Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/41155 )
Change subject: soc/intel/common/block/systemagent: Use TOUUD as base for MMIO above 4G ......................................................................
Patch Set 6:
(1 comment)
https://review.coreboot.org/c/coreboot/+/41155/6/src/soc/intel/common/block/... File src/soc/intel/common/block/systemagent/systemagent.c:
https://review.coreboot.org/c/coreboot/+/41155/6/src/soc/intel/common/block/... PS6, Line 123: sa_read_map_entry(sa_dev, &sa_memory_map[SA_TOUUD_REG], &gnvs->a4gb);
I've peeked into certain refcode, it doesn't seem to know that rule either. […]
What it says:
"BIOS Restriction: Minimum value for TOUUD is 4GB."
At first, I read it as, don't write anything below 4GiB here. But on second thought it could also mean, if the BIOS reads this, it should assume a 4GiB minimum.
I've also realized by now, that one would actually need less than 4GiB to run into problems here. Which is probably very rare with the supported chipsets? With exactly 4GiB, we would remap part of the memory and TOUUD would be the top of the remapping above 4G. Only if the memory is that small that there is nothing to remap, this could be an issue.