[PATCH 3/3] artist: only render dirty scanlines on the display surface

[Mark Cave-Ayland]

The framebuffer_update_display() function returns the dirty scanlines that were
touched since the last display update, however artist_update_display() always 
calls
dpy_gfx_update() with start and end scanlines of 0 and s->height causing the
entire display surface to be rendered on every update.

Update artist_update_display() so that dpy_gfx_update() only renders the dirty
scanlines on the display surface, bypassing the display surface rendering
completely if framebuffer_update_display() indicates no changes occurred.

This noticeably improves boot performance when the framebuffer is enabled on my
rather modest laptop here, including making the GTK UI usable.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
---
 hw/display/artist.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/hw/display/artist.c b/hw/display/artist.c
index 4a9f7b5e59..39fc0c4ca5 100644
--- a/hw/display/artist.c
+++ b/hw/display/artist.c
@@ -1243,7 +1243,9 @@ static void artist_update_display(void *opaque)
 
     artist_draw_cursor(s);
 
-    dpy_gfx_update(s->con, 0, 0, s->width, s->height);
+    if (first >= 0) {
+        dpy_gfx_update(s->con, 0, first, s->width, last - first + 1);
+    }
 }
 
 static void artist_invalidate(void *opaque)
-- 
2.20.1