[PATCH] drm/i915: Only dither on 6bpc panels

[Daniel Vetter]

In

commit d328c9d78d64ca11e744fe227096990430a88477
Author: Daniel Vetter <daniel.vetter at ffwll.ch>
Date:   Fri Apr 10 16:22:37 2015 +0200

    drm/i915: Select starting pipe bpp irrespective or the primary plane

we started to select the pipe bpp from sink capabilities and not from
the primary framebuffer - that one might change (and we don't want to
incur a modeset) and sprites might contain higher bpp content too.

Problem is that now if you have a 10bpc screen and display 24bpp rgb
primary then we select dithering, and apparently that mangles the high
8 bits even (even thought you'd expect dithering only to affect how
12bpc gets mapped into 10bpc). And that mangling upsets certain users.

Hence only enable dithering on 6bpc screens where we difinitely and
always want it.

Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
Reported-by: Mario Kleiner <mario.kleiner.de at gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/i915/intel_display.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 9a2f229a1c3a..128462e0a0b5 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -12186,7 +12186,9 @@ encoder_retry:
 		goto encoder_retry;
 	}
 
-	pipe_config->dither = pipe_config->pipe_bpp != base_bpp;
+	/* Dithering seems to not pass-through bits correctly when it should, so
+	 * only enable it on 6bpc panels. */
+	pipe_config->dither = pipe_config->pipe_bpp == 6*3;
 	DRM_DEBUG_KMS("plane bpp: %i, pipe bpp: %i, dithering: %i\n",
 		      base_bpp, pipe_config->pipe_bpp, pipe_config->dither);
 
-- 
2.5.0