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.
v2: Spelling and clarifications (Eric).
v3: Implement suggestion from Gabriel to fix the graph.
v4: Review from Laurent.
Reviewed-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
Cc: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxxxx>
Acked-by: Eric Anholt <eric@xxxxxxxxxx>
Cc: Eric Anholt <eric@xxxxxxxxxx>
Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
Cc: Harry Wentland <harry.wentland@xxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
Documentation/gpu/drm-kms-helpers.rst | 2 +
Documentation/gpu/drm-kms.rst | 84 ++++++++++++++++++++++++++++++++++-
2 files changed, 85 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
:internal:
+.. _drm_atomic_helper:
+
Atomic Modeset Helper Functions Reference
=========================================
diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index 1c42448c7aae..bfecd21a8cdf 100644
--- a/Documentation/gpu/drm-kms.rst
+++ b/Documentation/gpu/drm-kms.rst
@@ -189,8 +189,90 @@ multiple times to different objects using :c:func:`drm_object_attach_property()
.. kernel-doc:: drivers/gpu/drm/drm_mode_object.c
:export:
+Atomic Mode Setting
+===================
+
+
+.. 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"]
+ "duplicated drm_plane_state A" -> "drm_device"[style=invis]
+ }
+
+Atomic provides transactional modeset (including planes) updates, but a
+bit differently from the usual transactional approach of try-commit and
+rollback:
+
+- 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 allow setting and rollback of just the software state,
+ simplifying conversion of existing drivers. But auditing drivers for
+ correctness of the atomic_check code becomes really hard with that: Rolling
+ back changes in data structures all over the place is hard to get right.
+
+- 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 consequences 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 entirely 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 committed is it applied to the
+ driver and modeset objects. This way rolling back an update boils down to
+ releasing memory and unreferencing 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
:internal:
--
2.11.0
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