cpu/x86/64bit/mode_switch: Simplify assembly code

Drop the first argument specifying the number of arguments pushed
to the stack. Instead always push the 3 arguments to stack and use
the first one as function pointer to call while in protected mode.

While on it add more comments and simplify register restore code.

Tested:
- On qemu can call x86_32 function and pass argument and return
  value.
- Booted Lenovo X220 in x86_64 mode using x86_32 MRC.

Change-Id: I30809453a1800ba3c0df60acd7eca778841c520f
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79752
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

1 files changed, 14 insertions, 6 deletions

diff --git a/src/arch/x86/include/mode_switch.h b/src/arch/x86/include/mode_switch.h
index 24efb1ef58..4235db9be3 100644
--- a/src/arch/x86/include/mode_switch.h
+++ b/src/arch/x86/include/mode_switch.h
@@ -3,10 +3,18 @@
 #include <stdint.h>
 
 #if ENV_X86_64
-int protected_mode_call_narg(uint32_t arg_count,
-			     uint32_t func_ptr,
+/*
+ * Assembly code that drops into protected mode and calls the function
+ * specified as first argument, which must have been compiled for x86_32.
+ * After the function returns it enters long mode again.
+ * The function pointer destination must be below 4GiB in physical memory.
+ *
+ * The called function has 0-3 arguments and returns an int.
+ */
+int protected_mode_call_3arg(uint32_t func_ptr,
 			     uint32_t opt_arg1,
-			     uint32_t opt_arg2);
+			     uint32_t opt_arg2,
+			     uint32_t opt_arg3);
 
 /*
  * Drops into protected mode and calls the function, which must have been compiled for x86_32.
@@ -17,7 +25,7 @@ int protected_mode_call_narg(uint32_t arg_count,
  */
 static inline int protected_mode_call(void *func)
 {
-	return protected_mode_call_narg(0, (uintptr_t)func, 0, 0);
+	return protected_mode_call_3arg((uintptr_t)func, 0, 0, 0);
 }
 
 /*
@@ -30,7 +38,7 @@ static inline int protected_mode_call(void *func)
  */
 static inline int protected_mode_call_1arg(void *func, uint32_t arg1)
 {
-	return protected_mode_call_narg(1, (uintptr_t)func, arg1, 0);
+	return protected_mode_call_3arg((uintptr_t)func, arg1, 0, 0);
 }
 
 /*
@@ -43,7 +51,7 @@ static inline int protected_mode_call_1arg(void *func, uint32_t arg1)
  */
 static inline int protected_mode_call_2arg(void *func, uint32_t arg1, uint32_t arg2)
 {
-	return protected_mode_call_narg(2, (uintptr_t)func, arg1, arg2);
+	return protected_mode_call_3arg((uintptr_t)func, arg1, arg2, 0);
 }
 #else
 static inline int protected_mode_call(void *func)