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

[Jyri Sarha]

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, 1 byte per pixel color format appear 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: Jyri Sarha <jsarha at ti.com>
---
 drivers/gpu/drm/omapdrm/dss/dispc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/omapdrm/dss/dispc.c b/drivers/gpu/drm/omapdrm/dss/dispc.c
index 5554b72..61daef6 100644
--- a/drivers/gpu/drm/omapdrm/dss/dispc.c
+++ b/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_to_bpp(color_mode) > 8) {
+		/*
+		  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, 1 byte per pixel color format appear 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 (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;
-- 
1.9.1