Odd thing I noticed today: if I set Settings / Videos / Processing / HDR processing to “BT2020SDR”, and play an HDR video vfile, the video output is reported to the display as HLG HDR instead of SDR. I assume that’s not intentional?
Logs:
Sadly, it is intentional. AMLogic assume any HDR video that’s not HDR10[+] is HLG. And a couple of years ago they removed the ability to identify an HLG SEI from their microcode.
You’ve reminded me to have another look for a workaround.
I stand corrected. 
If you’re looking at HDR->SDR tone mapping, @grahamh , then there’s a change you’ve been saying you’ll make for a good couple of years, now, which is to increase the maximum allowed value of of Settings / System / Display / Display maximum luminance.