sh: Provide a sane valid_phys_addr_range() to prevent TLB reset with PMB.

With the PMB enabled, only P1SEG and up are covered by the PMB mappings,
meaning that situations where out-of-bounds physical addresses are read
from will lead to TLB reset after the PMB miss, allowing for use cases
like dd if=/dev/mem to reset the TLB.

Fix this up to make sure the reference is between __MEMORY_START (phys)
and __pa(high_memory). This is coherent across all variants of sh/sh64
with and without MMU, though the PMB bug itself is only applicable to
SH-4A parts.

Reported-by: Hideo Saito <saito@densan.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/mm/mmap.c

@@ -0,0 +1,31 @@
+/*
+ * arch/sh/mm/mmap.c
+ *
+ * Copyright (C) 2008  Paul Mundt
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ */
+#include <linux/io.h>
+#include <linux/mm.h>
+#include <asm/page.h>
+
+/*
+ * You really shouldn't be using read() or write() on /dev/mem.  This
+ * might go away in the future.
+ */
+int valid_phys_addr_range(unsigned long addr, size_t count)
+{
+	if (addr < (PAGE_OFFSET + (PFN_START << PAGE_SHIFT)))
+		return 0;
+	if (addr + count > __pa(high_memory))
+		return 0;
+
+	return 1;
+}
+
+int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
+{
+	return 1;
+}