Eric Anholt [off-list ref] writes:

The hardware clocks are not controllable by the ARM, so we have to
make requests to the firmware to do so from the VPU side.  This will
let us replace fixed clocks in our DT with actual clock control (and
correct frequency information).

Gordon from the Raspberry Pi Foundation just asked me "what do you mean,
you can't access the clocks from the ARM?"  I'd been assured I couldn't
by another developer (who was originally going to write a Linux driver
for this), and my own testing had also indicated I couldn't, but after a
new round of hacking together some tests, I see things that look a lot
like clockman registers.

Looks like we're going to get a native driver, instead.