spi: Skip zero-length transfers in spi_transfer_one_message()

With the introduction of 26751de ("spi: bcm2835: Micro-optimise
FIFO loops") it has become apparent that some users might initiate
zero-length SPI transfers. A fact the micro-optimization omitted, and
which turned out to cause crashes[1].

Instead of changing the micro-optimization itself, use a bigger hammer
and skip zero-length transfers altogether for drivers using the default
transfer_one_message() implementation.

Reported-by: Phil Elwell <[email protected]>
Fixes: 26751de ("spi: bcm2835: Micro-optimise FIFO loops")
Signed-off-by: Nicolas Saenz Julienne <[email protected]>

[1] raspberrypi/linux#4100
Link: https://lore.kernel.org/r/[email protected]

Signed-off-by: Mark Brown <[email protected]>

Author: Nicolas Saenz Julienne

drivers/spi/spi.c

@@ -1267,7 +1267,7 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
 			ptp_read_system_prets(xfer->ptp_sts);
 		}
 
-		if (xfer->tx_buf || xfer->rx_buf) {
+		if ((xfer->tx_buf || xfer->rx_buf) && xfer->len) {
 			reinit_completion(&ctlr->xfer_completion);
 
 fallback_pio: