Forgot to mention my setup. I'm using an ASUS R9 290, a Linux Mint base system
tracking the xorg-edgers PPA, and kernel 3.19-rc3 from here:
http://kernel.ubuntu.com/~kernel-ppa/mainline/v3.19-rc3-vivid/

I have the latest linux-firmware.

The mclk began changing after I upgraded from 3.18.1, but the sclk is still
stuck at the lowest level.