Dear @temalab,
Thanks for the update. I’ve made a mistake. There correct github link was to be this one: device-trees/imx6ull-colibri-eval-v3.dtsi at toradex_5.4.y · toradex/device-trees · GitHub. I sent to you the Downstream Kernel where it’s not enabled by default. You should be on this branch: toradex_5.4.y.
By the way, did you have some time to check this thread? How to use SPI on Colibri IMX6ULL - #18 by jaski.tx
On it, there is a patch that helped a customer use the SPI Dev interface.
Can you please have a look at it?
Finally, just as an additional check, did you make any additional change to your image that doesn’t include the kernel modifications you already commented on?