[S] Change in coreboot[main]: src/lib: Add Kconfig variable for fw_config default value

[Subrata Banik (Code Review)]

Attention is currently required from: Bora Guvendik, Cliff Huang, Julius Werner, Wonkyu Kim.

Subrata Banik has posted comments on this change by Cliff Huang. ( https://review.coreboot.org/c/coreboot/+/83153?usp=email )

Change subject: src/lib: Add Kconfig variable for fw_config default value
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Patch Set 3:

(1 comment)

File src/Kconfig:

https://review.coreboot.org/c/coreboot/+/83153/comment/275fe889_f5c6fc61?usp... :
PS2, Line 548: 0x00000
> I change default value to UNDEFINED_FW_CONFIG and this won't be used unless config.<board> has different value. We are using this to assign fw_config value in make build environment and using simulation environment.

"where no CBI value is synced from config.star", In this case, you do not select FW_CBI Kconfig in the board configuration. The issue with having a random FW_CONFIG value, such as 0xfffff, is that the code flow may take a different path than intended. For example, we have currently enabled VPU 0, but if there is no CBI, then VPU will be disabled.

I would prefer to ensure that CBI and FW_CONFIG are always in sync, and that we do not need to override FW_CONFIG_DEFAULT_VALUE from the coreboot Kconfig, because the source of truth for CBI is config.star, which represents the backend SKU details.