Re: [PATCH 17/28] drm/i915: Define segmented Lut and add capabilities to colorop

[Pekka Paalanen <pekka.paalanen@haloniitty.fi>]

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On Wed, 14 Feb 2024 07:28:37 +0000
"Shankar, Uma" <uma.shankar@intel.com> wrote:

> > -----Original Message-----
> > From: dri-devel <dri-devel-bounces@lists.freedesktop.org> On Behalf Of Pekka
> > Paalanen
> > Sent: Tuesday, February 13, 2024 3:07 PM
> > To: Shankar, Uma <uma.shankar@intel.com>
> > Cc: intel-gfx@lists.freedesktop.org; dri-devel@lists.freedesktop.org;
> > ville.syrjala@linux.intel.com; contact@emersion.fr; harry.wentland@amd.com;
> > mwen@igalia.com; jadahl@redhat.com; sebastian.wick@redhat.com;
> > shashank.sharma@amd.com; agoins@nvidia.com; joshua@froggi.es;
> > mdaenzer@redhat.com; aleixpol@kde.org; xaver.hugl@gmail.com;
> > victoria@system76.com; daniel@ffwll.ch; quic_naseer@quicinc.com;
> > quic_cbraga@quicinc.com; quic_abhinavk@quicinc.com; arthurgrillo@riseup.net;
> > marcan@marcan.st; Liviu.Dudau@arm.com; sashamcintosh@google.com;
> > sean@poorly.run
> > Subject: Re: [PATCH 17/28] drm/i915: Define segmented Lut and add capabilities
> > to colorop
> > 
> > On Tue, 13 Feb 2024 12:18:24 +0530
> > Uma Shankar <uma.shankar@intel.com> wrote:
> >   
> > > This defines the lut segments and create the color pipeline
> > >
> > > Signed-off-by: Uma Shankar <uma.shankar@intel.com>
> > > Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/display/intel_color.c | 109
> > > +++++++++++++++++++++
> > >  1 file changed, 109 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/i915/display/intel_color.c
> > > b/drivers/gpu/drm/i915/display/intel_color.c
> > > index e223edbe4c13..223cd1ff7291 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_color.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_color.c
> > > @@ -3811,6 +3811,105 @@ static const struct intel_color_funcs  
> > ilk_color_funcs = {  
> > >  	.get_config = ilk_get_config,
> > >  };
> > >
> > > +static const struct drm_color_lut_range xelpd_degamma_hdr[] = {
> > > +	/* segment 1 */
> > > +	{
> > > +		.flags = (DRM_MODE_LUT_REFLECT_NEGATIVE |
> > > +				DRM_MODE_LUT_INTERPOLATE |
> > > +				DRM_MODE_LUT_NON_DECREASING),  
> > 
> > Hi Uma,
> > 
> > is it a good idea to have these flags per-segment?
> > 
> > I would find it very strange, unusable really, if REFLECT_NEGATIVE applied on
> > some but not all segments, for example. Is such flexibility really necessary in the
> > hardware description?  
> 
> Hi Pekka,
> Idea to have these flags is to just have some option in case there are some differences
> across segments. Most cases this should not be the case, just helps to future proof
> the implementation.
> 
> Based on how the community feels on the usability of it, we can take a call on the flags
> and the expected interpretation for the same. We are open for suggestions on the same.
> 
> >   
> > > +		.count = 128,
> > > +		.input_bpc = 24, .output_bpc = 16,  
> > 
> > The same question about input_bpc and output_bpc.  
> 
> Same for these as well, userspace can just ignore these if no usage. However, for some clients
> it may help in Lut computations.
> The original idea for the structure came from Ville (missed to mention that in cover letter, will get that
> updated in next version).
> 
> @ville.syrjala@linux.intel.com Please share your inputs on the usability of these attributes.

Userspace will always need to evaluate whether each segment is good
enough individually, so maybe it's not that big deal.

