.. SPDX-License-Identifier: GPL-2.0

==============================
 drm/komeda Arm display driver
==============================

The drm/komeda driver supports the Arm display processor D71 and later products,
this document gives a brief overview of driver design: how it works and why
design it like that.

Overview of D71 like display IPs
================================

From D71, Arm display IP begins to adopt a flexible and modularized
architecture. A display pipeline is made up of multiple individual and
functional pipeline stages called components, and every component has some
specific capabilities that can give the flowed pipeline pixel data a
particular processing.

Typical D71 components:

Layer
-----
Layer is the first pipeline stage, which prepares the pixel data for the next
stage. It fetches the pixel from memory, decodes it if it's AFBC, rotates the
source image, unpacks or converts YUV pixels to the device internal RGB pixels,
then adjusts the color_space of pixels if needed.

Scaler
------
As its name suggests, scaler takes responsibility for scaling, and D71 also
supports image enhancements by scaler.
The usage of scaler is very flexible and can be connected to layer output
for layer scaling, or connected to compositor and scale the whole display
frame and then feed the output data into wb_layer which will then write it
into memory.

Compositor (compiz)
-------------------
Compositor blends multiple layers or pixel data flows into one single display
frame. its output frame can be fed into post image processor for showing it on
the monitor or fed into wb_layer and written to memory at the same time.
user can also insert a scaler between compositor and wb_layer to down scale
the display frame first and and then write to memory.

Writeback Layer (wb_layer)
--------------------------
Writeback layer does the opposite things of Layer, which connects to compiz
and writes the composition result to memory.

Post image processor (improc)
-----------------------------
Post image processor adjusts frame data like gamma and color space to fit the
requirements of the monitor.

Timing controller (timing_ctrlr)
--------------------------------
Final stage of display pipeline, Timing controller is not for the pixel
handling, but only for controlling the display timing.

Merger
------
D71 scaler mostly only has the half horizontal input/output capabilities
compared with Layer, like if Layer supports 4K input size, the scaler only can
support 2K input/output in the same time. To achieve the ful frame scaling, D71
introduces Layer Split, which splits the whole image to two half parts and feeds
them to two Layers A and B, and does the scaling independently. After scaling
the result need to be fed to merger to merge two part images together, and then
output merged result to compiz.

Splitter
--------
Similar to Layer Split, but Splitter is used for writeback, which splits the
compiz result to two parts and then feed them to two scalers.

Possible D71 Pipeline usage
===========================

Benefitting from the modularized architecture, D71 pipelines can be easily
adjusted to fit different usages. And D71 has two pipelines, which support two
types of working mode:

-   Dual display mode
    Two pipelines work independently and separately to drive two display outputs.

-   Single display mode
    Two pipelines work together to drive only one display output.

    On this mode, pipeline_B doesn't work indenpendently, but outputs its
    composition result into pipeline_A, and its pixel timing also derived from
    pipeline_A.timing_ctrlr. The pipeline_B works just like a "slave" of
    pipeline_A(master)

Single pipeline data flow
-------------------------

.. kernel-render:: DOT
   :alt: Single pipeline digraph
   :caption: Single pipeline data flow

   digraph single_ppl {
      rankdir=LR;

      subgraph {
         "Memory";
         "Monitor";
      }

      subgraph cluster_pipeline {
          style=dashed
          node [shape=box]
          {
              node [bgcolor=grey style=dashed]
              "Scaler-0";
              "Scaler-1";
              "Scaler-0/1"
          }

         node [bgcolor=grey style=filled]
         "Layer-0" -> "Scaler-0"
         "Layer-1" -> "Scaler-0"
         "Layer-2" -> "Scaler-1"
         "Layer-3" -> "Scaler-1"

         "Layer-0" -> "Compiz"
         "Layer-1" -> "Compiz"
         "Layer-2" -> "Compiz"
         "Layer-3" -> "Compiz"
         "Scaler-0" -> "Compiz"
         "Scaler-1" -> "Compiz"

         "Compiz" -> "Scaler-0/1" -> "Wb_layer"
         "Compiz" -> "Improc" -> "Timing Controller"
      }

      "Wb_layer" -> "Memory"
      "Timing Controller" -> "Monitor"
   }

Dual pipeline with Slave enabled
--------------------------------

.. kernel-render:: DOT
   :alt: Slave pipeline digraph
   :caption: Slave pipeline enabled data flow

   digraph slave_ppl {
      rankdir=LR;

      subgraph {
         "Memory";
         "Monitor";
      }
      node [shape=box]
      subgraph cluster_pipeline_slave {
          style=dashed
          label="Slave Pipeline_B"
          node [shape=box]
          {
              node [bgcolor=grey style=dashed]
              "Slave.Scaler-0";
              "Slave.Scaler-1";
          }

         node [bgcolor=grey style=filled]
         "Slave.Layer-0" -> "Slave.Scaler-0"
         "Slave.Layer-1" -> "Slave.Scaler-0"
         "Slave.Layer-2" -> "Slave.Scaler-1"
         "Slave.Layer-3" -> "Slave.Scaler-1"

