[PATCH RFC 2/4] block: Add BLK_FEAT_ATOMIC_WRITES flag

[John Garry <john.g.garry@xxxxxxxxxx>]

For stacking devices, not all underlying bottom devices may support atomic
writes, so a feature flag would be helpful in deciding whether the top
device may support atomic writes.

Furthermore, even if all bottom devices support atomic writes, a md/dm
personality may not.

Add flag BLK_FEAT_ATOMIC_WRITES to decide whether a block device supports
atomic writes.

Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
 block/blk-settings.c     |  3 ++-
 drivers/nvme/host/core.c |  3 +++
 drivers/scsi/sd.c        | 13 ++++++++++---
 include/linux/blkdev.h   |  3 +++
 4 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index cd8a8eabc9a5..036e67f73116 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -176,7 +176,7 @@ static void blk_validate_atomic_write_limits(struct queue_limits *lim)
 {
 	unsigned int boundary_sectors;
 
-	if (!lim->atomic_write_hw_max)
+	if (!(lim->features & BLK_FEAT_ATOMIC_WRITES) || !lim->atomic_write_hw_max)
 		goto unsupported;
 
 	boundary_sectors = lim->atomic_write_hw_boundary >> SECTOR_SHIFT;
@@ -217,6 +217,7 @@ static void blk_validate_atomic_write_limits(struct queue_limits *lim)
 	lim->atomic_write_boundary_sectors = 0;
 	lim->atomic_write_unit_min = 0;
 	lim->atomic_write_unit_max = 0;
+	lim->features &= ~BLK_FEAT_ATOMIC_WRITES;
 }
 
 /*
diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 0dc8bcc664f2..c70d8e7602bf 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1975,6 +1975,7 @@ static void nvme_update_atomic_write_disk_info(struct nvme_ns *ns,
 	lim->atomic_write_hw_boundary = boundary;
 	lim->atomic_write_hw_unit_min = bs;
 	lim->atomic_write_hw_unit_max = rounddown_pow_of_two(atomic_bs);
+	lim->features |= BLK_FEAT_ATOMIC_WRITES;
 }
 
 static u32 nvme_max_drv_segments(struct nvme_ctrl *ctrl)
@@ -2025,6 +2026,8 @@ static bool nvme_update_disk_info(struct nvme_ns *ns, struct nvme_id_ns *id,
 			atomic_bs = (1 + ns->ctrl->subsys->awupf) * bs;
 
 		nvme_update_atomic_write_disk_info(ns, id, lim, bs, atomic_bs);
+	} else {
+		lim->features &= ~BLK_FEAT_ATOMIC_WRITES;
 	}
 
 	if (id->nsfeat & NVME_NS_FEAT_IO_OPT) {
diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 9db86943d04c..aecd05165ee8 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -948,7 +948,7 @@ static void sd_config_atomic(struct scsi_disk *sdkp, struct queue_limits *lim)
 
 	if ((!sdkp->max_atomic && !sdkp->max_atomic_with_boundary) ||
 	    sdkp->protection_type == T10_PI_TYPE2_PROTECTION)
-		return;
+		goto out_unsupported;
 
 	physical_block_size_sectors = sdkp->physical_block_size /
 					sdkp->device->sector_size;
@@ -988,15 +988,22 @@ static void sd_config_atomic(struct scsi_disk *sdkp, struct queue_limits *lim)
 
 	if (sdkp->atomic_alignment > 1) {
 		if (unit_min > 1 && unit_min % sdkp->atomic_alignment)
-			return;
+			goto out_unsupported;
 		if (unit_max > 1 && unit_max % sdkp->atomic_alignment)
-			return;
+			goto out_unsupported;
 	}
 
 	lim->atomic_write_hw_max = max_atomic * logical_block_size;
 	lim->atomic_write_hw_boundary = 0;
 	lim->atomic_write_hw_unit_min = unit_min * logical_block_size;
 	lim->atomic_write_hw_unit_max = unit_max * logical_block_size;
+
+	lim->features |= BLK_FEAT_ATOMIC_WRITES;
+	return;
+
+out_unsupported:
+	lim->features &= ~BLK_FEAT_ATOMIC_WRITES;
+	return;
 }
 
 static blk_status_t sd_setup_write_same16_cmnd(struct scsi_cmnd *cmd,
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index af8434e391fa..c8c6a496a6ed 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -332,6 +332,9 @@ typedef unsigned int __bitwise blk_features_t;
 #define BLK_FEAT_RAID_PARTIAL_STRIPES_EXPENSIVE \
 	((__force blk_features_t)(1u << 15))
 
+/* device supports atomic writes */
+#define BLK_FEAT_ATOMIC_WRITES		((__force blk_features_t)(1u << 16))
+
 /*
  * Flags automatically inherited when stacking limits.
  */
-- 
2.31.1