vgabios: Add config option for assembler fixups

Add a kconfig build option (CONFIG_VGA_FIXUP_ASM) to allow users to
build the vgabios without the complex assembler fixups that work
around emulator bugs.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

2 files changed, 14 insertions, 0 deletions

diff --git a/vgasrc/Kconfig b/vgasrc/Kconfig
index 400e8da..27a24c9 100644
--- a/vgasrc/Kconfig
+++ b/vgasrc/Kconfig
@@ -90,6 +90,16 @@ menu "VGA ROM"
             Support emulating text mode features when only a
             framebuffer is available.
 
+    config VGA_FIXUP_ASM
+        bool "Fixup assembler to work with broken emulators"
+        default y
+        help
+            This option will cause the build to attempt to avoid
+            certain x86 machine instructions that are known to confuse
+            some emulators.  In particular, it works around
+            deficiencies in the Windows vgabios emulator and the
+            x86emu vgabios emulator (frequently used in Xorg).
+
     config VGA_ALLOCATE_EXTRA_STACK
         depends on BUILD_VGABIOS
         bool "Allocate an internal stack for 16bit interrupt entry point"
diff --git a/vgasrc/vgaentry.S b/vgasrc/vgaentry.S
index e0ab954..7ca550d 100644
--- a/vgasrc/vgaentry.S
+++ b/vgasrc/vgaentry.S
@@ -64,6 +64,7 @@ x86emu_fault:
         // This macro implements a call while avoiding instructions
         // that old versions of x86emu have problems with.
         .macro VGA_CALLL cfunc
+#if CONFIG_VGA_FIXUP_ASM
         // Make sure leal instruction works.
         movl $0x8000, %ecx
         leal (%ecx, %ecx, 1), %ecx
@@ -72,6 +73,9 @@ x86emu_fault:
         // Use callw instead of calll
         push %ax
         callw \cfunc
+#else
+        calll \cfunc
+#endif
         .endm
 
         // This macro is the same as ENTRY_ARG except VGA_CALLL is used.