Re: [RFC v2 05/22] drm/i915/xelpd: Define Degamma Lut range struct for HDR planes

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Thu, 11 Nov 2021 21:58:35 +0000
"Shankar, Uma" <uma.shankar@intel.com> wrote:

> > -----Original Message-----
> > From: Harry Wentland <harry.wentland@amd.com>
> > Sent: Friday, November 12, 2021 2:41 AM
> > To: Shankar, Uma <uma.shankar@intel.com>; Ville Syrjälä
> > <ville.syrjala@linux.intel.com>
> > Cc: intel-gfx@lists.freedesktop.org; dri-devel@lists.freedesktop.org;
> > ppaalanen@gmail.com; brian.starkey@arm.com; sebastian@sebastianwick.net;
> > Shashank.Sharma@amd.com
> > Subject: Re: [RFC v2 05/22] drm/i915/xelpd: Define Degamma Lut range struct for
> > HDR planes
> > 
> > 
> > 
> > On 2021-11-11 15:42, Shankar, Uma wrote:  
> > >
> > >  
> > >> -----Original Message-----
> > >> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > >> Sent: Thursday, November 11, 2021 10:13 PM
> > >> To: Harry Wentland <harry.wentland@amd.com>
> > >> Cc: Shankar, Uma <uma.shankar@intel.com>;
> > >> intel-gfx@lists.freedesktop.org; dri- devel@lists.freedesktop.org;
> > >> ppaalanen@gmail.com; brian.starkey@arm.com;
> > >> sebastian@sebastianwick.net; Shashank.Sharma@amd.com
> > >> Subject: Re: [RFC v2 05/22] drm/i915/xelpd: Define Degamma Lut range
> > >> struct for HDR planes
> > >>
> > >> On Thu, Nov 11, 2021 at 10:17:17AM -0500, Harry Wentland wrote:  
> > >>>
> > >>>
> > >>> On 2021-09-06 17:38, Uma Shankar wrote:  
> > >>>> Define the structure with XE_LPD degamma lut ranges. HDR and SDR
> > >>>> planes have different capabilities, implemented respective
> > >>>> structure for the HDR planes.
> > >>>>
> > >>>> Signed-off-by: Uma Shankar <uma.shankar@intel.com>
> > >>>> ---
> > >>>>  drivers/gpu/drm/i915/display/intel_color.c | 52
> > >>>> ++++++++++++++++++++++
> > >>>>  1 file changed, 52 insertions(+)
> > >>>>
> > >>>> diff --git a/drivers/gpu/drm/i915/display/intel_color.c
> > >>>> b/drivers/gpu/drm/i915/display/intel_color.c
> > >>>> index afcb4bf3826c..6403bd74324b 100644
> > >>>> --- a/drivers/gpu/drm/i915/display/intel_color.c
> > >>>> +++ b/drivers/gpu/drm/i915/display/intel_color.c
> > >>>> @@ -2092,6 +2092,58 @@ static void icl_read_luts(struct
> > >>>> intel_crtc_state  
> > >> *crtc_state)  
> > >>>>  	}
> > >>>>  }
> > >>>>
> > >>>> + /* FIXME input bpc? */
> > >>>> +__maybe_unused
> > >>>> +static const struct drm_color_lut_range d13_degamma_hdr[] = {
> > >>>> +	/* segment 1 */
> > >>>> +	{
> > >>>> +		.flags = (DRM_MODE_LUT_GAMMA |
> > >>>> +			  DRM_MODE_LUT_REFLECT_NEGATIVE |
> > >>>> +			  DRM_MODE_LUT_INTERPOLATE |
> > >>>> +			  DRM_MODE_LUT_NON_DECREASING),
> > >>>> +		.count = 128,
> > >>>> +		.input_bpc = 24, .output_bpc = 16,
> > >>>> +		.start = 0, .end = (1 << 24) - 1,
> > >>>> +		.min = 0, .max = (1 << 24) - 1,
> > >>>> +	},
> > >>>> +	/* segment 2 */
> > >>>> +	{
> > >>>> +		.flags = (DRM_MODE_LUT_GAMMA |
> > >>>> +			  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 << 27) - 1,
> > >>>> +	},
> > >>>> +	/* Segment 3 */
> > >>>> +	{
> > >>>> +		.flags = (DRM_MODE_LUT_GAMMA |
> > >>>> +			  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_GAMMA |
> > >>>> +			  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,
> > >>>> +	},
> > >>>> +};  
> > >>>
> > >>> If I understand this right, userspace would need this definition in
> > >>> order to populate the degamma blob. Should this sit in a UAPI header?  

Are you asking whether 'struct drm_color_lut_range` is defined in any
userspace visible header?

It seems to be in patch 2.

> > >
> > > Hi Harry, Pekka and Ville,
> > > Sorry for being a bit late on the replies, got side tracked with various issues.
> > > I am back on this. Apologies for delay.
> > >  
> > >> My original idea (not sure it's fully realized in this series) is to
> > >> have a new GAMMA_MODE/etc. enum property on each crtc (or plane) for
> > >> which each enum value points to a kernel provided blob that contains one of  
> > these LUT descriptors.  
> > >> Userspace can then query them dynamically and pick the best one for
> > >> its current use case.  
> > >
> > > We have this as part of the series Ville. Patch 3 of this series
> > > creates a DEGAMMA_MODE property just for this. With that userspace can
> > > just query the blob_id's and will get the various degamma mode possible and the  
> > respective segment and lut distributions.  
> > >
> > > This will be generic, so for userspace it should just be able to query
> > > this and parse and get the lut distribution and segment ranges.
> > >  
> > 
> > Thanks for the explanation.
> > 
> > Uma, have you had a chance to sketch some of this out in IGT? I'm trying to see how
> > userspace would do this in practice and will try to sketch an IGT test for this myself,
> > but if you have it already we could share the effort.  
> 
> Yes Harry, we do have some sample IGT's to test this. Will send those out and will copy
> you and all the stakeholders.
> 
> > >> The algorithm for choosing the best one might be something like:
> > >> - prefer LUT with bpc >= FB bpc, but perhaps not needlessly high bpc
> > >> - prefer interpolated vs. direct lookup based on current needs (eg. X
> > >>   could prefer direct lookup to get directcolor visuals).
> > >> - prefer one with extended range values if needed
> > >> - for HDR prefer smaller step size in dark tones,
> > >>   for SDR perhaps prefer a more uniform step size
> > >>
> > >> Or maybe we should include some kind of usage hints as well?  
> > >
> > > I think the segment range and distribution of lut should be enough for
> > > a userspace to pick the right ones, but we can add some examples in UAPI  
> > descriptions as hints.  
> > >  
> > 
> > The range might be enough, but we're already parsing hints like "GAMMA"
> > or "DEGAMMA". I wonder if it would make sense to add a flag for "HDR" or "SDR" as
> > well.  

What hints are GAMMA or DEGAMMA and who's parsing them? I thought they
are just arbitrary names to identify the element's position in the
abstract pipeline.

> 
> On Intel hardware, we differentiate this with precision and have HDR planes (they have extra
> Lut precision and samples) separately called out. We could add SDR/HDR FLAG as well.

What about wide color gamut SDR? That probably needs more precision
than "normal" SDR but is not HDR.

I can't think of how SDR/HDR flags would work or what they would mean.
Feels a bit too simple for practice. Maybe that concept should be
created by a hypothetical userspace helper library instead.


Thanks,
pq

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