I did a try to bring up mainline-bsp on a i.Mx8mn EVK (imx8mn-ddr4-evk) to try graphics. My EVK has a IMX-MIPI-HDMI adapter attached. When booting this setup with mainline-bsp (Linux imx8mn-ddr4-evk 5.10.25+g07073b4e6808) I get no drm device in the system. Digging in to this I can not find any mips-dsi on the aips4 bus in the device-tree and I can not find any driver for it in the kernel tree for linux-fslc (branch 5.10.x+fslc).
In linux-imx there is a driver in drivers/gpu/drm/imx/sec_mipi_dsim-imx.c that defines '.compatible = "fsl,imx8mn-mipi-dsim",' I can not find its counterpart in linux-fslc. My conclusion at the moment is that MIPI-DSI support for imx8mn is not ready upstream. Is this a correct conclusion or have I missed something?