Just for clarity for others searching, firmware versions to support crank length (applies to more than just P1’s, fwiw):
FR920XT: Requires 6.10 or higher
Fenix3: Requires 6.90 or higher (or under 6.00)
Fenix3 HR: Anything
FWIW, I keep track of all these in the compatibility section in my PowerTap P1 review. Though I do need to go back at test the BLE side for the handful of units that support BLE PM’s.
Compatibility charts at end of this section: http://www.dcrainmaker.com/2015/08/powertap-p1-pedals-review.html#head-unit-compatibility