[PATCH RFC 3/4] block: Support atomic writes limits for stacked devices

[John Garry]

Allow stacked devices to support atomic writes by aggregating the minimum
capacility of all bottom devices.

If a bottom device does not support atomic writes, then
BLK_FEAT_ATOMIC_WRITES should be cleared for that device, and the top
device then will also not support atomic writes.

Signed-off-by: John Garry <john.g.garry at oracle.com>
---
 block/blk-settings.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index 036e67f73116..aeb05fb24801 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -682,6 +682,25 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 		t->zone_write_granularity = 0;
 		t->max_zone_append_sectors = 0;
 	}
+	if (!(b->features & BLK_FEAT_ATOMIC_WRITES)) {
+		t->atomic_write_hw_max = 0;
+		t->atomic_write_hw_unit_max = 0;
+		t->atomic_write_hw_unit_min = 0;
+		t->atomic_write_hw_boundary = 0;
+		t->features &= ~BLK_FEAT_ATOMIC_WRITES;
+	} else if (t->features & BLK_FEAT_ATOMIC_WRITES) {
+		t->atomic_write_hw_max = min_not_zero(t->atomic_write_hw_max,
+						b->atomic_write_hw_max);
+		t->atomic_write_boundary_sectors =
+					min_not_zero(t->atomic_write_boundary_sectors,
+						b->atomic_write_boundary_sectors);
+		t->atomic_write_hw_unit_min = max(t->atomic_write_hw_unit_min,
+						b->atomic_write_hw_unit_min);
+		t->atomic_write_hw_unit_max =
+					min_not_zero(t->atomic_write_hw_unit_max,
+						b->atomic_write_hw_unit_max);
+	}
+
 	return ret;
 }
 EXPORT_SYMBOL(blk_stack_limits);
-- 
2.31.1