Rock64 micro-sd controller defective?

[z4v4l]

Your assumption is correct. In order to get the UHS-I capable on RK3328, the voltage switching is needed.

Is it possible on Rock64? Is the power domain of SDMMC0 (VCCIO3) sourced from a dedicated voltage regulator?

Of course it's not. I looked over the Rockchip dts files for rk3328, and none board had a dedicated voltage regulator for SDMMC0. Why they put UHS-I capable controller there then, if they made it impossible to use it? Funny, it's claimed to be compatible with rk3288's and that is SDR104 capable. wahoo. this thing could achieve ~75MB/s (given 150MHz limit exposed in the dts for the contoller functional frequence) but instead it is trice as slower.

Interesting, there is a regulator for a bunch of 1.8V domains, - LDO1. Why it was necessary to allocate a different one for eMMC, - LDO2? Couldn't it be fed from LDO1 as well and LDO2 instead would be used for SDMMC0? Were there technical reasons behind this decision? Isn't it possible to reevaluate it and consider allocating LDO2 for the SDMMC0 interface, bringing UHS-I at least to the future Rock64 revisions?