[Intel-gfx] [PATCH] drm/i915/display: Prevent double YUV range correction on HDR planes

[Ville Syrjälä]

On Tue, Dec 15, 2020 at 03:06:30PM -0500, Andres Calderon Jaramillo wrote:
> On Tue, Dec 15, 2020 at 1:01 PM Ville Syrjälä
> <ville.syrjala at linux.intel.com> wrote:
> >
> > On Mon, Dec 14, 2020 at 10:57:03PM +0000, Shankar, Uma wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: andrescj via sendgmr <andrescj at andrescj-cros-specialist.c.googlers.com>
> > > > On Behalf Of Andres Calderon Jaramillo
> > > > Sent: Tuesday, December 15, 2020 3:50 AM
> > > > To: intel-gfx at lists.freedesktop.org
> > > > Cc: Shankar, Uma <uma.shankar at intel.com>; Venkatesh Reddy, Sushma
> > > > <sushma.venkatesh.reddy at intel.com>; seanpaul at chromium.org; Andres
> > > > Calderon Jaramillo <andrescj at chromium.org>
> > > > Subject: [PATCH] drm/i915/display: Prevent double YUV range correction on HDR
> > > > planes
> > > >
> > > > From: Andres Calderon Jaramillo <andrescj at chromium.org>
> > > >
> > > > Prevent the ICL HDR plane pipeline from performing YUV color range correction
> > > > twice when the input is in limited range.
> > > >
> > > > Before this patch the following could happen: user space gives us a YUV buffer in
> > > > limited range; per the pipeline in [1], the plane would first go through a "YUV
> > > > Range correct" stage that expands the range; the plane would then go through
> > > > the "Input CSC" stage which would also expand the range because
> > > > icl_program_input_csc() would use a matrix and an offset that assume limited-
> > > > range input; this would ultimately cause dark and light colors to appear darker
> > > > and lighter than they should respectively.
> > > >
> > > > This is an issue because if a buffer switches between being scanned out and
> > > > being composited with the GPU, the user will see a color difference.
> > > > If this switching happens quickly and frequently, the user will perceive this as a
> > > > flickering.
> > > >
> > > > [1] https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-icllp-
> > > > vol12-displayengine_0.pdf#page=281
> > >
> > > Change looks good to me, double conversion should not be done.
> > > Plane input csc coefficients take care of the limited range.
> >
> > Might make sense to actually use the hw range correction instead.
> > Would avoid having to keep around two sets of coefficients.
> >
> > Also the question now becomes: How can our tests be passing if
> > we're doing the range correction twice?
> >
> 
> I also considered just removing the limited-range matrix/offsets from
> icl_program_input_csc(). However, I figured that since the "Input CSC"
> stage must happen regardless of range correction, maybe it would be a
> gain to disable the "YUV Range Correction" stage. However, I'm not
> familiar with the hardware details, so I don't know for sure. I don't
> feel strongly either way; let me know what you'd prefer.

IIRC we use the fixed function range compression + full range csc
approach on chv as well. Wouldn't hurt to do the same thing on
icl+ IMO.

> 
> I'm also curious about the testing. How do the tests work? I assume
> they are in Intel's CI and not open sourced? Do they use the DRM
> writeback connector (I didn't think this was implemented for i915
> yet)?

Tests are here:
git://anongit.freedesktop.org/xorg/app/intel-gpu-tools

In particular kms_plane/pixel-format tests should rather be failing I
think. Unless we've relaxed things a bit too much to get the crcs to
match when the results are close enough.

> 
> > >
> > > Reviewed-by: Uma Shankar <uma.shankar at intel.com>
> > >
> > > > Signed-off-by: Andres Calderon Jaramillo <andrescj at chromium.org>
> > > > ---
> > > >  drivers/gpu/drm/i915/display/intel_display.c | 7 +++++++
> > > >  1 file changed, 7 insertions(+)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/display/intel_display.c
> > > > b/drivers/gpu/drm/i915/display/intel_display.c
> > > > index 761be8deaa9b..aeea344b06ad 100644
> > > > --- a/drivers/gpu/drm/i915/display/intel_display.c
> > > > +++ b/drivers/gpu/drm/i915/display/intel_display.c
> > > > @@ -4811,6 +4811,13 @@ u32 glk_plane_color_ctl(const struct intel_crtc_state
> > > > *crtc_state,
> > > >                     plane_color_ctl |=
> > > > PLANE_COLOR_YUV_RANGE_CORRECTION_DISABLE;
> > > >     } else if (fb->format->is_yuv) {
> > > >             plane_color_ctl |= PLANE_COLOR_INPUT_CSC_ENABLE;
> > > > +
> > > > +           /*
> > > > +            * Disable the YUV range correction stage because the input CSC
> > > > +            * stage already takes care of range conversion by using separate
> > > > +            * matrices and offsets depending on the color range.
> > > > +            */
> > > > +           plane_color_ctl |=
> > > > PLANE_COLOR_YUV_RANGE_CORRECTION_DISABLE;
> > > >     }
> > > >
> > > >     return plane_color_ctl;
> > > > --
> > > > 2.29.2.684.gfbc64c5ab5-goog
> > >
> > > _______________________________________________
> > > Intel-gfx mailing list
> > > Intel-gfx at lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> >
> > --
> > Ville Syrjälä
> > Intel

-- 
Ville Syrjälä
Intel