[PATCH v3 1/2] virtio-blk: Don't use MAX_DISCARD_SEGMENTS if max_discard_seg is zero

[Xie Yongji <xieyongji@bytedance.com>]

Currently the value of max_discard_segment will be set to
MAX_DISCARD_SEGMENTS (256) with no basis in hardware if device
set 0 to max_discard_seg in configuration space. It's incorrect
since the device might not be able to handle such large descriptors.
To fix it, let's follow max_segments restrictions in this case.

Fixes: 1f23816b8eb8 ("virtio_blk: add discard and write zeroes support")
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
---
 drivers/block/virtio_blk.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c443cd64fc9b..7fc2c8b97077 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -925,9 +925,15 @@ static int virtblk_probe(struct virtio_device *vdev)
 
 		virtio_cread(vdev, struct virtio_blk_config, max_discard_seg,
 			     &v);
+
+		/*
+		 * max_discard_seg == 0 is out of spec but we always
+		 * handled it.
+		 */
+		if (!v)
+			v = sg_elems - 2;
 		blk_queue_max_discard_segments(q,
-					       min_not_zero(v,
-							    MAX_DISCARD_SEGMENTS));
+					       min(v, MAX_DISCARD_SEGMENTS));
 
 		blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
 	}
-- 
2.20.1