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 at redhat.com> Cc: Chris Wilson <chris at chris-wilson.co.uk> Cc: Daniel Vetter <daniel at ffwll.ch> Cc: Jani Nikula <jani.nikula at intel.com> Cc: Ville Syrjälä <ville.syrjala at 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
