Re: SPI loopback tests

[Krzysztof Kozlowski <krzk@kernel.org>]

On 10/03/2026 14:32, Francesco Dolcini wrote:
> 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.

Or the modern board file way: a module which in initcall creates several
SW nodes and instantiates the spidev. You still would need to insmode it
of course, but that way no DT would be directly involved.

> 
> 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?


Best regards,
Krzysztof