SPI loopback tests

[Francesco Dolcini <francesco@dolcini.it>]

Hello all,

I am writing to you SPI, test and DT people with reference to some need
we see in our testing infrastructure.

On our embedded boards we regularly (on mailine linux, on Linux LTS, and
on multiple hardware) run SPI tests, using a loopback connection (SPI
MISO/MOSI are connected together). The HW provide just some pin header
and there is no actual SPI slave device connected.

So far to test this we had some out-of-tree DT overlay abusing the spidev
compatible, however we'd like to move away from this approach and have a
solution that is 100% in mainline.

Manually unbinding/binding the driver in userspace does not seems as an
option, because there is no device node.

One option that I could think of would be to add a new compatible that
to describe this single wire loopback connection (something like
`linux,spi-miso-mosi-loopback`) that would bind to the spidev driver.

I am aware that you all DT maintainers shared in a pretty clear way your
view on the abuse of the spidev multiple times.

What would you be your advice to handle the need of an SPI loopback
test? Am I missing something and there is a solution already available?

Thanks in advance,
Francesco