drm/omapdrm: dispc: Refuse x-decimation above 4 for all but 8-bit formats

Let's disable all scaling that requires horizontal decimation with
higher factor than 4, until we have better estimates of what we can
and can not do. However, NV12 color format appears to work Ok with
all decimation factors.

When decimating horizontally by more that 4 the dss is not able to
fetch the data in burst mode. When this happens it is hard to tell if
there enough bandwidth. Despite what theory says this appears to be
true also for 16-bit color formats.
Signed-off-by: NJyri Sarha <jsarha@ti.com>
Acked-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/omapdrm/dss/dispc.c

@@ -2506,6 +2506,25 @@ static int dispc_ovl_calc_scaling_44xx(unsigned long pclk, unsigned long lclk,
 		return -EINVAL;
 	}
 
+	if (*decim_x > 4 && color_mode != OMAP_DSS_COLOR_NV12) {
+		/*
+		 * Let's disable all scaling that requires horizontal
+		 * decimation with higher factor than 4, until we have
+		 * better estimates of what we can and can not
+		 * do. However, NV12 color format appears to work Ok
+		 * with all decimation factors.
+		 *
+		 * When decimating horizontally by more that 4 the dss
+		 * is not able to fetch the data in burst mode. When
+		 * this happens it is hard to tell if there enough
+		 * bandwidth. Despite what theory says this appears to
+		 * be true also for 16-bit color formats.
+		 */
+		DSSERR("Not enough bandwidth, too much downscaling (x-decimation factor %d > 4)", *decim_x);
+
+		return -EINVAL;
+	}
+
 	*core_clk = dispc.feat->calc_core_clk(pclk, in_width, in_height,
 				out_width, out_height, mem_to_mem);
 	return 0;