OMAPDSS: DISPC: add check for scaling limits

On OMAP3/AM43xx some scaling factors cause underflows/synclosts. After
studying this, I found that sometimes the driver uses three-tap scaling
with downscaling factor smaller than x0.5. This causes issues, as x0.5
is the limit for three-tap scaling.

The driver has FEAT_PARAM_DOWNSCALE parameter, but that seems to be for
five-tap scaling, which allows scaling down to x0.25.

This patch adds checks for both horizontal and vertical scaling. For
horizontal the HW always uses 5 taps, so the limit is x0.25.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/fbdev/omap2/dss/dispc.c

@@ -2326,6 +2326,21 @@ static int dispc_ovl_calc_scaling_34xx(unsigned long pclk, unsigned long lclk,
 		error = (error || in_width > maxsinglelinewidth * 2 ||
 			(in_width > maxsinglelinewidth && *five_taps) ||
 			!*core_clk || *core_clk > dispc_core_clk_rate());
+
+		if (!error) {
+			/* verify that we're inside the limits of scaler */
+			if (in_width / 4 > out_width)
+					error = 1;
+
+			if (*five_taps) {
+				if (in_height / 4 > out_height)
+					error = 1;
+			} else {
+				if (in_height / 2 > out_height)
+					error = 1;
+			}
+		}
+
 		if (error) {
 			if (*decim_x == *decim_y) {
 				*decim_x = min_factor;