x86: bios: Synchronize stack between real and protected mode

PCI option rom may use different SS during its execution, so it is not
safe to assume esp pointed to the same location in the protected mode.

Signed-off-by: Jian Luo <jian.luo4@boschrexroth.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>

arch/x86/lib/bios_asm.S

@@ -246,6 +246,9 @@ __interrupt_handler_16bit = PTR_TO_REAL_MODE(.)
 	push	%fs
 	push	%gs
 
+	/* Save real mode SS */
+	movw	%ss, %cs:__realmode_ss
+
 	/* Clear DF to not break ABI assumptions */
 	cld
 
@@ -258,12 +261,29 @@ __interrupt_handler_16bit = PTR_TO_REAL_MODE(.)
 
 	enter_protected_mode
 
+	/*
+	 * Now we are in protected mode. We need compute the right ESP based
+	 * on saved real mode SS otherwise interrupt_handler() won't get
+	 * correct parameters from the stack.
+	 */
+	movzwl	%cs:__realmode_ss, %ecx
+	shll	$4, %ecx
+	addl	%ecx, %esp
+
 	/* Call the C interrupt handler */
 	movl	$interrupt_handler, %eax
 	call	*%eax
 
+	/* Restore real mode ESP based on saved SS */
+	movzwl	%cs:__realmode_ss, %ecx
+	shll	$4, %ecx
+	subl	%ecx, %esp
+
 	enter_real_mode
 
+	/* Restore real mode SS */
+	movw	%cs:__realmode_ss, %ss
+
 	/*
 	 * Restore all registers, including those manipulated by the C
 	 * handler
@@ -276,6 +296,9 @@ __interrupt_handler_16bit = PTR_TO_REAL_MODE(.)
 	popal
 	iret
 
+__realmode_ss = PTR_TO_REAL_MODE(.)
+	.word	0
+
 	.globl asm_realmode_code_size
 asm_realmode_code_size:
 	.long  . - asm_realmode_code