[PATCH] libata: don't initialize sg in ata_exec_internal() if DMA_NONE (take #2)

Calling sg_init_one() with NULL buf causes oops on certain
configurations.  Don't initialize sg in ata_exec_internal() if
DMA_NONE and make the function complain if @buf is NULL when dma_dir
isn't DMA_NONE.  While at it, fix comment.

The problem is discovered and initial patch was submitted by Arnd
Bergmann.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/libata-core.c

@@ -1332,7 +1332,7 @@ unsigned ata_exec_internal_sg(struct ata_device *dev,
 }
 
 /**
- *	ata_exec_internal_sg - execute libata internal command
+ *	ata_exec_internal - execute libata internal command
  *	@dev: Device to which the command is sent
  *	@tf: Taskfile registers for the command and the result
  *	@cdb: CDB for packet command
@@ -1353,11 +1353,17 @@ unsigned ata_exec_internal(struct ata_device *dev,
 			   struct ata_taskfile *tf, const u8 *cdb,
 			   int dma_dir, void *buf, unsigned int buflen)
 {
-	struct scatterlist sg;
+	struct scatterlist *psg = NULL, sg;
+	unsigned int n_elem = 0;
 
+	if (dma_dir != DMA_NONE) {
+		WARN_ON(!buf);
 		sg_init_one(&sg, buf, buflen);
+		psg = &sg;
+		n_elem++;
+	}
 
-	return ata_exec_internal_sg(dev, tf, cdb, dma_dir, &sg, 1);
+	return ata_exec_internal_sg(dev, tf, cdb, dma_dir, psg, n_elem);
 }
 
 /**