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: Tomi Valkeinen <tomi.valkeinen@ti.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/drivers/video/fbdev/omap2/dss/dispc.c b/drivers/video/fbdev/omap2/dss/dispc.c
index cc61513afdb4..ddce8fcfc5c1 100644
--- a/drivers/video/fbdev/omap2/dss/dispc.c
+++ b/drivers/video/fbdev/omap2/dss/dispc.c
@@ -2326,6 +2326,21 @@ again:
                 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;