rockpro64 fan speed control

I have acquired a rockpro64 SBC, an enclosure and disks to make a
raided NAS.  This system boots up perfectly with an image of HEAD from
nycdn.netbsd.org and I hope to get it to boot off the hard disk(s)
rather than the sdcard, something which looks fairly straightforward to
setup.

One thing I would like to control is the speed of cpu fan attached to
the pwm controller which shows as:

pwmfan0 at simplebus0: PWM Fan (levels 0% 39% 58% 78% 100%)

Looking at the source code of the controller it seems to attach the
device and then sets the fan speed to max (which is noisy!). There
doesn't appear to be any code to set the speed from userland and there
is no dev entry to control the device.

I was wondering if it would be fairly straightforward to add a sysctl
knob to query and set the fan speed. Would this be the best way
forward?

Cheers,
Dave