Since the ioremap operation is verifying that the specified address range
is NOT RAM, it will search the entire ioresource list if the condition
is true.  To make matters worse, it does this one 4k page at a time.
For a 128M BAR region this is 32 passes to determine the entire region
does not contain any RAM addresses.

This patch provides another resource lookup function, region_is_ram,
that searches for the entire region specified, verifying that it is
completely contained within the resource region.  If it is found, then
it is checked to be RAM or not, within a single pass.

The return result reflects if it was found or not (-1), and whether it is
RAM (1) or not (0).  This allows the caller to fallback to the previous
page by page search if it was not found.

...

--- linux.orig/kernel/resource.c
+++ linux/kernel/resource.c

@@ -494,6 +494,43 @@ int __weak page_is_ram(unsigned long pfn
 }
 EXPORT_SYMBOL_GPL(page_is_ram);
 
+/*
+ * Search for a resouce entry that fully contains the specified region.
+ * If found, return 1 if it is RAM, 0 if not.
+ * If not found, or region is not fully contained, return -1
+ *
+ * Used by the ioremap functions to insure user not remapping RAM and is as
+ * vast speed up over walking through the resource table page by page.
+ */
+int __weak region_is_ram(resource_size_t start, unsigned long size)
+{
+	struct resource *p;
+	resource_size_t end = start + size - 1;
+	int flags = IORESOURCE_MEM | IORESOURCE_BUSY;
+	const char *name = "System RAM";
+	int ret = -1;
+
+	read_lock(&resource_lock);
+	for (p = iomem_resource.child; p ; p = p->sibling) {
+		if (end < p->start)
+			continue;
+
+		if (p->start <= start && end <= p->end) {
+			/* resource fully contains region */
+			if ((p->flags != flags) || strcmp(p->name, name))
+				ret = 0;
+			else
+				ret = 1;
+			break;
+		}
+		if (p->end < start)
+			break;	/* not found */
+	}
+	read_unlock(&resource_lock);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(region_is_ram);