[coreboot] EARLY_CBMEM_INIT for Geode LX platforms

[Kyösti Mälkki]

Hi Piotr

I never worked with Geode LX codebase, my impression is that
EARLY_CBMEM_INIT may not be worth the trouble.

Visit comments in cpu/amd/geode_lx/msrinit.c: "set up about 500Mb of
memory", "doesn't really matter", "WARNING", "probably incorrectly set
here". That's enough to tell me you may be looking at some amount of
refactoring to have this feature sorted out.

First, you should be looking at MSR_GLIU0_SYSMEM and MSR_GLIU1_SYSMEM
to have cbmem_top() implemented for romstage. In its current state,
amd/lx/northbridgeinit.c : GLPCIInit() sets up these and other
relevant registers of memory map only late in ramstage. At the minimum
you will need to move that setting at the end of romstage with the
actual amount of installed DRAM on the system. I don't have Geode
datasheets at hand and I have no clue how many other MSRs are required
for correct DRAM routing.

Second, you need functional CAR migration to get any benefits of
EARLY_CBMEM_INIT, logs and timestamps from romstage get lost unless
you have exactly correct sequence for CAR teardown implemented. Given
that you don't have to worry about ACPI S3 support you have some room
for mistakes or taking shortcuts there. In general, LX cache
management code seems to be all over the place, or at least it is very
different from any Intel (native, MRC) or AMD (AGESA/binaryPI)
implementations I have reviewed in the past years.

If you decide to try hack something together, just push it out for
review with WIP or NOTFORMERGE. Make some logs available and I should
be able to explain why/where it fails without digging into the
datasheets myself. You need to be able to address all of DRAM at the
end of romstage and that is not currently happening, AFAICS.

Kyösti