[PATCH RFC 2/2] ACPI/IORT: work around num_ids ambiguity

[Ard Biesheuvel <ardb@xxxxxxxxxx>]

The ID mapping table structure of the IORT table describes the size of
a range using a num_ids field carrying the number of IDs in the region
minus one. This has been misinterpreted in the past in the parsing code,
and firmware is known to have shipped where this results in an ambiguity,
where regions that should be adjacent have an overlap of one value.

So let's work around this by detecting this case specifically: when
resolving an ID translation, allow one that matches right at the end of
a multi-ID region to be superseded by a subsequent one.

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 drivers/acpi/arm64/iort.c | 23 +++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
index 98be18266a73..d826dd9dc4c5 100644
--- a/drivers/acpi/arm64/iort.c
+++ b/drivers/acpi/arm64/iort.c
@@ -316,10 +316,19 @@ static int iort_id_map(struct acpi_iort_id_mapping *map, u8 type, u32 rid_in,
 	}
 
 	if (rid_in < map->input_base ||
-	    (rid_in >= map->input_base + map->id_count))
+	    (rid_in > map->input_base + map->id_count))
 		return -ENXIO;
 
 	*rid_out = map->output_base + (rid_in - map->input_base);
+
+	/*
+	 * Due to confusion regarding the meaning of the id_count field (which
+	 * carries the number of IDs *minus 1*), we may have to disregard this
+	 * match if it is at the end of the range, and overlaps with the start
+	 * of another one.
+	 */
+	if (map->id_count > 0 && rid_in == map->input_base + map->id_count)
+		return -EAGAIN;
 	return 0;
 }
 
@@ -404,7 +413,8 @@ static struct acpi_iort_node *iort_node_map_id(struct acpi_iort_node *node,
 	/* Parse the ID mapping tree to find specified node type */
 	while (node) {
 		struct acpi_iort_id_mapping *map;
-		int i, index;
+		int i, index, rc = 0;
+		u32 out_ref = 0, map_id = id;
 
 		if (IORT_TYPE_MASK(node->type) & type_mask) {
 			if (id_out)
@@ -438,15 +448,18 @@ static struct acpi_iort_node *iort_node_map_id(struct acpi_iort_node *node,
 			if (i == index)
 				continue;
 
-			if (!iort_id_map(map, node->type, id, &id))
+			rc = iort_id_map(map, node->type, map_id, &id);
+			if (!rc)
 				break;
+			if (rc == -EAGAIN)
+				out_ref = map->output_reference;
 		}
 
-		if (i == node->mapping_count)
+		if (i == node->mapping_count && rc != -EAGAIN)
 			goto fail_map;
 
 		node = ACPI_ADD_PTR(struct acpi_iort_node, iort_table,
-				    map->output_reference);
+				    rc ? out_ref : map->output_reference);
 	}
 
 fail_map:
-- 
2.17.1