I have found some problem in building kernel with radeonfb 16bpp support at 2.4.2-ac8.
here is patch. 
--HlL+5n6rz5pIUxbD
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Content-Disposition: attachment; filename="linux-2.4.2-ac8-radeonfb.patch"
--- linux/drivers/video/radeonfb.c.orig	Fri Mar  2 12:29:15 2001
+++ linux/drivers/video/radeonfb.c	Fri Mar  2 12:29:28 2001
@@ -845,9 +845,11 @@
 
 	rinfo->depth = disp->var.bits_per_pixel;	
         switch (disp->var.bits_per_pixel) {
+#ifdef FBCON_HAS_CFB8
                 case 8:
                         disp->dispsw = &fbcon_cfb8;
                         break;
+#endif			
 #ifdef FBCON_HAS_CFB16
                 case 16:
                         disp->dispsw = &fbcon_cfb16;
@@ -1322,8 +1322,8 @@
   		       		i = (regno << 8) | regno;
             			rinfo->con_cmap.cfb32[regno] = (i << 16) | i;
 		        	break;
-#endif
         		}
+#endif
 		}
         }
 #endif
--HlL+5n6rz5pIUxbD--
-
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