         "Slave.Layer-0" -> "Slave.Compiz"
         "Slave.Layer-1" -> "Slave.Compiz"
         "Slave.Layer-2" -> "Slave.Compiz"
         "Slave.Layer-3" -> "Slave.Compiz"
         "Slave.Scaler-0" -> "Slave.Compiz"
         "Slave.Scaler-1" -> "Slave.Compiz"
      }

      subgraph cluster_pipeline_master {
          style=dashed
          label="Master Pipeline_A"
          node [shape=box]
          {
              node [bgcolor=grey style=dashed]
              "Scaler-0";
              "Scaler-1";
              "Scaler-0/1"
          }

         node [bgcolor=grey style=filled]
         "Layer-0" -> "Scaler-0"
         "Layer-1" -> "Scaler-0"
         "Layer-2" -> "Scaler-1"
         "Layer-3" -> "Scaler-1"

         "Slave.Compiz" -> "Compiz"
         "Layer-0" -> "Compiz"
         "Layer-1" -> "Compiz"
         "Layer-2" -> "Compiz"
         "Layer-3" -> "Compiz"
         "Scaler-0" -> "Compiz"
         "Scaler-1" -> "Compiz"

         "Compiz" -> "Scaler-0/1" -> "Wb_layer"
         "Compiz" -> "Improc" -> "Timing Controller"
      }

      "Wb_layer" -> "Memory"
      "Timing Controller" -> "Monitor"
   }

Sub-pipelines for input and output
----------------------------------

A complete display pipeline can be easily divided into three sub-pipelines
according to the in/out usage.

Layer(input) pipeline
~~~~~~~~~~~~~~~~~~~~~

.. kernel-render:: DOT
   :alt: Layer data digraph
   :caption: Layer (input) data flow

   digraph layer_data_flow {
      rankdir=LR;
      node [shape=box]

      {
         node [bgcolor=grey style=dashed]
           "Scaler-n";
      }

      "Layer-n" -> "Scaler-n" -> "Compiz"
   }

.. kernel-render:: DOT
   :alt: Layer Split digraph
   :caption: Layer Split pipeline

   digraph layer_data_flow {
      rankdir=LR;
      node [shape=box]

      "Layer-0/1" -> "Scaler-0" -> "Merger"
      "Layer-2/3" -> "Scaler-1" -> "Merger"
      "Merger" -> "Compiz"
   }

Writeback(output) pipeline
~~~~~~~~~~~~~~~~~~~~~~~~~~
.. kernel-render:: DOT
   :alt: writeback digraph
   :caption: Writeback(output) data flow

   digraph writeback_data_flow {
      rankdir=LR;
      node [shape=box]

      {
         node [bgcolor=grey style=dashed]
           "Scaler-n";
      }

      "Compiz" -> "Scaler-n" -> "Wb_layer"
   }

.. kernel-render:: DOT
   :alt: split writeback digraph
   :caption: Writeback(output) Split data flow

   digraph writeback_data_flow {
      rankdir=LR;
      node [shape=box]

      "Compiz" -> "Splitter"
      "Splitter" -> "Scaler-0" -> "Merger"
      "Splitter" -> "Scaler-1" -> "Merger"
      "Merger" -> "Wb_layer"
   }

