From e69d38065080b50380e7fee96d5e8b481291d5ca Mon Sep 17 00:00:00 2001 From: Kris Bahnsen Date: Wed, 7 Dec 2022 15:08:53 -0800 Subject: [PATCH] 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: 4b859db2c606 ("spi: spi-gpio: add SPI_3WIRE support") Fixes: 5132b3d28371 ("spi: gpio: Support 3WIRE high-impedance turn-around") Signed-off-by: Kris Bahnsen Link: https://lore.kernel.org/r/20221207230853.6174-1-kris@embeddedTS.com Signed-off-by: Mark Brown Signed-off-by: Sasha Levin --- 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 0584f4d..3ffdab6 100644 --- a/drivers/spi/spi-gpio.c +++ b/drivers/spi/spi-gpio.c @@ -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); - 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 -- 2.7.4