soc/amd/common/psp_smm: add comments to psp_notify_smm

The reasoning behind this and the positive side effects of this aren't
too clear from the code, so point those out in a comment.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I4f4121031fc1ef600cdf5551f61f1ef4e03b56a5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83699
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>

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

@@ -47,6 +47,13 @@ static void clear_smm_flag(void)
 	smm_flag = 0;
 }
 
+/*
+ * The MBOX_BIOS_CMD_SMM_INFO PSP mailbox command doesn't necessarily need be sent from SMM,
+ * but doing so allows the linker to sort out the addresses of c2p_buffer, p2c_buffer and
+ * smm_flag without us needing to pass this info between ramstage and smm. In the PSP gen2 case
+ * this will also make sure that the PSP MMIO base will be cached in SMM before the OS takes
+ * over so no SMN accesses will be needed during OS runtime.
+ */
 int psp_notify_smm(void)
 {
 	msr_t msr;