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diff --git a/Documentation/gpu/amdgpu/display/display-manager.rst b/Documentation/gpu/amdgpu/display/display-manager.rst index 88e2c08c7014..b7abb18cfc82 100644 --- a/Documentation/gpu/amdgpu/display/display-manager.rst +++ b/Documentation/gpu/amdgpu/display/display-manager.rst @@ -83,3 +83,101 @@ schemas. **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 :ref:`DRM Plane Composition Properties <plane_composition_properties>`. + +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. |