From AN4467 i.MX 6 Series DDR Calibration, DQM looks to be evaluated during calibration in 64 bit operation.
"associated DQS, DQM signals (as well as DDR clock). DDR_PHY1 delay and status registers are used for calibration of DQ[63:32] and associated DQS, DQM signals."
How about in 32-bit operations?
Is DQM used in Sabre SDP?