[PATCH] drm/doc: atomic overview, with graph

Daniel Vetter daniel.vetter at ffwll.ch
Mon Dec 19 07:44:30 UTC 2016

I want to split up a few more things and document some details better
(like how exactly to subclass drm_atomic_state). And maybe also split
up the helpers a bit per-topic, but this should be a ok-ish start for
better atomic overview.

One thing I failed at is getting DOT to layout the overview graph how
I want it. The highlevel structure I want is:

	Free-standing State

	Current State

i.e. one over the other. Currently it lays it out side-by-side, but
not even that really - "Current State" is somewhat offset below. Makes
the graph look like garbage, and also way too wide for proper
rendering. Ideas appreciated.

Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Cc: Harry Wentland <harry.wentland at amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
 Documentation/gpu/drm-kms-helpers.rst |  2 +
 Documentation/gpu/drm-kms.rst         | 85 ++++++++++++++++++++++++++++++++++-
 2 files changed, 86 insertions(+), 1 deletion(-)

diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
index 050ebe81d256..ac53c0b893f6 100644
--- a/Documentation/gpu/drm-kms-helpers.rst
+++ b/Documentation/gpu/drm-kms-helpers.rst
@@ -42,6 +42,8 @@ Modeset Helper Reference for Common Vtables
 .. kernel-doc:: include/drm/drm_modeset_helper_vtables.h
+.. _drm_atomic_helper:
 Atomic Modeset Helper Functions Reference
diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index 07d288fa37b2..35c41cf84a1b 100644
--- a/Documentation/gpu/drm-kms.rst
+++ b/Documentation/gpu/drm-kms.rst
@@ -189,8 +189,91 @@ multiple times to different objects using :c:func:`drm_object_attach_property()
 .. kernel-doc:: drivers/gpu/drm/drm_mode_object.c
+Atomic Mode Setting
+.. FIXME: The I want the below graph to be laid out so that the 2 subgraph
+   clusters are below each another. But I failed.
+.. kernel-render:: DOT
+   :alt: Mode Objects and Properties
+   :caption: Mode Objects and Properties
+   digraph {
+      node [shape=box]
+      subgraph cluster_state {
+          style=dashed
+          label="Free-standing state"
+          "drm_atomic_state" -> "duplicated drm_plane_state A"
+          "drm_atomic_state" -> "duplicated drm_plane_state B"
+          "drm_atomic_state" -> "duplicated drm_crtc_state"
+          "drm_atomic_state" -> "duplicated drm_connector_state"
+          "drm_atomic_state" -> "duplicated driver private state"
+      }
+      subgraph cluster_current {
+          style=dashed
+          label="Current state"
+          "drm_device" -> "drm_plane A"
+          "drm_device" -> "drm_plane B"
+          "drm_device" -> "drm_crtc"
+          "drm_device" -> "drm_connector"
+          "drm_device" -> "driver private object"
+          "drm_plane A" -> "drm_plane_state A"
+          "drm_plane B" -> "drm_plane_state B"
+          "drm_crtc" -> "drm_crtc_state"
+          "drm_connector" -> "drm_connector_state"
+          "driver private object" -> "driver private state"
+      }
+      "drm_atomic_state" -> "drm_device" [label="atomic_commit"]
+   }
+Essentially atomic is transactional modeset (including planes) updates, but
+compared to the usual transactional approach of try-commit and rollback on
+failure atomic modesetting is a bit different:
+- Firstly, no hardware changes are allowed when the commit would fail. This
+  allows us to implement the DRM_MODE_ATOMIC_TEST_ONLY mode, which allows
+  userspace to explore whether certain configurations would work or not.
+- This would still allows setting and rollback of just the software state,
+  simplifying conversion of existing drivers. But auditing drivers for
+  correctness of the atomic_check code because really hard with that.
+- Lastly, for backwards compatibility and to support all use-cases, atomic
+  updates need to be incremental and be able to execute in parallel. Hardware
+  doesn't always allow it, but where possible plane updates on different CRTCs
+  should not interfere, and not get stalled due to output routing changing on
+  different CRTCs.
+Taken all together there's two consequence for the atomic design:
+- The overall state is split up into per-object state structures:
+  :c:type:`struct drm_plane_state <drm_plane_state>` for planes, :c:type:`struct
+  drm_crtc_state <drm_crtc_state>` for CRTCs and :c:type:`struct
+  drm_connector_state <drm_connector_state` for connectors. These are the only
+  objects with userspace-visible and settable state. For internal state drivers
+  can subclass these structures through embeddeding, or add entirely new state
+  structures for their globally shared hardware functions.
+- An atomic update is assembled and validated as an enterily free-standing pile
+  of structures within the :c:type:`drm_atomic_state <drm_atomic_state>`
+  container. Again drivers can subclass that container for their own state
+  structure tracking needs. Only when a state is commit is it applied to the
+  driver and modeset objects. This way rolling back an update boils down to
+  releasing memory and unreference objects like framebuffers.
+Read on in this chapter, and also in :ref:`drm_atomic_helper` for more detailed
+coverage of specific topics.
 Atomic Mode Setting Function Reference
 .. kernel-doc:: include/drm/drm_atomic.h

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