[PATCH 3/3] block: Export max_dev_sectors_kb in sysfs

["Martin K. Petersen" <martin.petersen@oracle.com>]

Some storage devices report a maximum transfer length which indicates
the maximum size of an I/O request that the device can process. This
limit is enforced in combination with the controller's max_hw_sectors
and DMA constraints to ensure that we do not issue a command too big for
the device.

Export the max_dev_sectors_kb queue limit in sysfs and update the
documentation accordingly.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
---
 Documentation/ABI/testing/sysfs-block |  9 +++++++++
 Documentation/block/queue-sysfs.txt   | 11 +++++++++--
 block/blk-settings.c                  |  4 +++-
 block/blk-sysfs.c                     | 13 +++++++++++++
 4 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
index 71d184dbb70d..4f284d38c085 100644
--- a/Documentation/ABI/testing/sysfs-block
+++ b/Documentation/ABI/testing/sysfs-block
@@ -235,3 +235,12 @@ Description:
 		write_same_max_bytes is 0, write same is not supported
 		by the device.
 
+What:		/sys/block/<disk>/queue/max_dev_sectors_kb
+Date:		December 2015
+Contact:	Martin K. Petersen <martin.petersen@oracle.com>
+Description:
+		Some storage devices report the maximum size that the
+		device can process in a single READ or WRITE
+		request. This limit is used in combination with
+		constraints set by the controller driver to limit the
+		size of filesystem requests.
diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt
index e5d914845be6..a078995f3eae 100644
--- a/Documentation/block/queue-sysfs.txt
+++ b/Documentation/block/queue-sysfs.txt
@@ -55,9 +55,15 @@ logical_block_size (RO)
 -----------------------
 This is the logcal block size of the device, in bytes.
 
+max_dev_sectors_kb (R)
+----------------------
+This is the maximum number of kilobytes supported by the storage device
+for a READ or WRITE request.
+
 max_hw_sectors_kb (RO)
 ----------------------
-This is the maximum number of kilobytes supported in a single data transfer.
+This is the maximum number of kilobytes supported by the storage
+controller in a single data transfer.
 
 max_integrity_segments (RO)
 ---------------------------
@@ -68,7 +74,8 @@ max_sectors_kb (RW)
 -------------------
 This is the maximum number of kilobytes that the block layer will allow
 for a filesystem request. Must be smaller than or equal to the maximum
-size allowed by the hardware.
+size allowed by the storage controller (max_hw_sectors_kb) and the
+maximum size allowed by the storage device (max_dev_sectors_kb).
 
 max_segments (RO)
 -----------------
diff --git a/block/blk-settings.c b/block/blk-settings.c
index dd4973583978..362b0179c86a 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -232,7 +232,9 @@ EXPORT_SYMBOL(blk_queue_bounce_limit);
  *    max_sectors is a soft limit imposed by the block layer for
  *    filesystem type requests.  This value can be overridden on a
  *    per-device basis in /sys/block/<device>/queue/max_sectors_kb.
- *    The soft limit can not exceed max_hw_sectors.
+ *
+ *    The soft limit's lower bound is the page cache size and it can not
+ *    exceed neither max_hw_sectors, nor max_dev_sectors.
  **/
 void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_sectors)
 {
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index e140cc487ce1..c289f9f6dcd9 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -225,6 +225,13 @@ static ssize_t queue_max_hw_sectors_show(struct request_queue *q, char *page)
 	return queue_var_show(max_hw_sectors_kb, (page));
 }
 
+static ssize_t queue_max_dev_sectors_show(struct request_queue *q, char *page)
+{
+	int max_dev_sectors_kb = q->limits.max_dev_sectors >> 1;
+
+	return queue_var_show(max_dev_sectors_kb, (page));
+}
+
 #define QUEUE_SYSFS_BIT_FNS(name, flag, neg)				\
 static ssize_t								\
 queue_show_##name(struct request_queue *q, char *page)			\
@@ -371,6 +378,11 @@ static struct queue_sysfs_entry queue_max_hw_sectors_entry = {
 	.show = queue_max_hw_sectors_show,
 };
 
+static struct queue_sysfs_entry queue_max_dev_sectors_entry = {
+	.attr = {.name = "max_dev_sectors_kb", .mode = S_IRUGO },
+	.show = queue_max_dev_sectors_show,
+};
+
 static struct queue_sysfs_entry queue_max_segments_entry = {
 	.attr = {.name = "max_segments", .mode = S_IRUGO },
 	.show = queue_max_segments_show,
@@ -483,6 +495,7 @@ static struct attribute *default_attrs[] = {
 	&queue_requests_entry.attr,
 	&queue_ra_entry.attr,
 	&queue_max_hw_sectors_entry.attr,
+	&queue_max_dev_sectors_entry.attr,
 	&queue_max_sectors_entry.attr,
 	&queue_max_segments_entry.attr,
 	&queue_max_integrity_segments_entry.attr,
-- 
2.5.0