imx8mm memory env in U-Boot

[Tim Harvey]

Greetings,

I'm trying to understand what the best memory usage is in U-Boot for
IMX8M boards for generic distro configs such as: loadaddr,
kernel_addr_r, fdt_addr_r, ramdisk_addr, scriptaddr.

My understanding is that the following is a good rule of thumb:
loadaddr = DDR start + 32MB (as FIT images may load kernel at DDR
start; but this only allows for a 32MB kernel)
kernel_addr_r = $loadaddr
fdt_addr_r = $kernel_addr_r + 128MB (allows you up to 128MB for your
kernel; handy if you want a kernel with internal ramdisk)
ramdisk_addr = fdt_addr_r + 512KB (512KB should be plenty for a dt)
scriptaddr = $loadaddr

Looking at the various imx8mm boards upstream they are kind of all
over the place but do follow some patterns likely due to some of us
just going with what prior boards used.

While I'm at it I've encountered a couple other questions:
- why on IMX8MM is CONFIG_LOADADDR is 0x40480000 when DDR starts at
0x40000000. Why the 4608KB offset?
- what is CONFIG_SYS_INIT_RAM_SIZE? Most boards are setting this to
2MB but a couple (cl-iot-gate/phycore) set it to 512KB
- what are people using for the load address for the kernel within FIT
images? I expect start of DDR is appropriate (0x40000000) however for
whatever reason I've been using 0x40200000. This plays into the env as
you can't overlap where you loaded the FIT image with where you told
the FIT image to relocate the kernel to.

Best regards,

Tim