[PATCH] drm/i915/dp: Stop enabling limited color ranges for everything

[Lyude <lyude@redhat.com>]

Until now, it seems we've been erroneously enabling limited color ranges
for the vast majority of DisplayPort monitors. I noticed this after
writing a frame dump comparison test for the Chamelium and noticing that
every i915 device I had was failing, while amdgpu machines were fine:

https://people.freedesktop.org/~lyudess/archive/01-03-2017/

It looks like this is because the DisplayPort spec tells us to use
limited color ranges whenever we detect a CEA mode in use. However, from
the looks of it there's another rather confusing part of the spec that
got missed: source devices are allowed to use the full range of values
for pixels -even- if the sink device declares that it's using a CEA
mode. It's up to the sink device to limit the pixel range to the CEA
ranges if it needs.

Signed-off-by: Lyude <lyude@redhat.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
I'm really surprised that this bug would have been around as long as it looks
like it has been without anyone noticing it, so I figured I'd just send a patch
to the mailing list so you guys can point out whether or not this is really the
correct thing to do.

 drivers/gpu/drm/i915/intel_dp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index d9bc19b..6642abd 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -1649,12 +1649,15 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 found:
 	if (intel_dp->color_range_auto) {
 		/*
+		 * We are required to use the limited CEA RGB range when a CEA
+		 * mode is declared in the EDID. However, limiting the pixel
+		 * value range is up to the sink, not the source. So, just
+		 * don't enable limited color ranges.
 		 * See:
 		 * CEA-861-E - 5.1 Default Encoding Parameters
 		 * VESA DisplayPort Ver.1.2a - 5.1.1.1 Video Colorimetry
 		 */
-		pipe_config->limited_color_range =
-			bpp != 18 && drm_match_cea_mode(adjusted_mode) > 1;
+		pipe_config->limited_color_range = false;
 	} else {
 		pipe_config->limited_color_range =
 			intel_dp->limited_color_range;
-- 
2.9.3