Reinstate "resource: avoid unnecessary lookups in find_next_iomem_res()"

Commit 97523a4 ("kernel/resource: remove first_lvl / siblings_only
logic") removed an optimization introduced by commit 7563987
("resource: avoid unnecessary lookups in find_next_iomem_res()").  That
was not called out in the message of the first commit explicitly so it's
not entirely clear whether removing the optimization happened
inadvertently or not.

As the original commit message of the optimization explains there is no
point considering the children of a subtree in find_next_iomem_res() if
the top level range does not match.

Reinstating the optimization results in performance improvements in
systems where /proc/iomem is ~5k lines long.  Calling mmap() on /dev/mem
in such platforms takes 700-1500μs without the optimisation and 10-50μs
with the optimisation.

Note that even though commit 97523a4 removed the 'sibling_only'
parameter from next_resource(), newer kernels have basically reinstated it
under the name 'skip_children'.

Link: https://lore.kernel.org/all/20251124165349.3377826-1-ilstam@amazon.com/T/#u
Fixes: 97523a4 ("kernel/resource: remove first_lvl / siblings_only logic")
Signed-off-by: Ilias Stamatis <ilstam@amazon.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Andriy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: "Huang, Ying" <huang.ying.caritas@gmail.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ilstam

kernel/resource.c

@@ -341,6 +341,8 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 			       unsigned long flags, unsigned long desc,
 			       struct resource *res)
 {
+	/* Skip children until we find a top level range that matches */
+	bool skip_children = true;
 	struct resource *p;
 
 	if (!res)
@@ -351,7 +353,7 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 
 	read_lock(&resource_lock);
 
-	for_each_resource(&iomem_resource, p, false) {
+	for_each_resource(&iomem_resource, p, skip_children) {
 		/* If we passed the resource we are looking for, stop */
 		if (p->start > end) {
 			p = NULL;
@@ -362,6 +364,12 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 		if (p->end < start)
 			continue;
 
+		/*
+		 * We found a top level range that matches what we are looking
+		 * for. Time to start checking children too.
+		 */
+		skip_children = false;
+
 		/* Found a match, break */
 		if (is_type_match(p, flags, desc))
 			break;