[PATCH] drm/i915/display: Fix RGB limited range handling for DP

[Ville Syrjälä]

On Tue, Jul 01, 2025 at 05:38:21PM +0000, Almahallawy, Khaled wrote:
> Thank you for your patch. This fix enables us to pass the DP1.4 Link
> Layer Test "4.2.2.8 EDID Read on IRQ HPD Event after Branch Device
> Detection," which was failing due to the following error:
> 
> "0003.610.625: Main Stream Attributes do not match expected values:
> 0003.610.669:   Received 28h color format differs from expected 20h."

If the test needs full range then set full range in the test.

> 
> Tested-by: Khaled Almahallawy <khaled.almahallawy at intel.com>
> 
> On Tue, 2025-07-01 at 14:15 +0000, Shankar, Uma wrote:
> > 
> > 
> > > -----Original Message-----
> > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > Sent: Tuesday, July 1, 2025 2:02 PM
> > > To: Shankar, Uma <uma.shankar at intel.com>
> > > Cc: intel-gfx at lists.freedesktop.org;
> > > intel-xe at lists.freedesktop.org; Borah,
> > > Chaitanya Kumar <chaitanya.kumar.borah at intel.com>; Almahallawy,
> > > Khaled
> > > <khaled.almahallawy at intel.com>
> > > Subject: Re: [PATCH] drm/i915/display: Fix RGB limited range
> > > handling for DP
> > > 
> > > On Tue, Jul 01, 2025 at 01:47:56PM +0530, Uma Shankar wrote:
> > > > RGB limited range should be selected only if explicitly asked by
> > > > userspace by the broadcast RGB property with LIMITED_RANGE. This
> > > > is
> > > > mostly enabled in case of CEA modes.
> > > > 
> > > > Display port by default uses Full Range, fixed the same. This
> > > > will
> > > > help set correct MSA information for colorimetry. Fixes a CTS
> > > > issue
> > > > wrt colorimetry.
> > > 
> > > If the CTS is expecting that then it's not following the actual DP
> > > spec.
> > > Originally the spec required limited range output for cea modes,
> > > but later version
> > > relaxed it to "should" and also notes that full range may also be
> > > used. Presumably
> > > it was realized that existing implementations had no consitency
> > > here and so both
> > > behaviours must be allowed.
> > 
> > Hi Ville,
> > Yes, there has been a flip flop. Ideally both should be allowed as
> > spec is not enforcing.
> > 
> > However, we can default to FULL RANGE and do Limited range if
> > userspace explicitly asks for it
> > using the broadcast RGB property. DP mostly have the FULL RANGE while
> > limited range is more
> > in the HDMI TVs. This will make CTS also happy. What do you suggest ?
> > 
> > Regards,
> > Uma Shankar
> > 
> > > > 
> > > > Signed-off-by: Uma Shankar <uma.shankar at intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/display/intel_dp.c | 21 +++++++------------
> > > > --
> > > >  1 file changed, 7 insertions(+), 14 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c
> > > > b/drivers/gpu/drm/i915/display/intel_dp.c
> > > > index f48912f308df..8758b9d60d5e 100644
> > > > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > > > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > > > @@ -2708,8 +2708,6 @@ bool intel_dp_limited_color_range(const
> > > > struct
> > > > intel_crtc_state *crtc_state,  {
> > > >  	const struct intel_digital_connector_state
> > > > *intel_conn_state =
> > > >  		to_intel_digital_connector_state(conn_state);
> > > > -	const struct drm_display_mode *adjusted_mode =
> > > > -		&crtc_state->hw.adjusted_mode;
> > > > 
> > > >  	/*
> > > >  	 * Our YCbCr output is always limited range.
> > > > @@ -2721,18 +2719,13 @@ bool intel_dp_limited_color_range(const
> > > > struct
> > > intel_crtc_state *crtc_state,
> > > >  	if (crtc_state->output_format !=
> > > > INTEL_OUTPUT_FORMAT_RGB)
> > > >  		return false;
> > > > 
> > > > -	if (intel_conn_state->broadcast_rgb ==
> > > INTEL_BROADCAST_RGB_AUTO) {
> > > > -		/*
> > > > -		 * See:
> > > > -		 * CEA-861-E - 5.1 Default Encoding Parameters
> > > > -		 * VESA DisplayPort Ver.1.2a - 5.1.1.1 Video
> > > > Colorimetry
> > > > -		 */
> > > > -		return crtc_state->pipe_bpp != 18 &&
> > > > -
> > > > 			drm_default_rgb_quant_range(adjusted_mode) ==
> > > > -			HDMI_QUANTIZATION_RANGE_LIMITED;
> > > > -	} else {
> > > > -		return intel_conn_state->broadcast_rgb ==
> > > > -			INTEL_BROADCAST_RGB_LIMITED;
> > > > +	switch (intel_conn_state->broadcast_rgb) {
> > > > +	case INTEL_BROADCAST_RGB_LIMITED:
> > > > +		return true;
> > > > +	case INTEL_BROADCAST_RGB_FULL:
> > > > +	case INTEL_BROADCAST_RGB_AUTO:
> > > > +	default:
> > > > +		return false;
> > > >  	}
> > > >  }
> > > > 
> > > > --
> > > > 2.42.0
> > > 
> > > --
> > > Ville Syrjälä
> > > Intel
> 

-- 
Ville Syrjälä
Intel