https://github.com/raspberrypi/firmware/issues/1375
> There are limits to how quickly the Pi 4 can boot - the SDRAM PHY has quite a lengthy training sequence, several seconds from what I remember.


0x7dc2_0000, ddr4 mmio base?
0x7dc2_4000, ddr4 offset used in memsys loading
0x7dc2_6010, ddr4, named it mmio4
0x7dc2_6100, ddr4 offset used in memsys loading, i named it mmio3
  a 6 * 32bit thing gets copied into mmio3?
  and then a function is ran?

https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=275419&start=25
> 2GB = D9WHZ = MT53D512M32D2DS-053 WT:D = 16Gb 1866MHz
> 4GB = D9WHV = MT53D1024M32D4DT-053 WT:D = 32Gb 1866MHz
> 8GB = D9ZCL = MT53E2G32D4NQ-046 WT:A = 64Gb 2133MHz