[PATCH v3] dm table: fix and simplify discard support processing

[Mike Snitzer <snitzer@redhat.com>]

Remove 'discards_supported' from the dm_table structure.  The same
information can be easily discovered from the table's target(s) in
dm_table_supports_discards().

Before this fix dm_table_supports_discards() would skip checking the
individual targets' 'discards_supported' flag if any one target in the
table didn't set num_discard_requests > 0.  Now the per-target
'discards_supported' flag is effective at insuring the final DM device
advertises discard support.  But, to be clear, targets that don't
support discards (!num_discard_requests) will not receive discard
requests.

Also DMWARN if a target sets 'discards_supported' override but forgets
to set 'num_discard_requests'.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-table.c |   13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

v3: rebased ontop the latest editing tree

Index: linux-2.6/drivers/md/dm-table.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-table.c
+++ linux-2.6/drivers/md/dm-table.c
@@ -54,7 +54,6 @@ struct dm_table {
 	sector_t *highs;
 	struct dm_target *targets;
 
-	unsigned discards_supported:1;
 	unsigned integrity_supported:1;
 	unsigned singleton:1;
 
@@ -209,7 +208,6 @@ int dm_table_create(struct dm_table **re
 	INIT_LIST_HEAD(&t->devices);
 	INIT_LIST_HEAD(&t->target_callbacks);
 	atomic_set(&t->holders, 0);
-	t->discards_supported = 1;
 
 	if (!num_targets)
 		num_targets = KEYS_PER_NODE;
@@ -809,8 +807,9 @@ int dm_table_add_target(struct dm_table 
 
 	t->highs[t->num_targets++] = tgt->begin + tgt->len - 1;
 
-	if (!tgt->num_discard_requests)
-		t->discards_supported = 0;
+	if (!tgt->num_discard_requests && tgt->discards_supported)
+		DMWARN("%s: %s: must set num_discard_requests if discards_supported, "
+		       "disabling discards", dm_device_name(t->md), type);
 
 	return 0;
 
@@ -1438,9 +1437,6 @@ bool dm_table_supports_discards(struct d
 	struct dm_target *ti;
 	unsigned i = 0;
 
-	if (!t->discards_supported)
-		return 0;
-
 	/*
 	 * Unless any target used by the table set discards_supported,
 	 * require at least one underlying device to support discards.
@@ -1451,6 +1447,9 @@ bool dm_table_supports_discards(struct d
 	while (i < dm_table_get_num_targets(t)) {
 		ti = dm_table_get_target(t, i++);
 
+		if (!ti->num_discard_requests)
+			continue;
+
 		if (ti->discards_supported)
 			return 1;