3w-9xxx: don't unmap bounce buffered commands

3w controller don't dma map small single SGL entry commands but instead
bounce buffer them.  Add a helper to identify these commands and don't
call scsi_dma_unmap for them.

Based on an earlier patch from James Bottomley.

Fixes: 118c85 ("3w-9xxx: fix command completion race")
Reported-by: Tóth Attila <atoth@atoth.sote.hu>
Tested-by: Tóth Attila <atoth@atoth.sote.hu>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>

drivers/scsi/3w-9xxx.c

@@ -212,6 +212,17 @@ static const struct file_operations twa_fops = {
 	.llseek		= noop_llseek,
 };
 
+/*
+ * The controllers use an inline buffer instead of a mapped SGL for small,
+ * single entry buffers.  Note that we treat a zero-length transfer like
+ * a mapped SGL.
+ */
+static bool twa_command_mapped(struct scsi_cmnd *cmd)
+{
+	return scsi_sg_count(cmd) != 1 ||
+		scsi_bufflen(cmd) >= TW_MIN_SGL_LENGTH;
+}
+
 /* This function will complete an aen request from the isr */
 static int twa_aen_complete(TW_Device_Extension *tw_dev, int request_id)
 {
@@ -1339,7 +1350,8 @@ static irqreturn_t twa_interrupt(int irq, void *dev_instance)
 				}
 
 				/* Now complete the io */
-				scsi_dma_unmap(cmd);
+				if (twa_command_mapped(cmd))
+					scsi_dma_unmap(cmd);
 				cmd->scsi_done(cmd);
 				tw_dev->state[request_id] = TW_S_COMPLETED;
 				twa_free_request_id(tw_dev, request_id);
@@ -1582,7 +1594,8 @@ static int twa_reset_device_extension(TW_Device_Extension *tw_dev)
 				struct scsi_cmnd *cmd = tw_dev->srb[i];
 
 				cmd->result = (DID_RESET << 16);
-				scsi_dma_unmap(cmd);
+				if (twa_command_mapped(cmd))
+					scsi_dma_unmap(cmd);
 				cmd->scsi_done(cmd);
 			}
 		}
@@ -1765,12 +1778,14 @@ static int twa_scsi_queue_lck(struct scsi_cmnd *SCpnt, void (*done)(struct scsi_
 	retval = twa_scsiop_execute_scsi(tw_dev, request_id, NULL, 0, NULL);
 	switch (retval) {
 	case SCSI_MLQUEUE_HOST_BUSY:
-		scsi_dma_unmap(SCpnt);
+		if (twa_command_mapped(SCpnt))
+			scsi_dma_unmap(SCpnt);
 		twa_free_request_id(tw_dev, request_id);
 		break;
 	case 1:
 		SCpnt->result = (DID_ERROR << 16);
-		scsi_dma_unmap(SCpnt);
+		if (twa_command_mapped(SCpnt))
+			scsi_dma_unmap(SCpnt);
 		done(SCpnt);
 		tw_dev->state[request_id] = TW_S_COMPLETED;
 		twa_free_request_id(tw_dev, request_id);
@@ -1831,8 +1846,7 @@ static int twa_scsiop_execute_scsi(TW_Device_Extension *tw_dev, int request_id,
 		/* Map sglist from scsi layer to cmd packet */
 
 		if (scsi_sg_count(srb)) {
-			if ((scsi_sg_count(srb) == 1) &&
-			    (scsi_bufflen(srb) < TW_MIN_SGL_LENGTH)) {
+			if (!twa_command_mapped(srb)) {
 				if (srb->sc_data_direction == DMA_TO_DEVICE ||
 				    srb->sc_data_direction == DMA_BIDIRECTIONAL)
 					scsi_sg_copy_to_buffer(srb,
@@ -1905,7 +1919,7 @@ static void twa_scsiop_execute_scsi_complete(TW_Device_Extension *tw_dev, int re
 {
 	struct scsi_cmnd *cmd = tw_dev->srb[request_id];
 
-	if (scsi_bufflen(cmd) < TW_MIN_SGL_LENGTH &&
+	if (!twa_command_mapped(cmd) &&
 	    (cmd->sc_data_direction == DMA_FROM_DEVICE ||
 	     cmd->sc_data_direction == DMA_BIDIRECTIONAL)) {
 		if (scsi_sg_count(cmd) == 1) {