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

* [PATCH] spi: spi-gpio: Don't set MOSI as an input if not 3WIRE mode
@ 2022-12-07 23:08 Kris Bahnsen
  2022-12-07 23:44 ` Mark Brown
  2022-12-08 13:14 ` Mark Brown
  0 siblings, 2 replies; 6+ messages in thread
From: Kris Bahnsen @ 2022-12-07 23:08 UTC (permalink / raw)
  To: Mark Brown, linux-spi; +Cc: linux-kernel, mark, Kris Bahnsen

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


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