IB/srp: Use a fake scatterlist for non-SG SCSI commands

Since the SCSI midlayer is moving towards entirely getting rid of
commands with use_sg == 0, we should treat this case as an exception.
Therefore, change the IB SRP initiator to create a fake scatterlist
for these commands with sg_init_one().  This simplifies the flow of
DMA mapping and unmapping, since SRP can just use dma_map_sg() and
dma_unmap_sg() unconditionally, rather than having to choose between
the dma_{map,unmap}_sg() and dma_{map,unmap}_single() variants.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/ulp/srp/ib_srp.h

@@ -38,6 +38,7 @@
 #include <linux/types.h>
 #include <linux/list.h>
 #include <linux/mutex.h>
+#include <linux/scatterlist.h>
 
 #include <scsi/scsi_host.h>
 #include <scsi/scsi_cmnd.h>
@@ -94,7 +95,11 @@ struct srp_request {
 	struct scsi_cmnd       *scmnd;
 	struct srp_iu	       *cmd;
 	struct srp_iu	       *tsk_mgmt;
-	DECLARE_PCI_UNMAP_ADDR(direct_mapping)
+	/*
+	 * Fake scatterlist used when scmnd->use_sg==0.  Can be killed
+	 * when the SCSI midlayer no longer generates non-SG commands.
+	 */
+	struct scatterlist	fake_sg;
 	struct completion	done;
 	short			next;
 	u8			cmd_done;