Create API to map between CPU physical and bus addresses

On some SoCs, DMA-capable peripherals see a different address space to
the CPU's physical address space. Create an API to allow platform-agnostic
drivers to convert between the two address spaces when programming DMA
operations.

This API will exist on all platforms, but will have a dummy implementation
when this feature is not required. Other platforms will enable
CONFIG_PHYS_TO_BUS and provide the required implementation.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>

include/phys2bus.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2015 Stephen Warren
+ *
+ * SPDX-License-Identifier:     GPL-2.0+
+ */
+
+#ifndef _BUS_ADDR_H
+#define _BUS_ADDR_H
+
+#ifdef CONFIG_PHYS_TO_BUS
+unsigned long phys_to_bus(unsigned long phys);
+unsigned long bus_to_phys(unsigned long bus);
+#else
+static inline unsigned long phys_to_bus(unsigned long phys)
+{
+	return phys;
+}
+
+static inline unsigned long bus_to_phys(unsigned long bus)
+{
+	return bus;
+}
+#endif
+
+#endif