Display output pipeline
~~~~~~~~~~~~~~~~~~~~~~~
.. kernel-render:: DOT
   :alt: display digraph
   :caption: display output data flow

   digraph single_ppl {
      rankdir=LR;
      node [shape=box]

      "Compiz" -> "Improc" -> "Timing Controller"
   }

In the following section we'll see these three sub-pipelines will be handled
by KMS-plane/wb_conn/crtc respectively.

Komeda Resource abstraction
===========================

struct komeda_pipeline/component
--------------------------------

To fully utilize and easily access/configure the HW, the driver side also uses
a similar architecture: Pipeline/Component to describe the HW features and
capabilities, and a specific component includes two parts:

-  Data flow controlling.
-  Specific component capabilities and features.

So the driver defines a common header struct komeda_component to describe the
data flow control and all specific components are a subclass of this base
structure.

.. kernel-doc:: drivers/gpu/drm/arm/display/komeda/komeda_pipeline.h
   :internal:

Resource discovery and initialization
=====================================

Pipeline and component are used to describe how to handle the pixel data. We
still need a @struct komeda_dev to describe the whole view of the device, and
the control-abilites of device.

We have &komeda_dev, &komeda_pipeline, &komeda_component. Now fill devices with
pipelines. Since komeda is not for D71 only but also intended for later products,
of course we’d better share as much as possible between different products. To
achieve this, split the komeda device into two layers: CORE and CHIP.

-   CORE: for common features and capabilities handling.
-   CHIP: for register programing and HW specific feature (limitation) handling.

CORE can access CHIP by three chip function structures:

-   struct komeda_dev_funcs
-   struct komeda_pipeline_funcs
-   struct komeda_component_funcs

.. kernel-doc:: drivers/gpu/drm/arm/display/komeda/komeda_dev.h
   :internal:

Format handling
===============

.. kernel-doc:: drivers/gpu/drm/arm/display/komeda/komeda_format_caps.h
   :internal:
.. kernel-doc:: drivers/gpu/drm/arm/display/komeda/komeda_framebuffer.h
   :internal:

Attach komeda_dev to DRM-KMS
============================

Komeda abstracts resources by pipeline/component, but DRM-KMS uses
crtc/plane/connector. One KMS-obj cannot represent only one single component,
since the requirements of a single KMS object cannot simply be achieved by a
single component, usually that needs multiple components to fit the requirement.
Like set mode, gamma, ctm for KMS all target on CRTC-obj, but komeda needs
compiz, improc and timing_ctrlr to work together to fit these requirements.
And a KMS-Plane may require multiple komeda resources: layer/scaler/compiz.

So, one KMS-Obj represents a sub-pipeline of komeda resources.

-   Plane: `Layer(input) pipeline`_
-   Wb_connector: `Writeback(output) pipeline`_
-   Crtc: `Display output pipeline`_

So, for komeda, we treat KMS crtc/plane/connector as users of pipeline and
component, and at any one time a pipeline/component only can be used by one
user. And pipeline/component will be treated as private object of DRM-KMS; the
state will be managed by drm_atomic_state as well.

How to map plane to Layer(input) pipeline
-----------------------------------------

Komeda has multiple Layer input pipelines, see:
-   `Single pipeline data flow`_
-   `Dual pipeline with Slave enabled`_

The easiest way is binding a plane to a fixed Layer pipeline, but consider the
komeda capabilities:

-   Layer Split, See `Layer(input) pipeline`_

    Layer_Split is quite complicated feature, which splits a big image into two
    parts and handles it by two layers and two scalers individually. But it
    imports an edge problem or effect in the middle of the image after the split.
    To avoid such a problem, it needs a complicated Split calculation and some
    special configurations to the layer and scaler. We'd better hide such HW
    related complexity to user mode.

-   Slave pipeline, See `Dual pipeline with Slave enabled`_

    Since the compiz component doesn't output alpha value, the slave pipeline
    only can be used for bottom layers composition. The komeda driver wants to
    hide this limitation to the user. The way to do this is to pick a suitable
    Layer according to plane_state->zpos.

So for komeda, the KMS-plane doesn't represent a fixed komeda layer pipeline,
but multiple Layers with same capabilities. Komeda will select one or more
Layers to fit the requirement of one KMS-plane.

Make component/pipeline to be drm_private_obj
---------------------------------------------

Add :c:type:`drm_private_obj` to :c:type:`komeda_component`, :c:type:`komeda_pipeline`

.. code-block:: c

    struct komeda_component {
        struct drm_private_obj obj;
        ...
    }

    struct komeda_pipeline {
        struct drm_private_obj obj;
        ...
    }

Tracking component_state/pipeline_state by drm_atomic_state
-----------------------------------------------------------

Add :c:type:`drm_private_state` and user to :c:type:`komeda_component_state`,
:c:type:`komeda_pipeline_state`

.. code-block:: c

    struct komeda_component_state {
        struct drm_private_state obj;
        void *binding_user;
        ...
    }

    struct komeda_pipeline_state {
        struct drm_private_state obj;
        struct drm_crtc *crtc;
        ...
    }

komeda component validation
---------------------------

Komeda has multiple types of components, but the process of validation are
similar, usually including the following steps:

.. code-block:: c

    int komeda_xxxx_validate(struct komeda_component_xxx xxx_comp,
                struct komeda_component_output *input_dflow,
                struct drm_plane/crtc/connector *user,
                struct drm_plane/crtc/connector_state, *user_state)
    {
         setup 1: check if component is needed, like the scaler is optional depending
                  on the user_state; if unneeded, just return, and the caller will
                  put the data flow into next stage.
         Setup 2: check user_state with component features and capabilities to see
                  if requirements can be met; if not, return fail.
         Setup 3: get component_state from drm_atomic_state, and try set to set
                  user to component; fail if component has been assigned to another
                  user already.
         Setup 3: configure the component_state, like set its input component,
                  convert user_state to component specific state.
         Setup 4: adjust the input_dflow and prepare it for the next stage.
    }

komeda_kms Abstraction
----------------------

.. kernel-doc:: drivers/gpu/drm/arm/display/komeda/komeda_kms.h
   :internal:

komde_kms Functions
-------------------
.. kernel-doc:: drivers/gpu/drm/arm/display/komeda/komeda_crtc.c
   :internal:
.. kernel-doc:: drivers/gpu/drm/arm/display/komeda/komeda_plane.c
   :internal:

Build komeda to be a Linux module driver
========================================

Now we have two level devices:

-   komeda_dev: describes the real display hardware.
-   komeda_kms_dev: attachs or connects komeda_dev to DRM-KMS.

All komeda operations are supplied or operated by komeda_dev or komeda_kms_dev,
the module driver is only a simple wrapper to pass the Linux command
(probe/remove/pm) into komeda_dev or komeda_kms_dev.