spi: Only call transfer_one() if we have buffers to transfer

Client drivers such as the ChomeOS EC driver sometimes use transfers with
no buffers and only a delay specified in order to allow a delay after the
assertion of /CS. Rather than require controller drivers handle this noop
case gracefully put checks in the core to ensure that we don't call into
the controller for such transfers.

Reported-by: Addy Ke <addy.ke@rock-chips.com>
Tested-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi.c

@@ -789,27 +789,35 @@ static int spi_transfer_one_message(struct spi_master *master,
 	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
 		trace_spi_transfer_start(msg, xfer);
 
-		reinit_completion(&master->xfer_completion);
-
-		ret = master->transfer_one(master, msg->spi, xfer);
-		if (ret < 0) {
-			dev_err(&msg->spi->dev,
-				"SPI transfer failed: %d\n", ret);
-			goto out;
-		}
+		if (xfer->tx_buf || xfer->rx_buf) {
+			reinit_completion(&master->xfer_completion);
+
+			ret = master->transfer_one(master, msg->spi, xfer);
+			if (ret < 0) {
+				dev_err(&msg->spi->dev,
+					"SPI transfer failed: %d\n", ret);
+				goto out;
+			}
 
-		if (ret > 0) {
-			ret = 0;
-			ms = xfer->len * 8 * 1000 / xfer->speed_hz;
-			ms += ms + 100; /* some tolerance */
+			if (ret > 0) {
+				ret = 0;
+				ms = xfer->len * 8 * 1000 / xfer->speed_hz;
+				ms += ms + 100; /* some tolerance */
 
-			ms = wait_for_completion_timeout(&master->xfer_completion,
-							 msecs_to_jiffies(ms));
-		}
+				ms = wait_for_completion_timeout(&master->xfer_completion,
+								 msecs_to_jiffies(ms));
+			}
 
-		if (ms == 0) {
-			dev_err(&msg->spi->dev, "SPI transfer timed out\n");
-			msg->status = -ETIMEDOUT;
+			if (ms == 0) {
+				dev_err(&msg->spi->dev,
+					"SPI transfer timed out\n");
+				msg->status = -ETIMEDOUT;
+			}
+		} else {
+			if (xfer->len)
+				dev_err(&msg->spi->dev,
+					"Bufferless transfer has length %u\n",
+					xfer->len);
 		}
 
 		trace_spi_transfer_stop(msg, xfer);