On Sat, Nov 07, 2020 at 05:13:51PM +0900, Damien Le Moal wrote:

> The resting state of MOSI is high when nothing is driving it. If we
> drive it low while recieving, it looks like we are transmitting 0x00
> instead of transmitting nothing. This can confuse slaves (like SD cards)
> which allow new commands to be sent over MOSI while they are returning
> data over MISO. The return of MOSI from 0 to 1 at the end of recieving
> a byte can look like a start bit and a transmission bit to an SD card.

If client devices are interpreting the transmitted data then I would
expect the drivers for that hardware to be ensuring that whatever we
transmit matches what the device is expecting.  We shouldn't be putting
a hack in a particular controller driver to paper over things, that will
mean that the device will break when used with other controllers and if
different devices have different requirements then obviously we can't
satisfy them.  There is not meaningfully a general specification for SPI
which says what happens when signals are idle, it's all specific to the
client device.

In this case it also looks like the controller hardware requires
transmit data and therefore should be setting SPI_MUST_TX and just
removing the in driver default anyway, though that will have no effect
one way or anther on the issue you're seeing.

Please also try to avoid the use of master/slave terminology where
reasonable, controller and device tend to work for SPI (though MOSI/MISO
are going to be harder to shift).