[PATCH] spi: spi-gpio: Don't set MOSI as an input if not 3WIRE mode

[Kris Bahnsen <kris@embeddedTS.com>]

The addition of 3WIRE support would affect MOSI direction even
when still in standard (4 wire) mode. This can lead to MOSI being
at an invalid logic level when a device driver sets an SPI
message with a NULL tx_buf.

spi.h states that if tx_buf is NULL then "zeros will be shifted
out ... " If MOSI is tristated then the data shifted out is subject
to pull resistors, keepers, or in the absence of those, noise.

This issue came to light when using spi-gpio connected to an
ADS7843 touchscreen controller. MOSI pulled high when clocking
MISO data in caused the SPI device to interpret this as a command
which would put the device in an unexpected and non-functional
state.

Fixes: 4b859db2c606 ("spi: spi-gpio: add SPI_3WIRE support")
Fixes: 5132b3d28371 ("spi: gpio: Support 3WIRE high-impedance turn-around")
Signed-off-by: Kris Bahnsen <kris@embeddedTS.com>
---

As an aside, I wasn't sure how to best put down the Fixes: tags.
4b859db2c606 ("spi: spi-gpio: add SPI_3WIRE support") introduced the
actual bug, but 5132b3d28371 ("spi: gpio: Support 3WIRE high-impedance turn-around")
modified that commit slightly and is what this patch actually applies
to. Let me know if marking both as fixes is incorrect and I can
create another patch.

 drivers/spi/spi-gpio.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/spi/spi-gpio.c b/drivers/spi/spi-gpio.c
index 4b12c4964a66..9c8c7948044e 100644
--- a/drivers/spi/spi-gpio.c
+++ b/drivers/spi/spi-gpio.c
@@ -268,9 +268,19 @@ static int spi_gpio_set_direction(struct spi_device *spi, bool output)
 	if (output)
 		return gpiod_direction_output(spi_gpio->mosi, 1);
 
-	ret = gpiod_direction_input(spi_gpio->mosi);
-	if (ret)
-		return ret;
+	/*
+	 * Only change MOSI to an input if using 3WIRE mode.
+	 * Otherwise, MOSI could be left floating if there is
+	 * no pull resistor connected to the I/O pin, or could
+	 * be left logic high if there is a pull-up. Transmitting
+	 * logic high when only clocking MISO data in can put some
+	 * SPI devices in to a bad state.
+	 */
+	if (spi->mode & SPI_3WIRE) {
+		ret = gpiod_direction_input(spi_gpio->mosi);
+		if (ret)
+			return ret;
+	}
 	/*
 	 * Send a turnaround high impedance cycle when switching
 	 * from output to input. Theoretically there should be
-- 
2.11.0