spi: spi_write_then_read() bugfixes

The "simplify spi_write_then_read()" patch included two regressions from
the 2.6.27 behaviors:

 - The data it wrote out during the (full duplex) read side
   of the transfer was not zeroed.

 - It fails completely on half duplex hardware, such as
   Microwire and most "3-wire" SPI variants.

So, revert that patch.  A revised version should be submitted at some
point, which can get the speedup on standard hardware (full duplex)
without breaking on less-capable half-duplex stuff.
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Cc: <stable@kernel.org>		[2.6.28.x, 2.6.29.x]
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/spi/spi.c

@@ -658,7 +658,7 @@ int spi_write_then_read(struct spi_device *spi,
 
 	int			status;
 	struct spi_message	message;
-	struct spi_transfer	x;
+	struct spi_transfer	x[2];
 	u8			*local_buf;
 
 	/* Use preallocated DMA-safe buffer.  We can't avoid copying here,
@@ -669,9 +669,15 @@ int spi_write_then_read(struct spi_device *spi,
 		return -EINVAL;
 
 	spi_message_init(&message);
-	memset(&x, 0, sizeof x);
-	x.len = n_tx + n_rx;
-	spi_message_add_tail(&x, &message);
+	memset(x, 0, sizeof x);
+	if (n_tx) {
+		x[0].len = n_tx;
+		spi_message_add_tail(&x[0], &message);
+	}
+	if (n_rx) {
+		x[1].len = n_rx;
+		spi_message_add_tail(&x[1], &message);
+	}
 
 	/* ... unless someone else is using the pre-allocated buffer */
 	if (!mutex_trylock(&lock)) {
@@ -682,15 +688,15 @@ int spi_write_then_read(struct spi_device *spi,
 		local_buf = buf;
 
 	memcpy(local_buf, txbuf, n_tx);
-	x.tx_buf = local_buf;
-	x.rx_buf = local_buf;
+	x[0].tx_buf = local_buf;
+	x[1].rx_buf = local_buf + n_tx;
 
 	/* do the i/o */
 	status = spi_sync(spi, &message);
 	if (status == 0)
-		memcpy(rxbuf, x.rx_buf + n_tx, n_rx);
+		memcpy(rxbuf, x[1].rx_buf, n_rx);
 
-	if (x.tx_buf == buf)
+	if (x[0].tx_buf == buf)
 		mutex_unlock(&lock);
 	else
 		kfree(local_buf);