Ignoring these is not an option for userspace, because that would mean
userspace does not know what it is getting. If UAPI contains a
parameter, then the onus is on userspace to ensure the value is
acceptable.

> > > +		.start = 0, .end = (1 << 24) - 1,
> > > +		.min = 0, .max = (1 << 24) - 1,
> > > +	},
> > > +	/* segment 2 */
> > > +	{
> > > +		.flags = (DRM_MODE_LUT_REFLECT_NEGATIVE |
> > > +				DRM_MODE_LUT_INTERPOLATE |
> > > +				DRM_MODE_LUT_REUSE_LAST |
> > > +				DRM_MODE_LUT_NON_DECREASING),
> > > +		.count = 1,
> > > +		.input_bpc = 24, .output_bpc = 16,
> > > +		.start = (1 << 24) - 1, .end = 1 << 24,  
> > 
> > What if there is a gap or overlap between the previous segment .end and the next
> > segment .start? Is it forbidden? Will the kernel common code verify that drivers
> > don't make mistakes? Or IGT?  
> 
> This is just to help give some reference to userspace.  As of now, driver trusts the values
> coming from userspace if it sends wrong values its on him and driver can't help much.
> However, we surely can have some sanity check like non decreasing luts etc. to driver.

But what will guarantee that the driver provided values are consistent?
That they actually describe a template of a well-formed sampled
curve? If they are not consistent, userspace cannot use the colorop.
Whose responsibility is it to ensure the consistency?

We have a few examples of drivers getting descriptive values like
these simply wrong until DRM common code started sanity-checking them,
the bitmasks of possible_clones and possible_crtcs for example.

There should also be DRM common code to verify that userspace provided
data matches the segmented LUT description rather than drivers just
trusting it. If it doesn't match, the atomic commit must fail rather
than silently malfunction. The same with programming hardware: if
hardware does not produce the intended result from a given segmented
LUT configuration, the atomic commit must fail instead of malfunction.

> 
> Ideally LUT values should not overlap, but we can indicate this explicitly with flag to
> hint the userspace (for overlap or otherwise) and also get a check in driver for the same.

Sorry? How could overlapping segments ever work? Or segments with a gap
between them?

If segments overlap, what's the rule for choosing which segment to use
for an input value hitting both? The segments can disagree on the
result.

If there are gaps, what is the rule how to handle an input value
hitting a gap?


