xtensa: make fixmap region addressing grow with index

It's much easier to reason about alignment and coloring of regions
located in the fixmap when fixmap index is just a PFN within the fixmap
region. Change fixmap addressing so that index 0 corresponds to
FIXADDR_START instead of the FIXADDR_TOP.

Signed-off-by: Max Filippov <[e-mail]>

arch/xtensa/include/asm/fixmap.h

···
  * Here we define all the compile-time 'special' virtual
  * addresses. The point is to have a constant address at
  * compile time, but to set the physical address only
- * in the boot process. We allocate these special  addresses
- * from the end of the consistent memory region backwards.
+ * in the boot process. We allocate these special addresses
+ * from the start of the consistent memory region upwards.
  * Also this lets us do fail-safe vmalloc(), we
  * can guarantee that these special addresses and
  * vmalloc()-ed addresses never overlap.
···
 #define FIXADDR_SIZE	(__end_of_fixed_addresses << PAGE_SHIFT)
 #define FIXADDR_START	((FIXADDR_TOP - FIXADDR_SIZE) & PMD_MASK)
 
-#include <asm-generic/fixmap.h>
+#define __fix_to_virt(x)	(FIXADDR_START + ((x) << PAGE_SHIFT))
+#define __virt_to_fix(x)	(((x) - FIXADDR_START) >> PAGE_SHIFT)
+
+#ifndef __ASSEMBLY__
+/*
+ * '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)
+{
+	BUILD_BUG_ON(idx >= __end_of_fixed_addresses);
+	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
 
 #define kmap_get_fixmap_pte(vaddr) \
 	pte_offset_kernel( \

arch/xtensa/mm/highmem.c

···
 	idx = type + KM_TYPE_NR * smp_processor_id();
 	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
 #ifdef CONFIG_DEBUG_HIGHMEM
-	BUG_ON(!pte_none(*(kmap_pte - idx)));
+	BUG_ON(!pte_none(*(kmap_pte + idx)));
 #endif
-	set_pte(kmap_pte - idx, mk_pte(page, PAGE_KERNEL_EXEC));
+	set_pte(kmap_pte + idx, mk_pte(page, PAGE_KERNEL_EXEC));
 
 	return (void *)vaddr;
 }
···
 		 * is a bad idea also, in case the page changes cacheability
 		 * attributes or becomes a protected page in a hypervisor.
 		 */
-		pte_clear(&init_mm, kvaddr, kmap_pte - idx);
+		pte_clear(&init_mm, kvaddr, kmap_pte + idx);
 		local_flush_tlb_kernel_range((unsigned long)kvaddr,
 					     (unsigned long)kvaddr + PAGE_SIZE);