[PATCH 4/4] Documentation/gpu/amdgpu/amdgpu_dm: add DM docs for pixel blend mode

Mon Jul 18 01:39:45 UTC 2022

On 16/07/2022 19:25, Melissa Wen wrote:
> AMD GPU display manager (DM) maps DRM pixel blend modes (None,
> Pre-multiplied, Coverage) to MPC hw blocks through blend configuration
> options. Describe relevant elements and how to set and test them to get
> the expected DRM blend mode on DCN hw.
> 
> Signed-off-by: Melissa Wen <mwen at igalia.com>
> ---
>   .../gpu/amdgpu/display/display-manager.rst    | 98 +++++++++++++++++++
>   Documentation/gpu/drm-kms.rst                 |  2 +
>   2 files changed, 100 insertions(+)
> 
> diff --git a/Documentation/gpu/amdgpu/display/display-manager.rst b/Documentation/gpu/amdgpu/display/display-manager.rst
> index 8960a5f1fa66..7a495ed1f69e 100644
> --- a/Documentation/gpu/amdgpu/display/display-manager.rst
> +++ b/Documentation/gpu/amdgpu/display/display-manager.rst
> @@ -84,3 +84,101 @@ families below.
>   **DCN 3.0 family color caps and mapping**
>   
>   .. kernel-figure:: dcn3_cm_drm_current.svg
> +
> +Blend Mode Properties
> +=====================
> +
> +Pixel blend mode is a DRM plane composition property of :c:type:`drm_plane` used to
> +describes how pixels from a foreground plane (fg) are composited with the
> +background plane (bg). Here, we present main concepts of DRM blend mode to help
> +to understand how this property is mapped to AMD DC interface. See more about
> +this DRM property and the alpha blending equations in :ref:`DRM Plane
> +Composition Properties <plane_composition_properties>`.
> +
> +Basically, a blend mode sets the alpha blending equation for plane
> +composition that fits the mode in which the alpha channel affects the state of
> +pixel color values and, therefore, the resulted pixel color. For
> +example, consider the following elements of the alpha blending equation:
> +
> +- *fg.rgb*: Each of the RGB component values from the foreground's pixel.
> +- *fg.alpha*: Alpha component value from the foreground's pixel.
> +- *bg.rgb*: Each of the RGB component values from the background.
> +- *plane_alpha*: Plane alpha value set by the **plane "alpha" property**, see
> +  more in `DRM Plane Composition Properties <plane_composition_properties>`.

You forgot to use :ref: in here.

> +
> +in the basic alpha blending equation::
> +
> +   out.rgb = alpha * fg.rgb + (1 - alpha) * bg.rgb
> +
> +the alpha channel value of each pixel in a plane is ignored and only the plane
> +alpha affects the resulted pixel color values.
> +
> +DRM has three blend mode to define the blend formula in the plane composition:
> +
> +* **None**: Blend formula that ignores the pixel alpha.
> +
> +* **Pre-multiplied**: Blend formula that assumes the pixel color values in a
> +  plane was already pre-multiplied by its own alpha channel before storage.
> +
> +* **Coverage**: Blend formula that assumes the pixel color values were not
> +  pre-multiplied with the alpha channel values.
> +
> +and pre-multiplied is the default pixel blend mode, that means, when no blend
> +mode property is created or defined, DRM considers the plane's pixels has
> +pre-multiplied color values. On IGT GPU tools, the kms_plane_alpha_blend test
> +provides a set of subtests to verify plane alpha and blend mode properties.
> +
> +The DRM blend mode and its elements are then mapped by AMDGPU display manager
> +(DM) to program the blending configuration of the Multiple Pipe/Plane Combined
> +(MPC), as follows:
> +
> +.. kernel-doc:: drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h
> +   :doc: mpc-overview
> +
> +.. kernel-doc:: drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h
> +   :functions: mpcc_blnd_cfg
> +
> +Therefore, the blending configuration for a single MPCC instance on the MPC
> +tree is defined by :c:type:`mpcc_blnd_cfg`, where
> +:c:type:`pre_multiplied_alpha` is the alpha pre-multiplied mode flag used to
> +set :c:type:`MPCC_ALPHA_MULTIPLIED_MODE`. It controls whether alpha is
> +multiplied (true/false), being only true for DRM pre-multiplied blend mode.
> +:c:type:`mpcc_alpha_blend_mode` defines the alpha blend mode regarding pixel
> +alpha and plane alpha values. It sets one of the three modes for
> +:c:type:`MPCC_ALPHA_BLND_MODE`, as described below.
> +
> +.. kernel-doc:: drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h
> +   :functions: mpcc_alpha_blend_mode
> +
> +DM then maps the elements of `enum mpcc_alpha_blend_mode` to those in the DRM
> +blend formula, as follows:
> +
> +* *MPC pixel alpha* matches *DRM fg.alpha* as the alpha component value
> +  from the plane's pixel
> +* *MPC global alpha* matches *DRM plane_alpha* when the pixel alpha should
> +  be ignored and, therefore, pixel values are not pre-multiplied
> +* *MPC global gain* assumes *MPC global alpha* value when both *DRM
> +  fg.alpha* and *DRM plane_alpha* participate in the blend equation
> +
> +In short, *fg.alpha* is ignored by selecting
> +:c:type:`MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA`. On the other hand, (plane_alpha *
> +fg.alpha) component becomes available by selecting
> +:c:type:`MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN`. And the
> +:c:type:`MPCC_ALPHA_MULTIPLIED_MODE` defines if the pixel color values are
> +pre-multiplied by alpha or not.
> +
> +Blend configuration flow
> +------------------------
> +
> +The alpha blending equation is configured from DRM to DC interface by the
> +following path:
> +
> +1. When updating a :c:type:`drm_plane_state <drm_plane_state>`, DM calls
> +   :c:type:`fill_blending_from_plane_state()` that maps
> +   :c:type:`drm_plane_state <drm_plane_state>` attributes to
> +   :c:type:`dc_plane_info <dc_plane_info>` struct to be handled in the
> +   OS-agnostic component (DC).
> +
> +2. On DC interface, :c:type:`struct mpcc_blnd_cfg <mpcc_blnd_cfg>` programs the
> +   MPCC blend configuration considering the :c:type:`dc_plane_info
> +   <dc_plane_info>` input from DPP.
> diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> index d14bf1c35d7e..7ec9dbe18b65 100644
> --- a/Documentation/gpu/drm-kms.rst
> +++ b/Documentation/gpu/drm-kms.rst
> @@ -532,6 +532,8 @@ Standard Plane Properties
>   .. kernel-doc:: drivers/gpu/drm/drm_plane.c
>      :doc: standard plane properties
>   
> +.. _plane_composition_properties:
> +
>   Plane Composition Properties
>   ----------------------------
>   

 From a beginner's perspective, the changes from this series are 
readable, but I cannot tell if there are any technical mistakes. So,
regarding readability and rendering htmldocs, apart from the nits I've 
raised, this whole series is

Reviewed-by: Tales Aparecida <tales.aparecida at gmail.com>

Thanks for the patch series, Melissa,
Tales Aparecida