[RFC PATCH v4 08/42] drm/doc/rfc: Describe why prescriptive color pipeline is needed

[Harry Wentland <harry.wentland@amd.com>]

v4:
 - Drop IOCTL docs since we dropped the IOCTLs (Pekka)
 - Clarify reading and setting of COLOR_PIPELINE prop (Pekka)
 - Add blurb about not requiring to reject a pipeline due to
   incompatible ops, as long as op can be bypassed (Pekka)
 - Dropped informational strings (such as input CSC) as they're
   not actually intended to be advertised (Pekka)

v3:
 - Describe DRM_CLIENT_CAP_PLANE_COLOR_PIPELINE (Sebastian)
 - Ask for clear documentation of colorop behavior (Sebastian)

v2:
 - Update colorop visualizations to match reality (Sebastian, Alex Hung)
 - Updated wording (Pekka)
 - Change BYPASS wording to make it non-mandatory (Sebastian)
 - Drop cover-letter-like paragraph from COLOR_PIPELINE Plane Property
   section (Pekka)
 - Use PQ EOTF instead of its inverse in Pipeline Programming example (Melissa)
 - Add "Driver Implementer's Guide" section (Pekka)
 - Add "Driver Forward/Backward Compatibility" section (Sebastian, Pekka)

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
---
 Documentation/gpu/rfc/color_pipeline.rst | 360 +++++++++++++++++++++++
 1 file changed, 360 insertions(+)
 create mode 100644 Documentation/gpu/rfc/color_pipeline.rst

diff --git a/Documentation/gpu/rfc/color_pipeline.rst b/Documentation/gpu/rfc/color_pipeline.rst
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+========================
+Linux Color Pipeline API
+========================
+
+What problem are we solving?
+============================
+
+We would like to support pre-, and post-blending complex color
+transformations in display controller hardware in order to allow for
+HW-supported HDR use-cases, as well as to provide support to
+color-managed applications, such as video or image editors.
+
+It is possible to support an HDR output on HW supporting the Colorspace
+and HDR Metadata drm_connector properties, but that requires the
+compositor or application to render and compose the content into one
+final buffer intended for display. Doing so is costly.
+
+Most modern display HW offers various 1D LUTs, 3D LUTs, matrices, and other
+operations to support color transformations. These operations are often
+implemented in fixed-function HW and therefore much more power efficient than
+performing similar operations via shaders or CPU.
+
+We would like to make use of this HW functionality to support complex color
+transformations with no, or minimal CPU or shader load.
+
+
+How are other OSes solving this problem?
+========================================
+
+The most widely supported use-cases regard HDR content, whether video or
+gaming.
+
+Most OSes will specify the source content format (color gamut, encoding transfer
+function, and other metadata, such as max and average light levels) to a driver.
+Drivers will then program their fixed-function HW accordingly to map from a
+source content buffer's space to a display's space.
+
+When fixed-function HW is not available the compositor will assemble a shader to
+ask the GPU to perform the transformation from the source content format to the
+display's format.
+
+A compositor's mapping function and a driver's mapping function are usually
+entirely separate concepts. On OSes where a HW vendor has no insight into
+closed-source compositor code such a vendor will tune their color management
+code to visually match the compositor's. On other OSes, where both mapping
+functions are open to an implementer they will ensure both mappings match.
+
+This results in mapping algorithm lock-in, meaning that no-one alone can
+experiment with or introduce new mapping algorithms and achieve
+consistent results regardless of which implementation path is taken.
+
+Why is Linux different?
+=======================
+
+Unlike other OSes, where there is one compositor for one or more drivers, on
+Linux we have a many-to-many relationship. Many compositors; many drivers.
+In addition each compositor vendor or community has their own view of how
+color management should be done. This is what makes Linux so beautiful.
+
+This means that a HW vendor can now no longer tune their driver to one
+compositor, as tuning it to one could make it look fairly different from
+another compositor's color mapping.
+
+We need a better solution.
