540 lines
16 KiB
ReStructuredText
540 lines
16 KiB
ReStructuredText
=========================
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Kernel Mode Setting (KMS)
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=========================
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Drivers must initialize the mode setting core by calling
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drmm_mode_config_init() on the DRM device. The function
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initializes the :c:type:`struct drm_device <drm_device>`
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mode_config field and never fails. Once done, mode configuration must
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be setup by initializing the following fields.
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- int min_width, min_height; int max_width, max_height;
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Minimum and maximum width and height of the frame buffers in pixel
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units.
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- struct drm_mode_config_funcs \*funcs;
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Mode setting functions.
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Overview
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========
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.. kernel-render:: DOT
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:alt: KMS Display Pipeline
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:caption: KMS Display Pipeline Overview
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digraph "KMS" {
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node [shape=box]
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subgraph cluster_static {
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style=dashed
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label="Static Objects"
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node [bgcolor=grey style=filled]
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"drm_plane A" -> "drm_crtc"
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"drm_plane B" -> "drm_crtc"
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"drm_crtc" -> "drm_encoder A"
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"drm_crtc" -> "drm_encoder B"
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}
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subgraph cluster_user_created {
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style=dashed
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label="Userspace-Created"
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node [shape=oval]
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"drm_framebuffer 1" -> "drm_plane A"
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"drm_framebuffer 2" -> "drm_plane B"
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}
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subgraph cluster_connector {
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style=dashed
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label="Hotpluggable"
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"drm_encoder A" -> "drm_connector A"
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"drm_encoder B" -> "drm_connector B"
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}
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}
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The basic object structure KMS presents to userspace is fairly simple.
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Framebuffers (represented by :c:type:`struct drm_framebuffer <drm_framebuffer>`,
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see `Frame Buffer Abstraction`_) feed into planes. Planes are represented by
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:c:type:`struct drm_plane <drm_plane>`, see `Plane Abstraction`_ for more
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details. One or more (or even no) planes feed their pixel data into a CRTC
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(represented by :c:type:`struct drm_crtc <drm_crtc>`, see `CRTC Abstraction`_)
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for blending. The precise blending step is explained in more detail in `Plane
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Composition Properties`_ and related chapters.
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For the output routing the first step is encoders (represented by
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:c:type:`struct drm_encoder <drm_encoder>`, see `Encoder Abstraction`_). Those
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are really just internal artifacts of the helper libraries used to implement KMS
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drivers. Besides that they make it unecessarily more complicated for userspace
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to figure out which connections between a CRTC and a connector are possible, and
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what kind of cloning is supported, they serve no purpose in the userspace API.
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Unfortunately encoders have been exposed to userspace, hence can't remove them
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at this point. Futhermore the exposed restrictions are often wrongly set by
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drivers, and in many cases not powerful enough to express the real restrictions.
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A CRTC can be connected to multiple encoders, and for an active CRTC there must
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be at least one encoder.
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The final, and real, endpoint in the display chain is the connector (represented
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by :c:type:`struct drm_connector <drm_connector>`, see `Connector
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Abstraction`_). Connectors can have different possible encoders, but the kernel
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driver selects which encoder to use for each connector. The use case is DVI,
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which could switch between an analog and a digital encoder. Encoders can also
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drive multiple different connectors. There is exactly one active connector for
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every active encoder.
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Internally the output pipeline is a bit more complex and matches today's
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hardware more closely:
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.. kernel-render:: DOT
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:alt: KMS Output Pipeline
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:caption: KMS Output Pipeline
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digraph "Output Pipeline" {
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node [shape=box]
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subgraph {
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"drm_crtc" [bgcolor=grey style=filled]
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}
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subgraph cluster_internal {
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style=dashed
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label="Internal Pipeline"
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{
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node [bgcolor=grey style=filled]
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"drm_encoder A";
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"drm_encoder B";
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"drm_encoder C";
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}
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{
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node [bgcolor=grey style=filled]
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"drm_encoder B" -> "drm_bridge B"
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"drm_encoder C" -> "drm_bridge C1"
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"drm_bridge C1" -> "drm_bridge C2";
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}
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}
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"drm_crtc" -> "drm_encoder A"
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"drm_crtc" -> "drm_encoder B"
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"drm_crtc" -> "drm_encoder C"
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subgraph cluster_output {
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style=dashed
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label="Outputs"
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"drm_encoder A" -> "drm_connector A";
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"drm_bridge B" -> "drm_connector B";
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"drm_bridge C2" -> "drm_connector C";
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"drm_panel"
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}
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}
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Internally two additional helper objects come into play. First, to be able to
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share code for encoders (sometimes on the same SoC, sometimes off-chip) one or
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more :ref:`drm_bridges` (represented by :c:type:`struct drm_bridge
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<drm_bridge>`) can be linked to an encoder. This link is static and cannot be
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changed, which means the cross-bar (if there is any) needs to be mapped between
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the CRTC and any encoders. Often for drivers with bridges there's no code left
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at the encoder level. Atomic drivers can leave out all the encoder callbacks to
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essentially only leave a dummy routing object behind, which is needed for
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backwards compatibility since encoders are exposed to userspace.
