[coreboot] problem booting AMD RS780 boards

[Scott Duplichan]

I found the AMD mahogany Family 10h coreboot is no longer booting on my
ECS-A780GM-M3 board. The first problem it encounters is a hang after warm
reset. The hang is due to HT training fail. Using the config option to
limit the HT speed works around the problem. Though older coreboot builds
do not have this problem with my board and processor, I am not too
concerned with it at the moment.

The more serious problem is a hang at "PCI: pci_scan_bus for bus 01".
This hang is due to commit http://review.coreboot.org/#/c/3606/.
If the changed default config access mechanism is reverted back to CF8 I/O,
then the board boots normally.

I noticed the comment for commit http://review.coreboot.org/#/c/3608/:
"On older AMD platform PCI MMIO may not be able to fully configure all
PCI devices/nodes, while MMIO_SUPPORT_DEFAULT would be preferred due
to its atomic nature. So those can be forced to IO config instead."
I tried removing "select MMCONF_SUPPORT_DEFAULT" from
src\cpu\amd\model_10xxx\Kconfig. With that change, the board can
sometimes boot, but boot progress is slow and often hangs before
completing.

Any suggestions for a proper fix? I may be able to look at it more
later next week.

Thanks,
Scott