ARM: kexec: fix crashkernel= handling

When the kernel crashkernel parameter is specified with just a size, we
are supposed to allocate a region from RAM to store the crashkernel.
However, ARM merely reserves physical address zero with no checking that
there is even RAM there.

Fix this by lifting similar code from x86, importing it to ARM with the
ARM specific parameters added.  In the absence of any platform specific
information, we allocate the crashkernel region from the first 512MB of
physical memory.

Update the kdump documentation to reflect this change.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Pratyush Anand <panand@redhat.com>

arch/arm/kernel/setup.c

@@ -938,6 +938,13 @@ static int __init init_machine_late(void)
 late_initcall(init_machine_late);
 
 #ifdef CONFIG_KEXEC
+/*
+ * The crash region must be aligned to 128MB to avoid
+ * zImage relocating below the reserved region.
+ */
+#define CRASH_ALIGN	(128 << 20)
+#define CRASH_ADDR_MAX	(PHYS_OFFSET + (512 << 20))
+
 static inline unsigned long long get_total_mem(void)
 {
 	unsigned long total;
@@ -965,6 +972,28 @@ static void __init reserve_crashkernel(void)
 	if (ret)
 		return;
 
+	if (crash_base <= 0) {
+		unsigned long long crash_max = CRASH_ADDR_MAX;
+		if (crash_max > (u32)~0)
+			crash_max = (u32)~0;
+		crash_base = memblock_find_in_range(CRASH_ALIGN, crash_max,
+						    crash_size, CRASH_ALIGN);
+		if (!crash_base) {
+			pr_err("crashkernel reservation failed - No suitable area found.\n");
+			return;
+		}
+	} else {
+		unsigned long long start;
+
+		start = memblock_find_in_range(crash_base,
+					       crash_base + crash_size,
+					       crash_size, SECTION_SIZE);
+		if (start != crash_base) {
+			pr_err("crashkernel reservation failed - memory is in use.\n");
+			return;
+		}
+	}
+
 	ret = memblock_reserve(crash_base, crash_size);
 	if (ret < 0) {
 		pr_warn("crashkernel reservation failed - memory is in use (0x%lx)\n",