On Wed, Dec 07, 2022 at 03:08:53PM -0800, Kris Bahnsen wrote:
> 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.
A cleaner fix which is probably marginally more performant would be to
make the setting of spi_gpio_set_direction() conditional on SPI_3WIRE -
then we won't call into the function at all when not doing 3 wire,
avoiding the issue entirely.
> 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.
That's fine, it doesn't really matter either way.
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