Marvin Rösch

Ah great, that is exactly what I did as well, so I guess I had misconfigured Kodi in some way that I didn't get any audio at all on my soundbar when using passthrough.
 
I've got at least a little further with my HDR DRM woes as well, the "support-multi-area" property in the device tree for vop was indeed what was required to make overlay planes show up. Unfortunately, all I get when trying to use these overlay planes from an application (e.g. moonlight-qt for game streaming with some adjustments to use a different pixel format) is a fully black or green screen. The HDR logo on my TV does show up, though, so it's a start! I haven't had too much time to look into it further the last few days, but I might investigate some more today.

Ahhh, thanks a bunch, @royk, I was missing this particular setting, now my TV switches to HDR when I play the right content. I also looked into why that setting is required at all, and it looks like the Rockchip DRM driver does not create any overlay planes that could be used as targets for intermediate HDR rendering.
 
I'm looking into why that might be, because as far as I can tell, the VOP2 driver that gets loaded for the RK3588 should set up at least a few overlay planes (see https://github.com/radxa/kernel/blob/linux-5.10-gen-rkr3.4/drivers/gpu/drm/rockchip/rockchip_vop2_reg.c and the actual implementation in drm_vop2.c). Maybe there's some option in the device tree that still needs to be enabled. There is something about multi area support in the driver source, possibly that is it.
 
I'm still looking into audio passthrough as well. I got the option enabled in Kodi and have enabled passthrough for various formats, but my soundbar still displays that it receives regular stereo LPCM, where it displays e.g. Dolby Digital if I pay something directly from my TV.