[PATCH 6.1 0521/1146] spi: spi-gpio: Dont set MOSI as an input if not 3WIRE mode

Greg Kroah-Hartman
2022-12-28T16:06:51Z

From: Kris Bahnsen <kris@embeddedTS.com>

[ Upstream commit 3a6f994f848a69deb2bf3cd9d130dd0c09730e55 ]

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>
Link: https://lore.kernel.org/r/20221207230853.6174-1-kris@embeddedTS.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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
--
2.35.1