vga: fix incorrect line height in 640x200x2 mode

When in CGA modes, QEMU wants to ignore the maximum scan field (bits 0..4) of
the maximum scan length register in the CRTC.  It is not clear why this is
needed---for example, Bochs ignores bit 7 instead.  The issue is that the
CGA modes are not detected correctly, and in particular mode 6 results in
multi_scan==3 according to how SeaBIOS programs it.  The right way to check
for CGA graphics modes is to check whether bit 13 of the address is special
cased by the CRT controller to achieve line interleaving, i.e. whether bit 0
of the CRTC mode control register is clear.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1020
Reported-by: Korneliusz Osmenda <korneliuszo@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/hw/display/vga.c b/hw/display/vga.c
index 5dca2d15284e5717cd2f865e6294eb79bc9f32c0..50ecb1ad0297c3f503e691706c37c05b05759aba 100644 (file)
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -1514,9 +1514,10 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
         force_shadow = true;
     }
 
+    /* bits 5-6: 0 = 16-color mode, 1 = 4-color mode, 2 = 256-color mode.  */
     shift_control = (s->gr[VGA_GFX_MODE] >> 5) & 3;
     double_scan = (s->cr[VGA_CRTC_MAX_SCAN] >> 7);
-    if (shift_control != 1) {
+    if (s->cr[VGA_CRTC_MODE] & 1) {
         multi_scan = (((s->cr[VGA_CRTC_MAX_SCAN] & 0x1f) + 1) << double_scan)
             - 1;
     } else {