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Commit 9e8440a6 authored by Kris Bahnsen's avatar Kris Bahnsen Committed by Greg Kroah-Hartman
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spi: spi-gpio: Don't set MOSI as an input if not 3WIRE mode 



[ 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: 4b859db2 ("spi: spi-gpio: add SPI_3WIRE support")
Fixes: 5132b3d2 ("spi: gpio: Support 3WIRE high-impedance turn-around")
Signed-off-by: default avatarKris Bahnsen <kris@embeddedTS.com>
Link: https://lore.kernel.org/r/20221207230853.6174-1-kris@embeddedTS.com


Signed-off-by: default avatarMark Brown <broonie@kernel.org>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent a00b4e0f

drivers/spi/spi-gpio.c

+13 −3
Original line number Diff line number Diff line
@@ -244,9 +244,19 @@ static int spi_gpio_set_direction(struct spi_device *spi, bool output) 
	if (output)

		return gpiod_direction_output(spi_gpio->mosi, 1);



	/*

	 * 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