[PATCH v3 3/6] block: Fix the DMA segment boundary mask check

[Bart Van Assche <bvanassche@acm.org>]

Commit d690cb8ae14b ("block: add an API to atomically update queue limits")
introduced the following code:

       /*
        * By default there is no limit on the segment boundary alignment,
        * but if there is one it can't be smaller than the page size as
        * that would break all the normal I/O patterns.
        */
       if (!lim->seg_boundary_mask)
               lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK;
       if (WARN_ON_ONCE(lim->seg_boundary_mask < PAGE_SIZE - 1))
               return -EINVAL;

The comment about "breaking normal I/O patterns" is no longer correct
since the block layer now supports DMA segments smaller than the page size.
Modify the check such that it still passes for all current block drivers.
The qedi iSCSI driver is an example of a driver that sets
.seg_boundary_mask to 0xfff.

This patch prepares for reducing the value of BLK_MIN_SEGMENT_SIZE.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-settings.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index 56017098d2c7..e900405d0cc3 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -450,14 +450,14 @@ int blk_validate_limits(struct queue_limits *lim)
 
 	/*
 	 * By default there is no limit on the segment boundary alignment,
-	 * but if there is one it can't be smaller than the page size as
-	 * that would break all the normal I/O patterns.
+	 * but if there is one, check that it allows at least 4 KiB of data to
+	 * be submitted at once. All known block device DMA controllers support
+	 * 4 KiB DMA segments that do not cross 4 KiB boundaries.
 	 */
 	if (!lim->seg_boundary_mask)
 		lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK;
-	if (WARN_ON_ONCE(lim->seg_boundary_mask < BLK_MIN_SEGMENT_SIZE - 1))
+	if (WARN_ON_ONCE(lim->seg_boundary_mask < SZ_4K - 1))
 		return -EINVAL;
-
 	/*
 	 * Stacking device may have both virtual boundary and max segment
 	 * size limit, so allow this setting now, and long-term the two