I booted the newest 4.9-rc1 kernel on my N900. (with also the device tree provided in mainline)
After it's booted, main functions can perform well, but I got "VDD1: ramp_delay not set" message frequently, which affected console usage.
Friendly advice from the regression tracker. This warning comes from here
$ grep -r "ramp_delay not set" *
drivers/regulator/core.c: rdev_warn(rdev, "ramp_delay not set\n");
Hence you might want to contact people using below mail addresses, as the relevant people are unlikely to see the problem here in bugzilla:
$ ./scripts/get_maintainer.pl drivers/regulator/core.c
Liam Girdwood <email@example.com> (supporter:VOLTAGE AND CURRENT REGULATOR FRAMEWORK)
Mark Brown <firstname.lastname@example.org> (supporter:VOLTAGE AND CURRENT REGULATOR FRAMEWORK)
email@example.com (open list:VOLTAGE AND CURRENT REGULATOR FRAMEWORK)
Maybe those are the wrong people to contact (it might happen due to change in a different area), but they should be able to point you in the right direction if you ask nicely.