[ARM] fixmap support

This is the minimum fixmap interface expected to be implemented by
architectures supporting highmem.

We have a second level page table already allocated and covering
0xfff00000-0xffffffff because the exception vector page is located
at 0xffff0000, and various cache tricks already use some entries above
0xffff0000. Therefore the PTEs covering 0xfff00000-0xfffeffff are free
to be used.

However the XScale cache flushing code already uses virtual addresses
between 0xfffe0000 and 0xfffeffff.

So this reserves the 0xfff00000-0xfffdffff range for fixmap stuff.

The Documentation/arm/memory.txt information is updated accordingly,
including the information about the actual top of DMA memory mapping
region which didn't match the code.

Signed-off-by: Nicolas Pitre <[e-mail]>

Documentation/arm/memory.txt

···
 				CPU supports vector relocation (control
 				register V bit.)
 
-ffc00000	fffeffff	DMA memory mapping region.  Memory returned
+fffe0000	fffeffff	XScale cache flush area.  This is used
+				in proc-xscale.S to flush the whole data
+				cache.  Free for other usage on non-XScale.
+
+fff00000	fffdffff	Fixmap mapping region.  Addresses provided
+				by fix_to_virt() will be located here.
+
+ffc00000	ffefffff	DMA memory mapping region.  Memory returned
 				by the dma_alloc_xxx functions will be
 				dynamically mapped here.
 

arch/arm/include/asm/fixmap.h

···
+#ifndef _ASM_FIXMAP_H
+#define _ASM_FIXMAP_H
+
+/*
+ * Nothing too fancy for now.
+ *
+ * On ARM we already have well known fixed virtual addresses imposed by
+ * the architecture such as the vector page which is located at 0xffff0000,
+ * therefore a second level page table is already allocated covering
+ * 0xfff00000 upwards.
+ *
+ * The cache flushing code in proc-xscale.S uses the virtual area between
+ * 0xfffe0000 and 0xfffeffff.
+ */
+
+#define FIXADDR_START		0xfff00000UL
+#define FIXADDR_TOP		0xfffe0000UL
+#define FIXADDR_SIZE		(FIXADDR_TOP - FIXADDR_START)
+
+#define FIX_KMAP_BEGIN		0
+#define FIX_KMAP_END		(FIXADDR_SIZE >> PAGE_SHIFT)
+
+#define __fix_to_virt(x)	(FIXADDR_START + ((x) << PAGE_SHIFT))
+#define __virt_to_fix(x)	(((x) - FIXADDR_START) >> PAGE_SHIFT)
+
+extern void __this_fixmap_does_not_exist(void);
+
+static inline unsigned long fix_to_virt(const unsigned int idx)
+{
+	if (idx >= FIX_KMAP_END)
+		__this_fixmap_does_not_exist();
+	return __fix_to_virt(idx);
+}
+
+static inline unsigned int virt_to_fix(const unsigned long vaddr)
+{
+	BUG_ON(vaddr >= FIXADDR_TOP || vaddr < FIXADDR_START);
+	return __virt_to_fix(vaddr);
+}
+
+#endif

arch/arm/mm/mm.h

···
-/* the upper-most page table pointer */
-
 #ifdef CONFIG_MMU
 
+/* the upper-most page table pointer */
 extern pmd_t *top_pmd;
 
 #define TOP_PTE(x)	pte_offset_kernel(top_pmd, x)