+
+
+Descriptive API
+===============
+
+An API that describes the source and destination colorspaces is a descriptive
+API. It describes the input and output color spaces but does not describe
+how precisely they should be mapped. Such a mapping includes many minute
+design decision that can greatly affect the look of the final result.
+
+It is not feasible to describe such mapping with enough detail to ensure the
+same result from each implementation. In fact, these mappings are a very active
+research area.
+
+
+Prescriptive API
+================
+
+A prescriptive API describes not the source and destination colorspaces. It
+instead prescribes a recipe for how to manipulate pixel values to arrive at the
+desired outcome.
+
+This recipe is generally an ordered list of straight-forward operations,
+with clear mathematical definitions, such as 1D LUTs, 3D LUTs, matrices,
+or other operations that can be described in a precise manner.
+
+
+The Color Pipeline API
+======================
+
+HW color management pipelines can significantly differ between HW
+vendors in terms of availability, ordering, and capabilities of HW
+blocks. This makes a common definition of color management blocks and
+their ordering nigh impossible. Instead we are defining an API that
+allows user space to discover the HW capabilities in a generic manner,
+agnostic of specific drivers and hardware.
+
+
+drm_colorop Object
+==================
+
+To support the definition of color pipelines we define the DRM core
+object type drm_colorop. Individual drm_colorop objects will be chained
+via the NEXT property of a drm_colorop to constitute a color pipeline.
+Each drm_colorop object is unique, i.e., even if multiple color
+pipelines have the same operation they won't share the same drm_colorop
+object to describe that operation.
+
+Note that drivers are not expected to map drm_colorop objects statically
+to specific HW blocks. The mapping of drm_colorop objects is entirely a
+driver-internal detail and can be as dynamic or static as a driver needs
+it to be. See more in the Driver Implementation Guide section below.
+
+Each drm_colorop has three core properties:
+
+TYPE: An enumeration property, definint the type of transformation, such as
+* enumerated curve
+* custom (uniform) 1D LUT
+* 3x3 matrix
+* 3x4 matrix
+* 3D LUT
+* etc.
+
+Depending on the type of transformation other properties will describe
+more details.
+
+BYPASS: A boolean property that can be used to easily put a block into
+bypass mode. While setting other properties might fail atomic check,
+setting the BYPASS property to true should never fail. The BYPASS
+property is not mandatory for a colorop, as long as the entire pipeline
+can get bypassed by setting the COLOR_PIPELINE on a plane to '0'.
+
+NEXT: The ID of the next drm_colorop in a color pipeline, or 0 if this
+drm_colorop is the last in the chain.
+
+An example of a drm_colorop object might look like one of these::
+
+    /* 1D enumerated curve */
+    Color operation 42
+    ├─ "TYPE": immutable enum {1D enumerated curve, 1D LUT, 3x3 matrix, 3x4 matrix, 3D LUT, etc.} = 1D enumerated curve
+    ├─ "BYPASS": bool {true, false}
+    ├─ "CURVE_1D_TYPE": enum {sRGB EOTF, sRGB inverse EOTF, PQ EOTF, PQ inverse EOTF, …}
+    └─ "NEXT": immutable color operation ID = 43
+
+    /* custom 4k entry 1D LUT */
+    Color operation 52
+    ├─ "TYPE": immutable enum {1D enumerated curve, 1D LUT, 3x3 matrix, 3x4 matrix, 3D LUT, etc.} = 1D LUT
+    ├─ "BYPASS": bool {true, false}
+    ├─ "LUT_1D_SIZE": immutable range = 4096
+    ├─ "LUT_1D": blob
+    └─ "NEXT": immutable color operation ID = 0
+
+    /* 17^3 3D LUT */
+    Color operation 72
+    ├─ "TYPE": immutable enum {1D enumerated curve, 1D LUT, 3x3 matrix, 3x4 matrix, 3D LUT, etc.} = 3D LUT
+    ├─ "BYPASS": bool {true, false}
+    ├─ "LUT_3D_SIZE": immutable range = 17
+    ├─ "LUT_3D": blob
+    └─ "NEXT": immutable color operation ID = 73
+
+
+COLOR_PIPELINE Plane Property
+=============================
+
+Color Pipelines are created by a driver and advertised via a new
+COLOR_PIPELINE enum property on each plane. Values of the property
+always include object id 0, which is the default and means all color
+processing is disabled. Additional values will be the object IDs of the
+first drm_colorop in a pipeline. A driver can create and advertise none,
+one, or more possible color pipelines. A DRM client will select a color
+pipeline by setting the COLOR PIPELINE to the respective value.
