dm error: add discard support

Add io_err_io_hints() and set discard limits so that the zero target
advertises support for discards.

The error target will return -EIO for discards.

This is useful when the user combines dm-error with other
discard-supporting targets in the same table; without dm-error
support, discards would be disabled for the whole combined device.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-target.c

@@ -119,6 +119,7 @@ static int io_err_ctr(struct dm_target *tt, unsigned int argc, char **args)
 	 * Return error for discards instead of -EOPNOTSUPP
 	 */
 	tt->num_discard_bios = 1;
+	tt->discards_supported = true;
 
 	return 0;
 }
@@ -145,6 +146,13 @@ static void io_err_release_clone_rq(struct request *clone,
 {
 }
 
+static void io_err_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	limits->max_discard_sectors = UINT_MAX;
+	limits->max_hw_discard_sectors = UINT_MAX;
+	limits->discard_granularity = 512;
+}
+
 static long io_err_dax_direct_access(struct dm_target *ti, pgoff_t pgoff,
 		long nr_pages, enum dax_access_mode mode, void **kaddr,
 		pfn_t *pfn)
@@ -154,13 +162,14 @@ static long io_err_dax_direct_access(struct dm_target *ti, pgoff_t pgoff,
 
 static struct target_type error_target = {
 	.name = "error",
-	.version = {1, 5, 0},
+	.version = {1, 6, 0},
 	.features = DM_TARGET_WILDCARD,
 	.ctr  = io_err_ctr,
 	.dtr  = io_err_dtr,
 	.map  = io_err_map,
 	.clone_and_map_rq = io_err_clone_and_map_rq,
 	.release_clone_rq = io_err_release_clone_rq,
+	.io_hints = io_err_io_hints,
 	.direct_access = io_err_dax_direct_access,
 };