I did change the code now, recompiled the driver and loaded the new module (after unloading the old one), and it works. As expected, it reversed the byte sending order and in that regards behaves like in JP5.
I am aware of that, but I was expecting that when NVIDIA starts developing a new Jetpack version, that they also migrate patches to the updated kernel, unless it is pushed to the mainline kernel anyway.
In case of JP6, I assume NVIDIA started of with the mainline kernel but missed to migrate lots of patches they did to the JP5 driver of the SPI. Comparing the commit history of the spi-tegra114 driver from JetPack 5’s kernel to the one from JetPack 6’s kernel shows that a lot of patches, for example from July 11th 2021, are not merged/included in the new kernel driver. Maybe, @KevinFFF, you have more insight into the development and can comment on that.
I see functionality missing and, judging by the commit history, even memory leaks not fixed in the JP6 driver. This leads us to probably go back to JP5 for now, until there is an update on that matter.