--- linux-2.6.21.orig/drivers/video/xenonfb.c	2007-05-02 20:03:43.906027946 +0200
+++ linux-2.6.21/drivers/video/xenonfb.c	2007-05-02 20:03:28.873088446 +0200
@@ -182,17 +182,20 @@
 	int nyres = (vyres - offset_y * 2) * 0x1000 / (scl_v/0x1000) + black_top * 2;
 	printk("native resolution: %d x %d\n", nxres, nyres);
 
-	gfx[0x44/4] = 0; // disable black bar
-	gfx[0x48/4] = 0;
-	gfx[0x4c/4] = 0;
-	gfx[0x50/4] = 0;
-
-	gfx[0x590/4] = 0; // disable scaling
-	gfx[0x584/4] = (nxres << 16) | nyres;
-	gfx[0x580/4] = 0; // disable offset
-	gfx[0x5e8/4] = (nxres * 4) / 0x10 - 1; // fix pitch
-	gfx[0x134/4] = nxres;
-	gfx[0x138/4] = nyres;
+	/* don't update interlaced displays */
+	if (gfx[0x30/4] == 0) {
+		gfx[0x44/4] = 0; // disable black bar
+		gfx[0x48/4] = 0;
+		gfx[0x4c/4] = 0;
+		gfx[0x50/4] = 0;
+
+		gfx[0x590/4] = 0; // disable scaling
+		gfx[0x584/4] = (nxres << 16) | nyres;
+		gfx[0x580/4] = 0; // disable offset
+		gfx[0x5e8/4] = (nxres * 4) / 0x10 - 1; // fix pitch
+		gfx[0x134/4] = nxres;
+		gfx[0x138/4] = nyres;
+	}
 
 	ai->base &= ~0xFFFF; // page-align.