[PATCH 3/6] drm/doc: Add KMS overview graphs

[Daniel Vetter <daniel.vetter@ffwll.ch>]

Oh, the shiny and pretties!

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 Documentation/gpu/drm-kms-helpers.rst |   4 ++
 Documentation/gpu/drm-kms.rst         | 132 ++++++++++++++++++++++++++++++++++
 2 files changed, 136 insertions(+)

diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
index 03040aa14fe8..012b6ff3ec3f 100644
--- a/Documentation/gpu/drm-kms-helpers.rst
+++ b/Documentation/gpu/drm-kms-helpers.rst
@@ -114,6 +114,8 @@ Framebuffer CMA Helper Functions Reference
 .. kernel-doc:: drivers/gpu/drm/drm_fb_cma_helper.c
    :export:
 
+.. _drm_bridges:
+
 Bridges
 =======
 
@@ -139,6 +141,8 @@ Bridge Helper Reference
 .. kernel-doc:: drivers/gpu/drm/drm_bridge.c
    :export:
 
+.. _drm_panel_helper:
+
 Panel Helper Reference
 ======================
 
diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index 4d4068855ec4..87d8162c9a34 100644
--- a/Documentation/gpu/drm-kms.rst
+++ b/Documentation/gpu/drm-kms.rst
@@ -17,6 +17,138 @@ be setup by initializing the following fields.
 
 Mode Configuration
 
+Overview
+========
+
+.. kernel-render:: DOT
+   :alt: KMS Display Pipeline
+   :caption: KMS Display Pipeline Overview
+
+   digraph "KMS" {
+      node [shape=box]
+
+      subgraph cluster_static {
+          style=dashed
+          label="Static Objects"
+
+          node [bgcolor=grey style=filled]
+          "drm_plane A" -> "drm_crtc"
+          "drm_plane B" -> "drm_crtc"
+          "drm_crtc" -> "drm_encoder A"
+          "drm_crtc" -> "drm_encoder B"
+      }
+
+      subgraph cluster_user_created {
+          style=dashed
+          label="Userspace-Created"
+
+          node [shape=oval]
+          "drm_framebuffer 1" -> "drm_plane A"
+          "drm_framebuffer 2" -> "drm_plane B"
+      }
+
+      subgraph cluster_connector {
+          style=dashed
+          label="Hotpluggable"
+
+          "drm_encoder A" -> "drm_connector A"
+          "drm_encoder B" -> "drm_connector B"
+      }
+   }
+
+The basic object structure KMS presents to userspace is fairly simple.
+Framebuffers (represented by :c:type:`struct drm_framebuffer <drm_framebuffer>`,
+see `Frame Buffer Abstraction`_) feed into planes. Multiple (or just one, or
+even no) planes feed their pixel data into a CRTC (represented by
+:c:type:`struct drm_crtc <drm_crtc>`, see `CRTC Abstraction`_) for blending. The
+precise blending step is explained in more detail in `Plane Composition
+Properties`_ and related chapter.
+
+For the output routing the first step are Encoders (represented by
+:c:type:`struct drm_encoder <drm_encoder>`, see `Encoder Abstraction`_). Those
+are really just internal artifacts of the helper libraries used to implement KMS
+drivers. But unfortunately encoders have been exposed to userspace. Besides that
+they make it unecessarily more complicated for userspace to figure out which
+connections between a CRTC and a connector, and what kind of cloning is
+supported, they serve no purpose in the userspace API. Futhermore the exposed
+restrictions are often wrongly set by drivers, and in many cases not powerful
+enough to express the real restrictions. A CRTC can be connected to multiple
+Encoders, but for an active CRTC there must be at least one encoder.
+
+The final, and real, endpoint in the display chain is the connector (represented
+by :c:type:`struct drm_connector <drm_connector>`, see `Connector
+Abstraction`_). Connectors can have different possible encoders, but the kernel
+driver does this selection. The use case is DVI, which could switch between an
+analog and a digital encoder. There is only ever one active connector for any
+encoder.
+
+Internally the output pipeline is a bit more complex and matches todays hardware
+more closely:
+
+.. kernel-render:: DOT
+   :alt: KMS Output Pipeline
+   :caption: KMS Output Pipeline
+
+   digraph "Output Pipeline" {
+      node [shape=box]
+
+      subgraph {
+          "drm_crtc" [bgcolor=grey style=filled]
+      }
+
+      subgraph cluster_internal {
+          style=dashed
+          label="Internal Pipeline"
+          {
+              node [bgcolor=grey style=filled]
+              "drm_encoder A";
+              "drm_encoder B";
+              "drm_encoder C";
+          }
+
+          {
+              node [bgcolor=grey style=filled]
+              "drm_encoder B" -> "drm_bridge B"
+              "drm_encoder C" -> "drm_bridge C1"
+              "drm_bridge C1" -> "drm_bridge C2";
+          }
+      }
+
+      "drm_crtc" -> "drm_encoder A"
+      "drm_crtc" -> "drm_encoder B"
+      "drm_crtc" -> "drm_encoder C"
+
+
+      subgraph cluster_output {
+          style=dashed
+          label="Outputs"
+
+          "drm_encoder A" -> "drm_connector A";
+          "drm_bridge B" -> "drm_connector B";
+          "drm_bridge C2" -> "drm_connector C";
+
+          "drm_panel"
+      }
+   }
+
+Internally two additional helper objects come into play. First, to be able to
+share code for encoders (sometimes on the same SoC, sometimes off-chip) one or
+more :ref:`drm_bridges` (represented by :c:type:`struct drm_bridge
+<drm_bridge>`) can be linked to an encoder. This link is static and cannot be
+changed, which means the cross-bar (if there is any) needs to be mapped between
+the CRTC and any encoders. Often for drivers with bridges there's no code left
+at the encoder level. Atomic drivers can leave out all the encoder callbacks to
+essentially only leave a dummy routing object behind, which is needed for
+backwards compatibility since encoders are exposed to userspace.
+
+The second objects are panels, represented by :c:type:`struct drm_panel
+<drm_panel>`, see :ref:`drm_panel_helper`. Panels do not have a fixed binding
+point, but are generally linked to the driver private structure which embeds
+:c:type:`struct drm_connector <drm_connector>`.
+
+Note that currently the bridge chaining and interactions with connectors and
+panels are still in-flux and not really fully sorted out yet.
+
 KMS Core Structures and Functions
 =================================
 
-- 
2.11.0

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