = "name";
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reg = <0x40031000 0x1000>;
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...
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big-endian;
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};
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Scenario 3 : CPU in BE mode & device in BE mode.
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dev: dev@40031000 {
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compatible = "name";
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reg = <0x40031000 0x1000>;
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...
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native-endian;
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};
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Scenario 4 : CPU in BE mode & device in LE mode.
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dev: dev@40031000 {
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compatible = "name";
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reg = <0x40031000 0x1000>;
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...
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little-endian;
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};
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Daisy-chained devices
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---------------------
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Many serially-attached GPIO and IIO devices are daisy-chainable. To the
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host controller, a daisy-chain appears as a single device, but the number
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of inputs and outputs it provides is the sum of inputs and outputs provided
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by all of its devices. The driver needs to know how many devices the
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daisy-chain comprises to determine the amount of data exchanged, how many
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inputs and outputs to register and so on.
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Optional properties:
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- #daisy-chained-devices: Number of devices in the daisy-chain (default is 1).
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Example:
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gpio@0 {
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compatible = "name";
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reg = <0>;
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gpio-controller;
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#gpio-cells = <2>;
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#daisy-chained-devices = <3>;
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};
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