+
+NOTE: Many DRM clients will set enumeration properties via the string
+value, often hard-coding it. Since this enumeration is generated based
+on the colorop object IDs it is important to perform the Color Pipeline
+Discovery, described below, instead of hard-coding color pipeline
+assignment. Drivers might generate the enum strings dynamically.
+Hard-coded strings might only work for specific drivers on a specific
+pieces of HW. Color Pipeline Discovery can work universally, as long as
+drivers implement the required color operations.
+
+The COLOR_PIPELINE property is only exposed when the
+DRM_CLIENT_CAP_PLANE_COLOR_PIPELINE is set. Drivers shall ignore any
+existing pre-blend color operations when this cap is set, such as
+COLOR_RANGE and COLOR_ENCODING. If drivers want to support COLOR_RANGE
+or COLOR_ENCODING functionality when the color pipeline client cap is
+set, they are expected to expose colorops in the pipeline to allow for
+the appropriate color transformation.
+
+Setting of the COLOR_PIPELINE plane property or drm_colorop properties
+is only allowed for userspace that sets this client cap.
+
+An example of a COLOR_PIPELINE property on a plane might look like this::
+
+    Plane 10
+    ├─ "TYPE": immutable enum {Overlay, Primary, Cursor} = Primary
+    ├─ …
+    └─ "COLOR_PIPELINE": enum {0, 42, 52} = 0
+
+
+Color Pipeline Discovery
+========================
+
+A DRM client wanting color management on a drm_plane will:
+
+1. Get the COLOR_PIPELINE property of the plane
+2. iterate all COLOR_PIPELINE enum values
+3. for each enum value walk the color pipeline (via the NEXT pointers)
+   and see if the available color operations are suitable for the
+   desired color management operations
+
+If userspace encounters an unknown or unsuitable color operation during
+discovery it does not need to reject the entire color pipeline outright,
+as long as the unknown or unsuitable colorop has a "BYPASS" property.
+Drivers will ensure that a bypassed block does not have any effect.
+
+An example of chained properties to define an AMD pre-blending color
+pipeline might look like this::
+
+    Plane 10
+    ├─ "TYPE" (immutable) = Primary
+    └─ "COLOR_PIPELINE": enum {0, 44} = 0
+
+    Color operation 44
+    ├─ "TYPE" (immutable) = 1D enumerated curve
+    ├─ "BYPASS": bool
+    ├─ "CURVE_1D_TYPE": enum {sRGB EOTF, PQ EOTF} = sRGB EOTF
+    └─ "NEXT" (immutable) = 45
+
+    Color operation 45
+    ├─ "TYPE" (immutable) = 3x4 Matrix
+    ├─ "BYPASS": bool
+    ├─ "MATRIX_3_4": blob
+    └─ "NEXT" (immutable) = 46
+
+    Color operation 46
+    ├─ "TYPE" (immutable) = 1D enumerated curve
+    ├─ "BYPASS": bool
+    ├─ "CURVE_1D_TYPE": enum {sRGB Inverse EOTF, PQ Inverse EOTF} = sRGB EOTF
+    └─ "NEXT" (immutable) = 47
+
+    Color operation 47
+    ├─ "TYPE" (immutable) = 1D LUT
+    ├─ "LUT_1D_SIZE": immutable range = 4096
+    ├─ "LUT_1D_DATA": blob
+    └─ "NEXT" (immutable) = 48
+
+    Color operation 48
+    ├─ "TYPE" (immutable) = 3D LUT
+    ├─ "LUT_3D_SIZE" (immutable) = 17
+    ├─ "LUT_3D_DATA": blob
+    └─ "NEXT" (immutable) = 49
+
+    Color operation 49
+    ├─ "TYPE" (immutable) = 1D enumerated curve
+    ├─ "BYPASS": bool
+    ├─ "CURVE_1D_TYPE": enum {sRGB EOTF, PQ EOTF} = sRGB EOTF
+    └─ "NEXT" (immutable) = 0
+
+
+Color Pipeline Programming
+==========================
+
+Once a DRM client has found a suitable pipeline it will:
+
+1. Set the COLOR_PIPELINE enum value to the one pointing at the first
+   drm_colorop object of the desired pipeline