Thanks,
pq

> 
> Regards,
> Uma Shankar
> 
> > 
> > Thanks,
> > pq
> >   
> > > +		.min = 0, .max = (1 << 27) - 1,
> > > +	},
> > > +	/* Segment 3 */
> > > +	{
> > > +		.flags = (DRM_MODE_LUT_REFLECT_NEGATIVE |
> > > +				DRM_MODE_LUT_INTERPOLATE |
> > > +				DRM_MODE_LUT_REUSE_LAST |
> > > +				DRM_MODE_LUT_NON_DECREASING),
> > > +		.count = 1,
> > > +		.input_bpc = 24, .output_bpc = 16,
> > > +		.start = 1 << 24, .end = 3 << 24,
> > > +		.min = 0, .max = (1 << 27) - 1,
> > > +	},
> > > +	/* Segment 4 */
> > > +	{
> > > +		.flags = (DRM_MODE_LUT_REFLECT_NEGATIVE |
> > > +				DRM_MODE_LUT_INTERPOLATE |
> > > +				DRM_MODE_LUT_REUSE_LAST |
> > > +				DRM_MODE_LUT_NON_DECREASING),
> > > +		.count = 1,
> > > +		.input_bpc = 24, .output_bpc = 16,
> > > +		.start = 3 << 24, .end = 7 << 24,
> > > +		.min = 0, .max = (1 << 27) - 1,
> > > +	}
> > > +};
> > > +
> > > +/* FIXME input bpc? */
> > > +static const struct drm_color_lut_range xelpd_gamma_hdr[] = {
> > > +	/*
> > > +	 * ToDo: Add Segment 1
> > > +	 * There is an optional fine segment added with 9 lut values
> > > +	 * Will be added later
> > > +	 */
> > > +
> > > +	/* segment 2 */
> > > +	{
> > > +		.flags = (DRM_MODE_LUT_REFLECT_NEGATIVE |
> > > +				DRM_MODE_LUT_INTERPOLATE |
> > > +				DRM_MODE_LUT_NON_DECREASING),
> > > +		.count = 32,
> > > +		.input_bpc = 24, .output_bpc = 16,
> > > +		.start = 0, .end = (1 << 24) - 1,
> > > +		.min = 0, .max = (1 << 24) - 1,
> > > +	},
> > > +	/* segment 3 */
> > > +	{
> > > +		.flags = (DRM_MODE_LUT_REFLECT_NEGATIVE |
> > > +				DRM_MODE_LUT_INTERPOLATE |
> > > +				DRM_MODE_LUT_REUSE_LAST |
> > > +				DRM_MODE_LUT_NON_DECREASING),
> > > +		.count = 1,
> > > +		.input_bpc = 24, .output_bpc = 16,
> > > +		.start = (1 << 24) - 1, .end = 1 << 24,
> > > +		.min = 0, .max = 1 << 24,
> > > +	},
> > > +	/* Segment 4 */
> > > +	{
> > > +		.flags = (DRM_MODE_LUT_REFLECT_NEGATIVE |
> > > +				DRM_MODE_LUT_INTERPOLATE |
> > > +				DRM_MODE_LUT_REUSE_LAST |
> > > +				DRM_MODE_LUT_NON_DECREASING),
> > > +		.count = 1,
> > > +		.input_bpc = 24, .output_bpc = 16,
> > > +		.start = 1 << 24, .end = 3 << 24,
> > > +		.min = 0, .max = (3 << 24),
> > > +	},
> > > +	/* Segment 5 */
> > > +	{
> > > +		.flags = (DRM_MODE_LUT_REFLECT_NEGATIVE |
> > > +				DRM_MODE_LUT_INTERPOLATE |
> > > +				DRM_MODE_LUT_REUSE_LAST |
> > > +				DRM_MODE_LUT_NON_DECREASING),
> > > +		.count = 1,
> > > +		.input_bpc = 24, .output_bpc = 16,
> > > +		.start = 3 << 24, .end = 7 << 24,
> > > +		.min = 0, .max = (7 << 24),
> > > +	},
> > > +};
> > > +
> > >  /* TODO: Move to another file */
> > >  struct intel_plane_colorop *intel_colorop_alloc(void)  { @@ -3865,6
> > > +3964,11 @@ int intel_plane_tf_pipeline_init(struct drm_plane *plane, struct  
> > drm_prop_enum_l  
> > >  	if (ret)
> > >  		return ret;
> > >
> > > +	if (icl_is_hdr_plane(i915, to_intel_plane(plane)->id)) {
> > > +		drm_colorop_lutcaps_init(&colorop->base, plane,  
> > xelpd_degamma_hdr,  
> > > +					 sizeof(xelpd_degamma_hdr));
> > > +	}
> > > +
> > >  	list->type = colorop->base.base.id;
> > >  	list->name = kasprintf(GFP_KERNEL, "Color Pipeline %d",
> > > colorop->base.base.id);
> > >
> > > @@ -3886,6 +3990,11 @@ int intel_plane_tf_pipeline_init(struct drm_plane  
> > *plane, struct drm_prop_enum_l  
> > >  	if (ret)
> > >  		return ret;
> > >
> > > +	if (icl_is_hdr_plane(i915, to_intel_plane(plane)->id)) {
> > > +		drm_colorop_lutcaps_init(&colorop->base, plane,  
> > xelpd_gamma_hdr,  
> > > +					 sizeof(xelpd_gamma_hdr));
> > > +	}
> > > +
> > >  	drm_colorop_set_next_property(prev_op, &colorop->base);
> > >
> > >  	return 0;  
> 


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