diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 0a537a0..d055450 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -439,10 +439,7 @@
 
 	act_len = sg_copy_from_buffer(sdb->table.sgl, sdb->table.nents,
 				      arr, arr_len);
-	if (sdb->resid)
-		sdb->resid -= act_len;
-	else
-		sdb->resid = scsi_bufflen(scp) - act_len;
+	sdb->resid = scsi_bufflen(scp) - act_len;
 
 	return 0;
 }
@@ -1693,24 +1690,48 @@
 	return 0;
 }
 
+/* Returns number of bytes copied or -1 if error. */
 static int do_device_access(struct scsi_cmnd *scmd,
 			    struct sdebug_dev_info *devi,
 			    unsigned long long lba, unsigned int num, int write)
 {
 	int ret;
 	unsigned long long block, rest = 0;
-	int (*func)(struct scsi_cmnd *, unsigned char *, int);
+	struct scsi_data_buffer *sdb;
+	enum dma_data_direction dir;
+	size_t (*func)(struct scatterlist *, unsigned int, void *, size_t,
+		       off_t);
 
-	func = write ? fetch_to_dev_buffer : fill_from_dev_buffer;
+	if (write) {
+		sdb = scsi_out(scmd);
+		dir = DMA_TO_DEVICE;
+		func = sg_pcopy_to_buffer;
+	} else {
+		sdb = scsi_in(scmd);
+		dir = DMA_FROM_DEVICE;
+		func = sg_pcopy_from_buffer;
+	}
+
+	if (!sdb->length)
+		return 0;
+	if (!(scsi_bidi_cmnd(scmd) || scmd->sc_data_direction == dir))
+		return -1;
 
 	block = do_div(lba, sdebug_store_sectors);
 	if (block + num > sdebug_store_sectors)
 		rest = block + num - sdebug_store_sectors;
 
-	ret = func(scmd, fake_storep + (block * scsi_debug_sector_size),
-		   (num - rest) * scsi_debug_sector_size);
-	if (!ret && rest)
-		ret = func(scmd, fake_storep, rest * scsi_debug_sector_size);
+	ret = func(sdb->table.sgl, sdb->table.nents,
+		   fake_storep + (block * scsi_debug_sector_size),
+		   (num - rest) * scsi_debug_sector_size, 0);
+	if (ret != (num - rest) * scsi_debug_sector_size)
+		return ret;
+
+	if (rest) {
+		ret += func(sdb->table.sgl, sdb->table.nents,
+			    fake_storep, rest * scsi_debug_sector_size,
+			    (num - rest) * scsi_debug_sector_size);
+	}
 
 	return ret;
 }
@@ -1849,7 +1870,12 @@
 	read_lock_irqsave(&atomic_rw, iflags);
 	ret = do_device_access(SCpnt, devip, lba, num, 0);
 	read_unlock_irqrestore(&atomic_rw, iflags);
-	return ret;
+	if (ret == -1)
+		return DID_ERROR << 16;
+
+	scsi_in(SCpnt)->resid = scsi_bufflen(SCpnt) - ret;
+
+	return 0;
 }
 
 void dump_sector(unsigned char *buf, int len)