Subject: libata: fix oops with sparsemem

libata incorrectly passes NULL arguments to sg_set_buf, which
crashes on powerpc64 when looking for the corresponding mem_section.

This introduces a new ata_exec_nodma() wrapper that takes no buffer
arguments and does not call sg_set_buf either. In order to make it
easier to detect this sort of problem, it also adds a WARN_ON(!buf)
to sg_set_buf() so we get a log message even platforms without
sparsemem.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Index: linux-2.6/drivers/ata/libata-core.c
===================================================================
--- linux-2.6.orig/drivers/ata/libata-core.c
+++ linux-2.6/drivers/ata/libata-core.c
@@ -1332,7 +1332,7 @@ unsigned ata_exec_internal_sg(struct ata
 }
 
 /**
- *	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
@@ -1361,6 +1361,25 @@ unsigned ata_exec_internal(struct ata_de
 }
 
 /**
+ *	ata_exec_nodma - execute libata internal command
+ *	@dev: Device to which the command is sent
+ *	@tf: Taskfile registers for the command and the result
+ *
+ *	Wrapper around ata_exec_internal_sg() which takes no
+ *	data buffer.
+ *
+ *	LOCKING:
+ *	None.  Should be called with kernel context, might sleep.
+ *
+ *	RETURNS:
+ *	Zero on success, AC_ERR_* mask on failure
+ */
+static unsigned ata_exec_nodma(struct ata_device *dev, struct ata_taskfile *tf)
+{
+	return ata_exec_internal_sg(dev, tf, NULL, DMA_NONE, NULL, 0);
+}
+
+/**
  *	ata_do_simple_cmd - execute simple internal command
  *	@dev: Device to which the command is sent
  *	@cmd: Opcode to execute
@@ -1384,7 +1403,7 @@ unsigned int ata_do_simple_cmd(struct at
 	tf.flags |= ATA_TFLAG_DEVICE;
 	tf.protocol = ATA_PROT_NODATA;
 
-	return ata_exec_internal(dev, &tf, NULL, DMA_NONE, NULL, 0);
+	return ata_exec_nodma(dev, &tf);
 }
 
 /**
@@ -3475,7 +3494,7 @@ static unsigned int ata_dev_set_xfermode
 	tf.protocol = ATA_PROT_NODATA;
 	tf.nsect = dev->xfer_mode;
 
-	err_mask = ata_exec_internal(dev, &tf, NULL, DMA_NONE, NULL, 0);
+	err_mask = ata_exec_nodma(dev, &tf);
 
 	DPRINTK("EXIT, err_mask=%x\n", err_mask);
 	return err_mask;
@@ -3513,7 +3532,7 @@ static unsigned int ata_dev_init_params(
 	tf.nsect = sectors;
 	tf.device |= (heads - 1) & 0x0f; /* max head = num. of heads - 1 */
 
-	err_mask = ata_exec_internal(dev, &tf, NULL, DMA_NONE, NULL, 0);
+	err_mask = ata_exec_nodma(dev, &tf);
 
 	DPRINTK("EXIT, err_mask=%x\n", err_mask);
 	return err_mask;
Index: linux-2.6/include/linux/scatterlist.h
===================================================================
--- linux-2.6.orig/include/linux/scatterlist.h
+++ linux-2.6/include/linux/scatterlist.h
@@ -8,6 +8,8 @@
 static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
 			      unsigned int buflen)
 {
+	WARN_ON(!buf); /* virt_to_page(NULL) crashes with sparsemem */
+
 	sg->page = virt_to_page(buf);
 	sg->offset = offset_in_page(buf);
 	sg->length = buflen;