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AMDgpu Display Manager

Documentation/gpu/amdgpu/display/display-manager.rst

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====================== AMDgpu Display Manager

.. contents:: Table of Contents :depth: 3

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c :doc: overview

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h :internal:

Lifecycle

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c :doc: DM Lifecycle

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c :functions: dm_hw_init dm_hw_fini

Interrupts

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_irq.c :doc: overview

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_irq.c :internal:

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c :functions: register_hpd_handlers dm_crtc_high_irq dm_pflip_high_irq

Atomic Implementation

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c :doc: atomic

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c :functions: amdgpu_dm_atomic_check amdgpu_dm_atomic_commit_tail

Color Management Properties

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c :doc: overview

.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c :internal:

DC Color Capabilities between DCN generations

DRM/KMS framework defines three CRTC color correction properties: degamma, color transformation matrix (CTM) and gamma, and two properties for degamma and gamma LUT sizes. AMD DC programs some of the color correction features pre-blending but DRM/KMS has not per-plane color correction properties.

In general, the DRM CRTC color properties are programmed to DC, as follows: CRTC gamma after blending, and CRTC degamma pre-blending. Although CTM is programmed after blending, it is mapped to DPP hw blocks (pre-blending). Other color caps available in the hw is not currently exposed by DRM interface and are bypassed.

.. kernel-doc:: drivers/gpu/drm/amd/display/dc/dc.h :doc: color-management-caps

.. kernel-doc:: drivers/gpu/drm/amd/display/dc/dc.h :internal:

The color pipeline has undergone major changes between DCN hardware generations. What's possible to do before and after blending depends on hardware capabilities, as illustrated below by the DCN 2.0 and DCN 3.0 families schemas.

DCN 2.0 family color caps and mapping

.. kernel-figure:: dcn2_cm_drm_current.svg

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:amdgpu_dm_plane_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.