linux/include/asm-x86/fixmap.h

#ifndef _ASM_FIXMAP_H
#define _ASM_FIXMAP_H

#ifdef CONFIG_X86_32
# include "fixmap_32.h"
#else
# include "fixmap_64.h"
#endif

extern int fixmaps_set;

extern void __set_fixmap(enum fixed_addresses idx,
			 unsigned long phys, pgprot_t flags);
#define set_fixmap(idx, phys)				\
	__set_fixmap(idx, phys, PAGE_KERNEL)

/*
 * Some hardware wants to get fixmapped without caching.
 */
#define set_fixmap_nocache(idx, phys)			\
	__set_fixmap(idx, phys, PAGE_KERNEL_NOCACHE)

#define clear_fixmap(idx)			\
	__set_fixmap(idx, 0, __pgprot(0))

#define __fix_to_virt(x)	(FIXADDR_TOP - ((x) << PAGE_SHIFT))
#define __virt_to_fix(x)	((FIXADDR_TOP - ((x)&PAGE_MASK)) >> PAGE_SHIFT)

extern void __this_fixmap_does_not_exist(void);

/*
 * 'index to address' translation. If anyone tries to use the idx
 * directly without translation, we catch the bug with a NULL-deference
 * kernel oops. Illegal ranges of incoming indices are caught too.
 */
static __always_inline unsigned long fix_to_virt(const unsigned int idx)
{
	/*
	 * this branch gets completely eliminated after inlining,
	 * except when someone tries to use fixaddr indices in an
	 * illegal way. (such as mixing up address types or using
	 * out-of-range indices).
	 *
	 * If it doesn't get removed, the linker will complain
	 * loudly with a reasonably clear error message..
	 */
	if (idx >= __end_of_fixed_addresses)
		__this_fixmap_does_not_exist();

	return __fix_to_virt(idx);
}

static inline unsigned long virt_to_fix(const unsigned long vaddr)
{
	BUG_ON(vaddr >= FIXADDR_TOP || vaddr < FIXADDR_START);
	return __virt_to_fix(vaddr);
}
#endif