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The second object is for panels, represented by :c:type:`struct drm_panel
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<drm_panel>`, see :ref:`drm_panel_helper`. Panels do not have a fixed binding
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point, but are generally linked to the driver private structure that embeds
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:c:type:`struct drm_connector <drm_connector>`.
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Note that currently the bridge chaining and interactions with connectors and
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panels are still in-flux and not really fully sorted out yet.
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KMS Core Structures and Functions
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=================================
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.. kernel-doc:: include/drm/drm_mode_config.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_mode_config.c
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:export:
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Modeset Base Object Abstraction
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===============================
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.. kernel-render:: DOT
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:alt: Mode Objects and Properties
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:caption: Mode Objects and Properties
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digraph {
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node [shape=box]
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"drm_property A" -> "drm_mode_object A"
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"drm_property A" -> "drm_mode_object B"
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"drm_property B" -> "drm_mode_object A"
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}
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The base structure for all KMS objects is :c:type:`struct drm_mode_object
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<drm_mode_object>`. One of the base services it provides is tracking properties,
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which are especially important for the atomic IOCTL (see `Atomic Mode
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Setting`_). The somewhat surprising part here is that properties are not
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directly instantiated on each object, but free-standing mode objects themselves,
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represented by :c:type:`struct drm_property <drm_property>`, which only specify
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the type and value range of a property. Any given property can be attached
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multiple times to different objects using drm_object_attach_property().
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.. kernel-doc:: include/drm/drm_mode_object.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_mode_object.c
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:export:
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Atomic Mode Setting
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===================
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.. kernel-render:: DOT
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:alt: Mode Objects and Properties
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:caption: Mode Objects and Properties
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digraph {
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node [shape=box]
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subgraph cluster_state {
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style=dashed
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label="Free-standing state"
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"drm_atomic_state" -> "duplicated drm_plane_state A"
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"drm_atomic_state" -> "duplicated drm_plane_state B"
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"drm_atomic_state" -> "duplicated drm_crtc_state"
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"drm_atomic_state" -> "duplicated drm_connector_state"
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"drm_atomic_state" -> "duplicated driver private state"
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}
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subgraph cluster_current {
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style=dashed
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label="Current state"
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"drm_device" -> "drm_plane A"
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"drm_device" -> "drm_plane B"
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"drm_device" -> "drm_crtc"
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"drm_device" -> "drm_connector"
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"drm_device" -> "driver private object"
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"drm_plane A" -> "drm_plane_state A"
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"drm_plane B" -> "drm_plane_state B"
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"drm_crtc" -> "drm_crtc_state"
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"drm_connector" -> "drm_connector_state"
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"driver private object" -> "driver private state"
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}
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"drm_atomic_state" -> "drm_device" [label="atomic_commit"]
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"duplicated drm_plane_state A" -> "drm_device"[style=invis]
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}
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Atomic provides transactional modeset (including planes) updates, but a
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bit differently from the usual transactional approach of try-commit and
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rollback:
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- Firstly, no hardware changes are allowed when the commit would fail. This
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allows us to implement the DRM_MODE_ATOMIC_TEST_ONLY mode, which allows
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userspace to explore whether certain configurations would work or not.
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- This would still allow setting and rollback of just the software state,
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simplifying conversion of existing drivers. But auditing drivers for
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correctness of the atomic_check code becomes really hard with that: Rolling
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back changes in data structures all over the place is hard to get right.
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- Lastly, for backwards compatibility and to support all use-cases, atomic
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updates need to be incremental and be able to execute in parallel. Hardware
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doesn't always allow it, but where possible plane updates on different CRTCs
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should not interfere, and not get stalled due to output routing changing on
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different CRTCs.
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Taken all together there's two consequences for the atomic design:
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- The overall state is split up into per-object state structures:
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:c:type:`struct drm_plane_state <drm_plane_state>` for planes, :c:type:`struct
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drm_crtc_state <drm_crtc_state>` for CRTCs and :c:type:`struct
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drm_connector_state <drm_connector_state>` for connectors. These are the only
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objects with userspace-visible and settable state. For internal state drivers
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can subclass these structures through embeddeding, or add entirely new state
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structures for their globally shared hardware functions, see :c:type:`struct
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drm_private_state<drm_private_state>`.
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- An atomic update is assembled and validated as an entirely free-standing pile
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of structures within the :c:type:`drm_atomic_state <drm_atomic_state>`
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container. Driver private state structures are also tracked in the same
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structure; see the next chapter. Only when a state is committed is it applied
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to the driver and modeset objects. This way rolling back an update boils down
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to releasing memory and unreferencing objects like framebuffers.
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Locking of atomic state structures is internally using :c:type:`struct
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drm_modeset_lock <drm_modeset_lock>`. As a general rule the locking shouldn't be
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exposed to drivers, instead the right locks should be automatically acquired by
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any function that duplicates or peeks into a state, like e.g.
