Documentation/gpu/komeda-kms.rst - linux - Git at Google

 .. 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 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.
	.. 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 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.