How to control R7 370 voltage?

Does anyone know if it is possible to control the voltage of R7 370s under Linux. Atitweak doesn't work for undervolting.
I changed PL 1 & 4 from 1.188 to 1.1V for adapter 1:

Code:

1. AMD Radeon (TM) R7 370 Series  (:0.1)
    engine clock range is 150 - 1150MHz
    memory clock range is 75 - 1500MHz
    core voltage range is 0.875 - 1.188VDC
    performance level 0: engine clock 300MHz, memory clock 150MHz, core voltage 0.875VDC
    performance level 1: engine clock 1150MHz, memory clock 1500MHz, core voltage 1.1VDC

But according to atitweak -s, nothing changed:

Code:

1. AMD Radeon (TM) R7 370 Series  (:0.1)
    engine clock 1150MHz, memory clock 1500MHz, core voltage 1.188VDC, performance level 4, utilization 99%

It also seems impossible to over-volt beyond 1.188V. I searched for R7 370 BIOS editors, and couldn't find anything either. Any ideas?