RDMA/srp: Increase max_segment_size

The default behavior of the SCSI core is to set the block layer request
queue parameter max_segment_size to 64 KB. That means that elements of
scatterlists are limited to 64 KB. Since RDMA adapters support larger
sizes, increase max_segment_size for the SRP initiator.

Notes:
- The SCSI max_segment_size parameter was introduced in kernel v5.0. See
  also commit 50c2e910 ("scsi: introduce a max_segment_size
  host_template parameters").
- Some other block drivers already set max_segment_size to UINT_MAX,
  e.g. nbd and rbd.
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/srp/ib_srp.c

@@ -3823,6 +3823,7 @@ static ssize_t srp_create_target(struct device *dev,
 	target_host->max_id      = 1;
 	target_host->max_lun     = -1LL;
 	target_host->max_cmd_len = sizeof ((struct srp_cmd *) (void *) 0L)->cdb;
+	target_host->max_segment_size = ib_dma_max_seg_size(ibdev);
 
 	target = host_to_target(target_host);
 

include/rdma/ib_verbs.h

@@ -3715,6 +3715,19 @@ static inline unsigned int ib_sg_dma_len(struct ib_device *dev,
 	return sg_dma_len(sg);
 }
 
+/**
+ * ib_dma_max_seg_size - Return the size limit of a single DMA transfer
+ * @dev: The device to query
+ *
+ * The returned value represents a size in bytes.
+ */
+static inline unsigned int ib_dma_max_seg_size(struct ib_device *dev)
+{
+	struct device_dma_parameters *p = dev->dma_device->dma_parms;
+
+	return p ? p->max_segment_size : UINT_MAX;
+}
+
 /**
  * ib_dma_sync_single_for_cpu - Prepare DMA region to be accessed by CPU
  * @dev: The device for which the DMA address was created