On 11/24/2014 06:36 PM, Kevin Cernekee wrote:
> These apply to newly converted drivers, like serial8250/libahci/...
> The examples were adapted from the regmap bindings document.
>
> Signed-off-by: Kevin Cernekee <cernekee@xxxxxxxxx>
> ---
> .../devicetree/bindings/common-properties.txt | 60 ++++++++++++++++++++++
> 1 file changed, 60 insertions(+)
> create mode 100644 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 0000000..21044a4
> --- /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. This is the
> + default.
There is a fundamental problem with specifying the default in DT bindings.
How can drivers which are currently native-endian support big-endian?
If the driver is converted to support big-endian, every previous
devicetree will be invalid with the new kernel (because those devicetrees
don't specify 'native-endian').
IOW, consider if the default were 'native-endian'. How would the 8250
driver support existing devicetrees?
Regards,
Peter Hurley
> + - 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.
> +
> +Note that regmap, in contrast, defaults to native-endian. But this
> +document is targeted for existing drivers, most of which currently use
> +readl/writel because they expect to be accessing PCI/PCIe devices rather
> +than memory-mapped SoC peripherals. Since the readl/writel accessors
> +perform a byteswap on BE systems, this means that the drivers in question
> +are implicitly "little-endian".
> +
> +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;
> +};
>
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