of: Add helper function to check MMIO register endianness

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.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/base.c

@@ -567,6 +567,29 @@ 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

include/linux/of.h

@@ -305,6 +305,7 @@ 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);
@@ -466,6 +467,11 @@ 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)