linux/arch/arm/include/asm/swab.h

/*
 *  arch/arm/include/asm/byteorder.h
 *
 * ARM Endian-ness.  In little endian mode, the data bus is connected such
 * that byte accesses appear as:
 *  0 = d0...d7, 1 = d8...d15, 2 = d16...d23, 3 = d24...d31
 * and word accesses (data or instruction) appear as:
 *  d0...d31
 *
 * When in big endian mode, byte accesses appear as:
 *  0 = d24...d31, 1 = d16...d23, 2 = d8...d15, 3 = d0...d7
 * and word accesses (data or instruction) appear as:
 *  d0...d31
 */
#ifndef __ASM_ARM_SWAB_H
#define __ASM_ARM_SWAB_H

#include <linux/compiler.h>
#include <linux/types.h>

#if !defined(__STRICT_ANSI__) || defined(__KERNEL__)
#  define __SWAB_64_THRU_32__
#endif

#if defined(__KERNEL__) && __LINUX_ARM_ARCH__ >= 6

static inline __attribute_const__ __u16 __arch_swab16(__u16 x)
{
	__asm__ ("rev16 %0, %1" : "=r" (x) : "r" (x));
	return x;
}
#define __arch_swab16 __arch_swab16

static inline __attribute_const__ __u32 __arch_swab32(__u32 x)
{
	__asm__ ("rev %0, %1" : "=r" (x) : "r" (x));
	return x;
}
#define __arch_swab32 __arch_swab32

#else

static inline __attribute_const__ __u32 __arch_swab32(__u32 x)
{
	__u32 t;

#ifndef __thumb__
	if (!__builtin_constant_p(x)) {
		/*
		 * The compiler needs a bit of a hint here to always do the
		 * right thing and not screw it up to different degrees
		 * depending on the gcc version.
		 */
		asm ("eor\t%0, %1, %1, ror #16" : "=r" (t) : "r" (x));
	} else
#endif
		t = x ^ ((x << 16) | (x >> 16)); /* eor r1,r0,r0,ror #16 */

	x = (x << 24) | (x >> 8);		/* mov r0,r0,ror #8      */
	t &= ~0x00FF0000;			/* bic r1,r1,#0x00FF0000 */
	x ^= (t >> 8);				/* eor r0,r0,r1,lsr #8   */

	return x;
}
#define __arch_swab32 __arch_swab32

#endif

#endif