I'm working with a custom board based on the MCIMX6UL-EVK (14x14 evk), using the iMX6UL (MCIMX6G3CVM05AB) and the same RAM chip (MT41K256M16LY-093). I've run the NXP DDR Stress Test tool on the board using the default MX6UL DDR3 script, setting MR1 to 0x0004 and the DDR freq to 400MHz, but the calibration fails. I then tried calibrating at 300MHz (see calibration result in this post), and was able to run the stress test overnight.
I've been trying to compile mainline U-boot using the mx6ul_14x14_evk_defconfig. However, I need to be able to reduce the DDR clock from the default 400Mhz down to 300MHz. Where can this be done?