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

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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: 4b859db2c6 ("spi: spi-gpio: add SPI_3WIRE support")
Fixes: 5132b3d283 ("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>
This commit is contained in:
Kris Bahnsen 2022-12-07 15:08:53 -08:00 committed by Mark Brown
parent 8330e9e826
commit 3a6f994f84
No known key found for this signature in database
GPG key ID: 24D68B725D5487D0
1 changed files with 13 additions and 3 deletions
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16
drivers/spi/spi-gpio.c
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@ -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