From d54eaa5a0fde0a202e4e91f200f818edcef15bee Mon Sep 17 00:00:00 2001
From: Mike Snitzer <snitzer@redhat.com>
Date: Fri, 21 Dec 2012 20:23:36 +0000
Subject: [PATCH] dm: prepare to support WRITE SAME

Allow targets to opt in to WRITE SAME support by setting
'num_write_same_requests' in the dm_target structure.

A dm device will only advertise WRITE SAME support if all its
targets and all its underlying devices support it.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
---
 drivers/md/dm-table.c         | 30 +++++++++++++++++++++++++++++-
 include/linux/device-mapper.h |  5 +++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index fa2955790031..6be58b696377 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -1414,6 +1414,33 @@ static bool dm_table_all_devices_attribute(struct dm_table *t,
 	return 1;
 }
 
+static int device_not_write_same_capable(struct dm_target *ti, struct dm_dev *dev,
+					 sector_t start, sector_t len, void *data)
+{
+	struct request_queue *q = bdev_get_queue(dev->bdev);
+
+	return q && !q->limits.max_write_same_sectors;
+}
+
+static bool dm_table_supports_write_same(struct dm_table *t)
+{
+	struct dm_target *ti;
+	unsigned i = 0;
+
+	while (i < dm_table_get_num_targets(t)) {
+		ti = dm_table_get_target(t, i++);
+
+		if (!ti->num_write_same_requests)
+			return false;
+
+		if (!ti->type->iterate_devices ||
+		    !ti->type->iterate_devices(ti, device_not_write_same_capable, NULL))
+			return false;
+	}
+
+	return true;
+}
+
 void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 			       struct queue_limits *limits)
 {
@@ -1445,7 +1472,8 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 	else
 		queue_flag_clear_unlocked(QUEUE_FLAG_NONROT, q);
 
-	q->limits.max_write_same_sectors = 0;
+	if (!dm_table_supports_write_same(t))
+		q->limits.max_write_same_sectors = 0;
 
 	dm_table_set_integrity(t);
 
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 38d27a10aa5d..d1f6cd8486f2 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -205,6 +205,11 @@ struct dm_target {
 	 */
 	unsigned num_discard_requests;
 
+	/*
+	 * The number of WRITE SAME requests that will be submitted to the target.
+	 */
+	unsigned num_write_same_requests;
+
 	/* target specific data */
 	void *private;