i'm trying to put a new box together to run at home, and was interested
in keeping an eye on the power consumption of the system. the most
common way to do this is with a power monitoring plug thing, but it
seems like a no brainer to me that this functionality would be part of
the actual power supply in the computer.

turns out the no brainer is actually quite niche, and there's only a
couple of manufacturers that do it and only on a subset of their
products. corsair is one of those, and one of these psus was on ebay so
i grabbed it.

the psu has a usb socket which you can plug into your system, and then
you can ask it for information. 

the following is a fairly quick and dirty driver to talk to my psu. it's
based on information figured out by Wilken Gottwalt, as per
https://github.com/wgottwalt/corsair-psu/tree/main.

i say it's quick and dirty cos it won't cope with the usb device being
unplugged while the sensors are being read, and apparently different
PSUs can present different subsets of the possible information. eg, mine
does not appear to know how much current i'm pulling from mains, and
doesn't do any power calculations. i should probably omit those values
if i detect that situation.

these feel like things we could work on in the tree, if the driver is
acceptibe in the first place.

the readings are presented as kstats like this:

crsri0:0:corsair-psu:0
         product: RM850i
        vrm-temp: 47.00 degC
       case-temp: 41.25 degC
       fan-speed: 0
     input-volts: 230.00 VAC
   input-current: 0.000 A
       12v-volts: 12.06 VDC
     12v-current: 1.250 A
       12v-power: 0.000 W
        5v-volts: 5.03 VDC
      5v-current: 1.938 A
        5v-power: 0.000 W
      3.3v-volts: 3.33 VDC
    3.3v-current: 0.688 A
      3.3v-power: 0.000 W