I'm actually surprised Marcus Smart hasn't been fined by the league yet. Here's the official description of the NBA's flopping rule, which can result in a warning followed by a series of increasing fines:
'Flopping' will be defined as any physical act that appears to have been intended to cause the referees to call a foul on another player. The primary factor in determining whether a player committed a flop is whether his physical reaction to contact with another player is inconsistent with what would reasonably be expected given the force or direction of the contact.