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drm_atomic_get_crtc_state(). Locking only protects the software data
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structure, ordering of committing state changes to hardware is sequenced using
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:c:type:`struct drm_crtc_commit <drm_crtc_commit>`.
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Read on in this chapter, and also in :ref:`drm_atomic_helper` for more detailed
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coverage of specific topics.
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Handling Driver Private State
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-----------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
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:doc: handling driver private state
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Atomic Mode Setting Function Reference
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--------------------------------------
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.. kernel-doc:: include/drm/drm_atomic.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
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:export:
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Atomic Mode Setting IOCTL and UAPI Functions
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--------------------------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_atomic_uapi.c
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:doc: overview
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.. kernel-doc:: drivers/gpu/drm/drm_atomic_uapi.c
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:export:
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CRTC Abstraction
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================
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.. kernel-doc:: drivers/gpu/drm/drm_crtc.c
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:doc: overview
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CRTC Functions Reference
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--------------------------------
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.. kernel-doc:: include/drm/drm_crtc.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_crtc.c
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:export:
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Frame Buffer Abstraction
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========================
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.. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c
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:doc: overview
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Frame Buffer Functions Reference
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--------------------------------
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.. kernel-doc:: include/drm/drm_framebuffer.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c
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:export:
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DRM Format Handling
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===================
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.. kernel-doc:: include/uapi/drm/drm_fourcc.h
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:doc: overview
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Format Functions Reference
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--------------------------
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.. kernel-doc:: include/drm/drm_fourcc.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_fourcc.c
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:export:
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Dumb Buffer Objects
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===================
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.. kernel-doc:: drivers/gpu/drm/drm_dumb_buffers.c
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:doc: overview
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Plane Abstraction
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=================
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.. kernel-doc:: drivers/gpu/drm/drm_plane.c
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:doc: overview
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Plane Functions Reference
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-------------------------
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.. kernel-doc:: include/drm/drm_plane.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_plane.c
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:export:
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Display Modes Function Reference
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================================
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.. kernel-doc:: include/drm/drm_modes.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_modes.c
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:export:
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Connector Abstraction
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=====================
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.. kernel-doc:: drivers/gpu/drm/drm_connector.c
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:doc: overview
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Connector Functions Reference
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-----------------------------
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.. kernel-doc:: include/drm/drm_connector.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_connector.c
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:export:
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Writeback Connectors
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--------------------
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.. kernel-doc:: include/drm/drm_writeback.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
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:doc: overview
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.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
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:export:
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Encoder Abstraction
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===================
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.. kernel-doc:: drivers/gpu/drm/drm_encoder.c
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:doc: overview
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Encoder Functions Reference
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---------------------------
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.. kernel-doc:: include/drm/drm_encoder.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_encoder.c
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:export:
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KMS Locking
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===========
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.. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c
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:doc: kms locking
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.. kernel-doc:: include/drm/drm_modeset_lock.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c
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:export:
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KMS Properties
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==============
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Property Types and Blob Property Support
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----------------------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_property.c
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:doc: overview
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.. kernel-doc:: include/drm/drm_property.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_property.c
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:export:
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Standard Connector Properties
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-----------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_connector.c
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:doc: standard connector properties
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HDMI Specific Connector Properties
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----------------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_connector.c
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:doc: HDMI connector properties
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Plane Composition Properties
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----------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_blend.c
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:doc: overview
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.. kernel-doc:: drivers/gpu/drm/drm_blend.c
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:export:
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FB_DAMAGE_CLIPS
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~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/gpu/drm/drm_damage_helper.c
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:doc: overview
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.. kernel-doc:: drivers/gpu/drm/drm_damage_helper.c
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:export:
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.. kernel-doc:: include/drm/drm_damage_helper.h
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:internal:
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Color Management Properties
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---------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
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:doc: overview
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.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
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:export:
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.. kernel-doc:: include/drm/drm_color_mgmt.h
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:internal:
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Tile Group Property
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-------------------
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.. kernel-doc:: drivers/gpu/drm/drm_connector.c
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:doc: Tile group
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Explicit Fencing Properties
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---------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_atomic_uapi.c
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:doc: explicit fencing properties
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Variable Refresh Properties
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---------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_connector.c
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:doc: Variable refresh properties
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Existing KMS Properties
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-----------------------
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The following table gives description of drm properties exposed by various
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modules/drivers. Because this table is very unwieldy, do not add any new
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properties here. Instead document them in a section above.
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.. csv-table::
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:header-rows: 1
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:file: kms-properties.csv
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Vertical Blanking
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=================
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.. kernel-doc:: drivers/gpu/drm/drm_vblank.c
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:doc: vblank handling
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Vertical Blanking and Interrupt Handling Functions Reference
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------------------------------------------------------------
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.. kernel-doc:: include/drm/drm_vblank.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_vblank.c
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:export:
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