From: Marc-André Lureau <marcandre.lureau@redhat.com>

When bit 3 of the VGA Attribute Mode Control register is set, attribute
bit 7 switches from selecting bright background colors to enabling
character blink.

Implement this by tracking a separate blink phase timer that toggles
every 32 frames (matching real VGA hardware frame counter bit 5 @60hz),
and rendering blinking characters by replacing their foreground with
background during the off phase.

As with cursor, no VMState migration of the fields, as they are
transient display-side states.

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/1585
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 hw/display/vga_int.h |  2 ++
 hw/display/vga.c     | 24 +++++++++++++++++++++---
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/hw/display/vga_int.h b/hw/display/vga_int.h
index 747b5cc6cf8..5664317ecd6 100644
--- a/hw/display/vga_int.h
+++ b/hw/display/vga_int.h
@@ -130,6 +130,8 @@ typedef struct VGACommonState {
     uint8_t cursor_start, cursor_end;
     bool cursor_visible_phase;
     int64_t cursor_blink_time;
+    bool blink_visible_phase;
+    int64_t blink_time;
     uint32_t cursor_offset;
     const GraphicHwOps *hw_ops;
     bool full_update_text;
diff --git a/hw/display/vga.c b/hw/display/vga.c
index 0ac4bf37310..ea01a889f20 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -45,8 +45,10 @@
 
 bool have_vga = true;
 
-/* 16 state changes per vertical frame @60 Hz */
+/* frame counter bit 4: cursor blink toggles every 16 frames @60 Hz */
 #define VGA_TEXT_CURSOR_PERIOD_MS       (1000 * 2 * 16 / 60)
+/* frame counter bit 5: character blink toggles every 32 frames @60 Hz */
+#define VGA_TEXT_BLINK_PERIOD_MS        (1000 * 2 * 32 / 60)
 
 /* Address mask for non-VESA modes.  */
 #define VGA_VRAM_SIZE                   (256 * KiB)
@@ -1286,6 +1288,13 @@ static void vga_draw_text(VGACommonState *s, int full_update)
         s->cursor_blink_time = now + VGA_TEXT_CURSOR_PERIOD_MS / 2;
         s->cursor_visible_phase = !s->cursor_visible_phase;
     }
+    if (now >= s->blink_time) {
+        s->blink_time = now + VGA_TEXT_BLINK_PERIOD_MS / 2;
+        s->blink_visible_phase = !s->blink_visible_phase;
+        if (s->ar[VGA_ATC_MODE] & 0x08) {
+            full_update = 1;
+        }
+    }
 
     dest = surface_data(surface);
     linesize = surface_stride(surface);
@@ -1317,8 +1326,17 @@ static void vga_draw_text(VGACommonState *s, int full_update)
 #endif
                 font_ptr = font_base[(cattr >> 3) & 1];
                 font_ptr += 32 * 4 * ch;
-                bgcol = palette[cattr >> 4];
-                fgcol = palette[cattr & 0x0f];
+                if (s->ar[VGA_ATC_MODE] & 0x08) {
+                    bgcol = palette[(cattr >> 4) & 0x07];
+                    if ((cattr & 0x80) && !s->blink_visible_phase) {
+                        fgcol = bgcol;
+                    } else {
+                        fgcol = palette[cattr & 0x0f];
+                    }
+                } else {
+                    bgcol = palette[cattr >> 4];
+                    fgcol = palette[cattr & 0x0f];
+                }
                 if (cw == 16) {
                     vga_draw_glyph16(d1, linesize,
                                      font_ptr, cheight, fgcol, bgcol);
-- 
2.54.0