[M] Change in coreboot[main]: soc/amd: add PSP SMI handler stub

[Nico Huber (Code Review)]

Attention is currently required from: Felix Held, Fred Reitberger, Jason Glenesk, Martin L Roth, Matt DeVillier, Varshit Pandya, ritul guru.

Nico Huber has posted comments on this change by Felix Held. ( https://review.coreboot.org/c/coreboot/+/83739?usp=email )

Change subject: soc/amd: add PSP SMI handler stub
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Patch Set 2: Code-Review+1

(3 comments)

File src/soc/amd/cezanne/smihandler.c:

https://review.coreboot.org/c/coreboot/+/83739/comment/d121087d_efae0491?usp... :
PS2, Line 116: psp_smi_handler
Stubbing `psp_smi_handler()` in the header file once would avoid the `#if`
in all the C files. It would add some unnecessary data to the binary when
the Kconfig is disabled, though. So no strong preference on my end.

File src/soc/amd/common/block/psp/Kconfig:

https://review.coreboot.org/c/coreboot/+/83739/comment/6be9e14c_9121f854?usp... :
PS2, Line 108: 	  SPI flash after the BOOT_DONE PSP command.
NB. Making this some kind of append-only and only erase during boot,
for instance, could avoid the problem.

File src/soc/amd/common/block/psp/psp_smm.c:

https://review.coreboot.org/c/coreboot/+/83739/comment/ba48727a_1aa39f08?usp... :
PS2, Line 94: 		configure_psp_smi();
Is it necessary to do this in SMM?

Don't know much about the amd/soc/ code, but after a quick look,
it doesn't seem like configure_smi() is usually called from SMM.