Re: [PATCH V2 05/10] of: Add helper function to check MMIO register endianness

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On Wed, 12 Nov 2014 12:54:02 -0800
, Kevin Cernekee <cernekee@xxxxxxxxx>
 wrote:
> SoC peripherals can come in several different flavors:
> 
>  - little-endian: registers always need to be accessed in LE mode (so the
>    kernel should perform a swap if the CPU is running BE)
> 
>  - big-endian: registers always need to be accessed in BE mode (so the
>    kernel should perform a swap if the CPU is running LE)
> 
>  - native-endian: the bus will automatically swap accesses, so the kernel
>    should never swap
> Introduce a function that checks an OF device node to see whether it
> contains a "big-endian" or "native-endian" property.  For the former case,
> always return true.  For the latter case, return true iff the kernel was
> built for BE (implying that the BE MMIO accessors do not perform a swap).
> Otherwise return false, assuming LE registers.
> 
> LE registers are assumed by default because most existing drivers (libahci,
> serial8250, usb) always use readl/writel in the absence of instructions
> to the contrary, so that will be our fallback.

This is proposing a new binding, or at least a common pattern to be used
by other bindings. It should be documented under
Documentation/devicetree/bindings/. I suggest creating a new file under
bindings/common-properties.txt

Otherwise looks fine to me. We'll give it a week to see if there are any
other comments, but I think this looks fine.

g.

> 
> Signed-off-by: Kevin Cernekee <cernekee@xxxxxxxxx>
> ---
>  drivers/of/base.c  | 23 +++++++++++++++++++++++
>  include/linux/of.h |  6 ++++++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/drivers/of/base.c b/drivers/of/base.c
> index 81c095f..35d95a1 100644
> --- a/drivers/of/base.c
> +++ b/drivers/of/base.c
> @@ -552,6 +552,29 @@ bool of_device_is_available(const struct device_node *device)
>  EXPORT_SYMBOL(of_device_is_available);
>  
>  /**
> + *  of_device_is_big_endian - check if a device has BE registers
> + *
> + *  @device: Node to check for endianness
> + *
> + *  Returns true if the device has a "big-endian" property, or if the kernel
> + *  was compiled for BE *and* the device has a "native-endian" property.
> + *  Returns false otherwise.
> + *
> + *  Callers would nominally use ioread32be/iowrite32be if
> + *  of_device_is_big_endian() == true, or readl/writel otherwise.
> + */
> +bool of_device_is_big_endian(const struct device_node *device)
> +{
> +	if (of_property_read_bool(device, "big-endian"))
> +		return true;
> +	if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN) &&
> +	    of_property_read_bool(device, "native-endian"))
> +		return true;
> +	return false;
> +}
> +EXPORT_SYMBOL(of_device_is_big_endian);
> +
> +/**
>   *	of_get_parent - Get a node's parent if any
>   *	@node:	Node to get parent
>   *
> diff --git a/include/linux/of.h b/include/linux/of.h
> index 7aaaa59..fc70b01 100644
> --- a/include/linux/of.h
> +++ b/include/linux/of.h
> @@ -276,6 +276,7 @@ extern int of_property_read_string_helper(struct device_node *np,
>  extern int of_device_is_compatible(const struct device_node *device,
>  				   const char *);
>  extern bool of_device_is_available(const struct device_node *device);
> +extern bool of_device_is_big_endian(const struct device_node *device);
>  extern const void *of_get_property(const struct device_node *node,
>  				const char *name,
>  				int *lenp);
> @@ -431,6 +432,11 @@ static inline bool of_device_is_available(const struct device_node *device)
>  	return false;
>  }
>  
> +static inline bool of_device_is_big_endian(const struct device_node *device)
> +{
> +	return false;
> +}
> +
>  static inline struct property *of_find_property(const struct device_node *np,
>  						const char *name,
>  						int *lenp)
> -- 
> 2.1.1
> 

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