[PATCHv3 2/6] block: export dma_alignment attribute

[Keith Busch <kbusch@fb.com>]

From: Keith Busch <kbusch@kernel.org>

User space may want to know how to align their buffers to avoid
bouncing. Export the queue attribute.

Signed-off-by: Keith Busch <kbusch@kernel.org>
---
v2->v3:

  Added Documentation (Bart)

 Documentation/ABI/stable/sysfs-block | 9 +++++++++
 block/blk-sysfs.c                    | 7 +++++++
 2 files changed, 16 insertions(+)

diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
index e8797cd09aff..08f6d00df138 100644
--- a/Documentation/ABI/stable/sysfs-block
+++ b/Documentation/ABI/stable/sysfs-block
@@ -142,6 +142,15 @@ Description:
 		Default value of this file is '1'(on).
 
 
+What:		/sys/block/<disk>/queue/dma_alignment
+Date:		May 2022
+Contact:	linux-block@vger.kernel.org
+Description:
+		Reports the alignment that user space addresses must have to be
+		used for raw block device access with O_DIRECT and other driver
+		specific passthrough mechanisms.
+
+
 What:		/sys/block/<disk>/queue/chunk_sectors
 Date:		September 2016
 Contact:	Hannes Reinecke <hare@suse.com>
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 88bd41d4cb59..14607565d781 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -274,6 +274,11 @@ static ssize_t queue_virt_boundary_mask_show(struct request_queue *q, char *page
 	return queue_var_show(q->limits.virt_boundary_mask, page);
 }
 
+static ssize_t queue_dma_alignment_show(struct request_queue *q, char *page)
+{
+	return queue_var_show(queue_dma_alignment(q), page);
+}
+
 #define QUEUE_SYSFS_BIT_FNS(name, flag, neg)				\
 static ssize_t								\
 queue_##name##_show(struct request_queue *q, char *page)		\
@@ -606,6 +611,7 @@ QUEUE_RO_ENTRY(queue_dax, "dax");
 QUEUE_RW_ENTRY(queue_io_timeout, "io_timeout");
 QUEUE_RW_ENTRY(queue_wb_lat, "wbt_lat_usec");
 QUEUE_RO_ENTRY(queue_virt_boundary_mask, "virt_boundary_mask");
+QUEUE_RO_ENTRY(queue_dma_alignment, "dma_alignment");
 
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
 QUEUE_RW_ENTRY(blk_throtl_sample_time, "throttle_sample_time");
@@ -667,6 +673,7 @@ static struct attribute *queue_attrs[] = {
 	&blk_throtl_sample_time_entry.attr,
 #endif
 	&queue_virt_boundary_mask_entry.attr,
+	&queue_dma_alignment_entry.attr,
 	NULL,
 };
 
-- 
2.30.2