So, I haven’t seen fixed undervolting support yet, and I don’t know if it’s possible, but:
I can confirm that ryzenadj recognizes the 7840U on my system and I messed around with it a bit, but haven’t tested out the AMD “Curve Optimizer” feature (I plan to), which is kind of an undervolt, but not quite.
Here’s an explanation on Zen2, here’s @TheTRUEAsian’s experience on Windows with Curve Optimizer + the rest of that thread, and here’s my explanation on what it does, from research.
For Curve Optimizer on ryzenadj, it looks like it’s not documented in the readme currently, but if you run just ryzenadj without any parameters, it will list all the possible options including:
--set-coall=<u32> All core Curve Optimiser
--set-coper=<u32> Per core Curve Optimiser
--set-cogfx=<u32> iGPU Curve Optimiser
The code is here, and it looks like All core Curve Optimiser support was added for Phoenix in this commit, and Per core Curve Optimiser and iGPU Curve Optimiser was added in this commit bundled with Hawk Point. So it looks like those options should be supported on this platform currently.
Also, there’s some great info on 7840 CPU Power efficiency testing that may help, and hey! The creator of that now closed issue happened to post on that thread and is on here — hi @GreyXor 
wondering if you have any insight?