Rihanna's pops found out about his soon-to-be grandkid same as everyone else -- while watchin' her Super Bowl halftime show ... and he's hoping, once again, for 2 X chromosomes.
Ronald Fenty tells TMZ ... he's over the moon about his daughter and A$AP Rocky's new kiddo, and with 4 grandsons already, he's hoping RiRi can shake things up with a girl.
He says he's excited at the prospect of having a granddaughter ... face it, he had great luck in that department with his famous daughter.