[XS] Change in coreboot[main]: arch/x86/include/smm: improve documentation of call_smm

[Felix Held (Code Review)]

Attention is currently required from: Jérémy Compostella.

Felix Held has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/79834?usp=email )

Change subject: arch/x86/include/smm: improve documentation of call_smm
......................................................................

arch/x86/include/smm: improve documentation of call_smm

Since the inline assembly code doesn't make it exactly obvious how this
function to call the APMC SMI handler works in detail and especially how
it passes the sub-command and argument pointer to the SMI handler, add a
more detailed explanation as comment.

Signed-off-by: Felix Held 
Change-Id: I3566af191492ce00a3033335ff80e01c33e98e63
---
M src/arch/x86/include/smm_call.h
1 file changed, 4 insertions(+), 1 deletion(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/34/79834/1

diff --git a/src/arch/x86/include/smm_call.h b/src/arch/x86/include/smm_call.h
index 66925c4..d940d0b 100644
--- a/src/arch/x86/include/smm_call.h
+++ b/src/arch/x86/include/smm_call.h
@@ -4,7 +4,10 @@
 #include <cpu/x86/smm.h>

/*
- * calls into SMM with the given cmd and subcmd in eax, and arg in ebx
+ * Call the APMC SMI handler that resides in SMM. First, the command and sub-command are stored
+ * in eax, and the argument pointer is stored in ebx, then the command byte is written to the
+ * APMC IO port to trigger the SMI. The APMC SMI handler can then read the command from the
+ * APMC IO port and the contents of eax and ebx from the SMM state save area.
  *
  * static inline because the resulting assembly is often smaller than
  * the call sequence due to constant folding.