With the introduction of 26751de25d25 ("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 <redacted>
Fixes: 26751de25d25 ("spi: bcm2835: Micro-optimise FIFO loops")
Signed-off-by: Nicolas Saenz Julienne <redacted>

[1] https://github.com/raspberrypi/linux/issues/4100

---

NOTE: I've reviewed a bunch of drivers and couldn't find a compelling
reason why zero-length transfers should be passed into them. But I
might be missing something.

 drivers/spi/spi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 7745eec994fd..b08efe88ccd6 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c

@@ -1269,7 +1269,7 @@ 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:

-- 
2.30.0