Re: [PATCH v1 1/2] dt-bindings: trivial-devices: add spi-miso-mosi-loopback

[Krzysztof Kozlowski <krzk@kernel.org>]

On 18/03/2026 15:14, Francesco Dolcini wrote:
> 
>>> linux,spi-loopback-test as a compatible.
>>>
>>> On the actual HW I have a pins header and a wire between MISO/MOSI.
>>
>> pins header, if I understood correctly what you are talking about, is a
>> connector. Connecting there a jumper to short circuit there is out of
>> scope, because it is not part of the board. Thus cannot be send upstream
>> DTS, unless it is really embedded in the design like USB-A dongles which
>> could not be removed by end user.
>>
>> Otherwise you could say that every evalkit board can have such device,
>> because they all have pin headers...
>>
>>>
>>> I am clearly missing something obvious, any advise is appreciated.
>>
>> Describe your hardware... Nothing in this patchset helps in
>> understanding it.
> 
> Let me describe the need first, and after that I'll describe the HW.
> 
> The need we have is to have a way, with Linux mainline kernel, to test the SPI
> functionality (subsystem, driver and the actual HW). So far this was
> done using the spidev out-of-tree and with a loopback between the SPI MOSI and
> MISO signals.
> 
> We have a variety of HW that have various signals available on some sort of
> connector or pins headers [1].
> 
> This https://docs.toradex.com/112710-mallow-carrier-board_back.png is just one
> example, I know you are familiar with this kind of HW, I am writing it here the
> link just for completness and for everyone else.

Looks like a specific device (hat), not a jumper connecting two pins on
the header.

> 
> So yes, your understanding is correct.
> 
> About the HW, we have various test boards, that plugs into this connector(s),
> as test fixture, and, with traces on the PCB, they do connect together the SPI
> MISO and MOSI signals.
> 
> Here a couple of picture as an example, of the "Aquila Lava HAT",
> https://gist.github.com/dolcini/33c332f0b2174a9d495baf7c79341abf
I don't think this is anyhow different than all existing cases of people
submitting spidev patches.

You can have only two choices:
1. Connector. You cannot send bindings for spidev or loopback claiming
it is a connector.

2. Actual device plugged there. You need specific compatible for this
actual device. The compatible cannot be used for anything else.

Best regards,
Krzysztof