[XS] Change in coreboot[main]: soc/amd/common/psp_smm: add comments to psp_notify_smm

[Felix Held (Code Review)]

Felix Held has submitted this change. ( https://review.coreboot.org/c/coreboot/+/83699?usp=email )

Change subject: soc/amd/common/psp_smm: add comments to psp_notify_smm
......................................................................

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 
Change-Id: I4f4121031fc1ef600cdf5551f61f1ef4e03b56a5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83699
Tested-by: build bot (Jenkins) 
Reviewed-by: Matt DeVillier 
---
M src/soc/amd/common/block/psp/psp_smm.c
1 file changed, 7 insertions(+), 0 deletions(-)

Approvals:
  Matt DeVillier: Looks good to me, approved
  build bot (Jenkins): Verified

diff --git a/src/soc/amd/common/block/psp/psp_smm.c b/src/soc/amd/common/block/psp/psp_smm.c
index 114cf12..eecdf02 100644
--- a/src/soc/amd/common/block/psp/psp_smm.c
+++ b/src/soc/amd/common/block/psp/psp_smm.c
@@ -47,6 +47,13 @@
    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;