+2. Set the properties for all drm_colorop objects in the pipeline to the
+   desired values, setting BYPASS to true for unused drm_colorop blocks,
+   and false for enabled drm_colorop blocks
+3. Perform (TEST_ONLY or not) atomic commit with all the other KMS
+   states it wishes to change
+
+To configure the pipeline for an HDR10 PQ plane and blending in linear
+space, a compositor might perform an atomic commit with the following
+property values::
+
+    Plane 10
+    └─ "COLOR_PIPELINE" = 42
+
+    Color operation 42
+    └─ "BYPASS" = true
+
+    Color operation 44
+    └─ "BYPASS" = true
+
+    Color operation 45
+    └─ "BYPASS" = true
+
+    Color operation 46
+    └─ "BYPASS" = true
+
+    Color operation 47
+    ├─ "LUT_3D_DATA" = Gamut mapping + tone mapping + night mode
+    └─ "BYPASS" = false
+
+    Color operation 48
+    ├─ "CURVE_1D_TYPE" = PQ EOTF
+    └─ "BYPASS" = false
+
+
+Driver Implementer's Guide
+==========================
+
+What does this all mean for driver implementations? As noted above the
+colorops can map to HW directly but don't need to do so. Here are some
+suggestions on how to think about creating your color pipelines:
+
+- Try to expose pipelines that use already defined colorops, even if
+  your hardware pipeline is split differently. This allows existing
+  userspace to immediately take advantage of the hardware.
+
+- Additionally, try to expose your actual hardware blocks as colorops.
+  Define new colorop types where you believe it can offer significant
+  benefits if userspace learns to program them.
+
+- Avoid defining new colorops for compound operations with very narrow
+  scope. If you have a hardware block for a special operation that
+  cannot be split further, you can expose that as a new colorop type.
+  However, try to not define colorops for "use cases", especially if
+  they require you to combine multiple hardware blocks.
+
+- Design new colorops as prescriptive, not descriptive; by the
+  mathematical formula, not by the assumed input and output.
+
+A defined colorop type must be deterministic. The exact behavior of the
+colorop must be documented entirely, whether via a mathematical formula
+or some other description. Its operation can depend only on its
+properties and input and nothing else, allowed error tolerance
+notwithstanding.
+
+
+Driver Forward/Backward Compatibility
+=====================================
+
+As this is uAPI drivers can't regress color pipelines that have been
+introduced for a given HW generation. New HW generations are free to
+abandon color pipelines advertised for previous generations.
+Nevertheless, it can be beneficial to carry support for existing color
+pipelines forward as those will likely already have support in DRM
+clients.
+
+Introducing new colorops to a pipeline is fine, as long as they can be
+bypassed or are purely informational. DRM clients implementing support
+for the pipeline can always skip unknown properties as long as they can
+be confident that doing so will not cause unexpected results.
+
+If a new colorop doesn't fall into one of the above categories
+(bypassable or informational) the modified pipeline would be unusable
+for user space. In this case a new pipeline should be defined.
+
+
+References
+==========
+
+1. https://lore.kernel.org/dri-devel/QMers3awXvNCQlyhWdTtsPwkp5ie9bze_hD5nAccFW7a_RXlWjYB7MoUW_8CKLT2bSQwIXVi5H6VULYIxCdgvryZoAoJnC5lZgyK1QWn488=@emersion.fr/
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