Mike Banon has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/38224 )
Change subject: [TESTME]pcengines/apu1: fix the VGA-related PCI resource allocation problems ......................................................................
Patch Set 3:
You cannot lower it however you like it, because it changes the memory map. This setting should be used carefully with good understanding of what it does.
Theoretically this setting could badly affect MTRR and system performance, so I'm going to check if there's any difference at Linux Kernel messages between 0xD0, 0x80 and 0x40 settings. However, I can do that only for fam15h and fam16h boards, don't have any fam14h. As APU1 is fam14h, it would be nice if you could do this at least for 0x40. I hope for a common default setting for fam14/15/16 boards, and this change above is going to be superseded by CB:38472