scsi: scsi_debug: do_device_access add sg offset argument

WRITE SCATTERED needs to take several "bites" out of the data-out buffer.
Expand the do_device_access() function to take a sg_skip argument.
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_debug.c

@@ -2415,8 +2415,8 @@ static int check_device_access_params(struct scsi_cmnd *scp,
 }
 
 /* Returns number of bytes copied or -1 if error. */
-static int do_device_access(struct scsi_cmnd *scmd, u64 lba, u32 num,
-			    bool do_write)
+static int do_device_access(struct scsi_cmnd *scmd, u32 sg_skip, u64 lba,
+			    u32 num, bool do_write)
 {
 	int ret;
 	u64 block, rest = 0;
@@ -2442,14 +2442,15 @@ static int do_device_access(struct scsi_cmnd *scmd, u64 lba, u32 num,
 
 	ret = sg_copy_buffer(sdb->table.sgl, sdb->table.nents,
 		   fake_storep + (block * sdebug_sector_size),
-		   (num - rest) * sdebug_sector_size, 0, do_write);
+		   (num - rest) * sdebug_sector_size, sg_skip, do_write);
 	if (ret != (num - rest) * sdebug_sector_size)
 		return ret;
 
 	if (rest) {
 		ret += sg_copy_buffer(sdb->table.sgl, sdb->table.nents,
 			    fake_storep, rest * sdebug_sector_size,
-			    (num - rest) * sdebug_sector_size, do_write);
+			    sg_skip + ((num - rest) * sdebug_sector_size),
+			    do_write);
 	}
 
 	return ret;
@@ -2710,7 +2711,7 @@ static int resp_read_dt0(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 		}
 	}
 
-	ret = do_device_access(scp, lba, num, false);
+	ret = do_device_access(scp, 0, lba, num, false);
 	read_unlock_irqrestore(&atomic_rw, iflags);
 	if (unlikely(ret == -1))
 		return DID_ERROR << 16;
@@ -2998,7 +2999,7 @@ static int resp_write_dt0(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 		}
 	}
 
-	ret = do_device_access(scp, lba, num, true);
+	ret = do_device_access(scp, 0, lba, num, true);
 	if (unlikely(scsi_debug_lbp()))
 		map_region(lba, num);
 	write_unlock_irqrestore(&atomic_rw, iflags);
@@ -3239,7 +3240,7 @@ static int resp_comp_write(struct scsi_cmnd *scp,
 	 * from data-in into arr. Safe (atomic) since write_lock held. */
 	fake_storep_hold = fake_storep;
 	fake_storep = arr;
-	ret = do_device_access(scp, 0, dnum, true);
+	ret = do_device_access(scp, 0, 0, dnum, true);
 	fake_storep = fake_storep_hold;
 	if (ret == -1) {
 		retval = DID_ERROR << 16;