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

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Date:   Wed, 07 Dec 2022 16:36:41 -0800
From:   Kris Bahnsen <kris@...eddedTS.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
        mark@...eddedts.com
Subject: Re: [PATCH] spi: spi-gpio: Don't set MOSI as an input if not 3WIRE
 mode

On Wed, 2022-12-07 at 23:44 +0000, Mark Brown wrote:
> 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.

That makes sense to me. I was operating under the assumption that 3WIRE
mode could be switched in to at a later time via ioctl(), but with the
death of spidev that is presumably no longer a concern.

I'll get a v2 put together and probably sent in tomorrow. Thanks.

> 
> > 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.