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

[Andres Calderon Jaramillo]

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.

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)?

> >
> > 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
> >
> > _______________________________________________
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> > Intel-gfx at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
>
> --
> Ville Syrjälä
> Intel