Re: [PATCH 02/21] drm: Add Plane Degamma Mode property

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Fri, 4 Jun 2021 14:24:28 -0400
Harry Wentland <harry.wentland@amd.com> wrote:

> On 2021-06-01 6:51 a.m., Uma Shankar wrote:
> > Add Plane Degamma Mode as an enum property. Create a helper
> > function for all plane color management features.
> > 
> > This is an enum property with values as blob_id's and exposes
> > the various gamma modes supported and the lut ranges. Getting
> > the blob id in userspace, user can get the mode supported and
> > also the range of gamma mode supported with number of lut
> > coefficients. It can then set one of the modes using this
> > enum property.
> > 
> > Lut values will be sent through separate GAMMA_LUT blob property.
> > 
> > Signed-off-by: Uma Shankar <uma.shankar@intel.com>
> > ---
> >  Documentation/gpu/drm-kms.rst             | 90 ++++++++++++++++++++++
> >  drivers/gpu/drm/drm_atomic.c              |  1 +
> >  drivers/gpu/drm/drm_atomic_state_helper.c |  2 +
> >  drivers/gpu/drm/drm_atomic_uapi.c         |  4 +
> >  drivers/gpu/drm/drm_color_mgmt.c          | 93 ++++++++++++++++++++++-
> >  include/drm/drm_mode_object.h             |  2 +-
> >  include/drm/drm_plane.h                   | 23 ++++++
> >  7 files changed, 212 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> > index 87e5023e3f55..752be545e7d7 100644
> > --- a/Documentation/gpu/drm-kms.rst
> > +++ b/Documentation/gpu/drm-kms.rst
> > @@ -514,9 +514,99 @@ Damage Tracking Properties
> >  Color Management Properties
> >  ---------------------------
> >  
> > +Below is how a typical hardware pipeline for color
> > +will look like:
> > +
> > +.. kernel-render:: DOT
> > +   :alt: Display Color Pipeline
> > +   :caption: Display Color Pipeline Overview
> > +
> > +   digraph "KMS" {
> > +      node [shape=box]
> > +
> > +      subgraph cluster_static {
> > +          style=dashed
> > +          label="Display Color Hardware Blocks"
> > +
> > +          node [bgcolor=grey style=filled]
> > +          "Plane Degamma A" -> "Plane CSC/CTM A"
> > +          "Plane CSC/CTM A" -> "Plane Gamma A"
> > +          "Pipe Blender" [color=lightblue,style=filled, width=5.25, height=0.75];
> > +          "Plane Gamma A" -> "Pipe Blender"
> > +	  "Pipe Blender" -> "Pipe DeGamma"
> > +          "Pipe DeGamma" -> "Pipe CSC/CTM"
> > +          "Pipe CSC/CTM" -> "Pipe Gamma"
> > +          "Pipe Gamma" -> "Pipe Output"
> > +      }
> > +  
> 
> It might be worthwhile to also highlight the YCbCr coefficient matrix in the pipeline,
> between the FB and Plane degamma, i.e.
>   YCbCr coefficients > plane degamma > csc > ...
> 
> One problem with this view is that not all HW will support all (or any) of these
> CM blocks on all planes. For example, on AMD HW cursors are very different from
> other planes and don't really have full CM support.
> 
> > +      subgraph cluster_static {
> > +          style=dashed
> > +
> > +          node [shape=box]
> > +          "Plane Degamma B" -> "Plane CSC/CTM B"
> > +          "Plane CSC/CTM B" -> "Plane Gamma B"
> > +          "Plane Gamma B" -> "Pipe Blender"
> > +      }
> > +
> > +      subgraph cluster_static {
> > +          style=dashed
> > +
> > +          node [shape=box]
> > +          "Plane Degamma C" -> "Plane CSC/CTM C"
> > +          "Plane CSC/CTM C" -> "Plane Gamma C"
> > +          "Plane Gamma C" -> "Pipe Blender"
> > +      }
> > +
> > +      subgraph cluster_fb {
> > +          style=dashed
> > +          label="RAM"
> > +
> > +          node [shape=box width=1.7 height=0.2]
> > +
> > +          "FB 1" -> "Plane Degamma A"
> > +          "FB 2" -> "Plane Degamma B"
> > +          "FB 3" -> "Plane Degamma C"
> > +      }
> > +   }
> > +
> > +In real world usecases,
> > +
> > +1. Plane Degamma can be used to linearize a non linear gamma
> > +encoded framebuffer. This is needed to do any linear math like
> > +color space conversion. For ex, linearize frames encoded in SRGB
> > +or by HDR curve.
> > +
> > +2. Later Plane CTM block can convert the content to some different
> > +colorspace. For ex, SRGB to BT2020 etc.
> > +
> > +3. Plane Gamma block can be used later to re-apply the non-linear
> > +curve. This can also be used to apply Tone Mapping for HDR usecases.
> > +  
> 
> This would mean you're blending in gamma space which is likely not what
> most compositors expect. There are numerous articles that describe why
> blending in gamma space is problematic, such as [1]
> 
> [1] https://ninedegreesbelow.com/photography/linear-gamma-blur-normal-blend.html
> 
> To blend in linear space this should be configured to do
> 
>   Plane Degamma > Plane CTM > CRTC Gamma
> 
> I think it would also be good if we moved away from calling this gamma. It's
> really only gamma for legacy SDR scenarios. For HDR cases I would never expect
> these to use gamma and even though the sRGB transfer function is based on gamma
> functions it's complicated [2].
> 
> [2] https://en.wikipedia.org/wiki/SRGB
> 
> A better way to describe these would be as "transfer function" and "inverse
> transfer function." The space at various stages could then be described as linear
> or non-linear, specifically PQ, HLG, sRGB, BT709, or using another transfer
> function.

Hi,

incidentally, I, Sebastian and Vitaly are discussing on what to call
these pieces in Weston:
https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/582#note_947292

They might not even be the transfer functions but just some curves,
e.g. optimized to keep a 3D LUT small without sacrificing precision.


Thanks,
pq

> > +All the layers or framebuffers need to be converted to same color
> > +space and format before blending. The plane color hardware blocks
> > +can help with this. Once the Data is blended, similar color processing
> > +can be done on blended output using pipe color hardware blocks.
> > +
> > +DRM Properties have been created to define and expose all these
> > +hardware blocks to userspace. A userspace application (compositor
> > +or any color app) can use these interfaces and define policies to
> > +efficiently use the display hardware for such color operations.
> > +
> > +Pipe Color Management Properties
> > +---------------------------------
> > +
> >  .. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
> >     :doc: overview
> >  
> > +Plane Color Management Properties
> > +---------------------------------
> > +
> > +.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
> > +   :doc: Plane Color Properties
> > +
> > +.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
> > +   :doc: export
> > +
> >  Tile Group Property
> >  -------------------

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