Documentation/gpu/drm-kms.rst
Drivers must initialize the mode setting core by calling
drmm_mode_config_init() on the DRM device. The function
initializes the :c:type:struct drm_device <drm_device>
mode_config field and never fails. Once done, mode configuration must
be setup by initializing the following fields.
int min_width, min_height; int max_width, max_height; Minimum and maximum width and height of the frame buffers in pixel units.
struct drm_mode_config_funcs *funcs; Mode setting functions.
.. 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. Planes are represented by
:c:type:struct drm_plane <drm_plane>, see Plane Abstraction for more
details. One or more (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 chapters.
For the output routing the first step is 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. Besides that they make it unnecessarily more complicated for userspace
to figure out which connections between a CRTC and a connector are possible, and
what kind of cloning is supported, they serve no purpose in the userspace API.
Unfortunately encoders have been exposed to userspace, hence can't remove them
at this point. Furthermore 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, and 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 selects which encoder to use for each connector. The use case is DVI,
which could switch between an analog and a digital encoder. Encoders can also
drive multiple different connectors. There is exactly one active connector for
every active encoder.
Internally the output pipeline is a bit more complex and matches today's 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 object is for 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 that 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.
.. kernel-doc:: include/drm/drm_mode_config.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_mode_config.c :export:
.. _kms_base_object_abstraction:
.. kernel-render:: DOT :alt: Mode Objects and Properties :caption: Mode Objects and Properties
digraph { node [shape=box]
"drm_property A" -> "drm_mode_object A"
"drm_property A" -> "drm_mode_object B"
"drm_property B" -> "drm_mode_object A"
}
The base structure for all KMS objects is :c:type:struct drm_mode_object <drm_mode_object>. One of the base services it provides is tracking properties,
which are especially important for the atomic IOCTL (see Atomic Mode Setting_). The somewhat surprising part here is that properties are not
directly instantiated on each object, but free-standing mode objects themselves,
represented by :c:type:struct drm_property <drm_property>, which only specify
the type and value range of a property. Any given property can be attached
multiple times to different objects using drm_object_attach_property().
.. kernel-doc:: include/drm/drm_mode_object.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_mode_object.c :export:
.. 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 embedding, or add entirely new state
structures for their globally shared hardware functions, see :c:type:struct drm_private_state<drm_private_state>.
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. Driver private state structures are also tracked in the same
structure; see the next chapter. 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.
Locking of atomic state structures is internally using :c:type:struct drm_modeset_lock <drm_modeset_lock>. As a general rule the locking shouldn't be
exposed to drivers, instead the right locks should be automatically acquired by
any function that duplicates or peeks into a state, like e.g.
drm_atomic_get_crtc_state(). Locking only protects the software data
structure, ordering of committing state changes to hardware is sequenced using
:c:type:struct drm_crtc_commit <drm_crtc_commit>.
Read on in this chapter, and also in :ref:drm_atomic_helper for more detailed
coverage of specific topics.
.. kernel-doc:: drivers/gpu/drm/drm_atomic.c :doc: handling driver private state
.. kernel-doc:: include/drm/drm_atomic.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_atomic.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_atomic_uapi.c :doc: overview
.. kernel-doc:: drivers/gpu/drm/drm_atomic_uapi.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_crtc.c :doc: overview
.. kernel-doc:: include/drm/drm_crtc.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_crtc.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c :export:
.. kernel-doc:: include/drm/drm_color_mgmt.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c :doc: overview
.. kernel-doc:: include/drm/drm_framebuffer.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c :export:
.. kernel-doc:: include/uapi/drm/drm_fourcc.h :doc: overview
.. kernel-doc:: include/drm/drm_fourcc.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_fourcc.c :export:
.. _kms_dumb_buffer_objects:
.. kernel-doc:: drivers/gpu/drm/drm_dumb_buffers.c :doc: overview
.. kernel-doc:: drivers/gpu/drm/drm_plane.c :doc: overview
.. kernel-doc:: include/drm/drm_plane.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_plane.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_blend.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_damage_helper.c :export:
.. kernel-doc:: include/drm/drm_damage_helper.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_panic.c :doc: overview
.. kernel-doc:: include/drm/drm_panic.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_panic.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_colorop.c :doc: overview
.. kernel-doc:: include/drm/drm_colorop.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_colorop.c :export:
.. kernel-doc:: include/drm/drm_modes.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_modes.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_connector.c :doc: overview
.. kernel-doc:: include/drm/drm_connector.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_connector.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_writeback.c :doc: overview
.. kernel-doc:: include/drm/drm_writeback.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_writeback.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_encoder.c :doc: overview
.. kernel-doc:: include/drm/drm_encoder.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_encoder.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c :doc: kms locking
.. kernel-doc:: include/drm/drm_modeset_lock.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c :export:
This section of the documentation is primarily aimed at user-space developers. For the driver APIs, see the other sections.
KMS drivers might need to add extra properties to support new features. Each new property introduced in a driver needs to meet a few requirements, in addition to the one mentioned above:
It must be standardized, documenting:
It must provide a generic helper in the core code to register that property on the object it attaches to.
Its content must be decoded by the core and provided in the object's associated state structure. That includes anything drivers might want to precompute, like struct drm_clip_rect for planes.
Its initial state must match the behavior prior to the property introduction. This might be a fixed value matching what the hardware does, or it may be inherited from the state the firmware left the system in during boot.
An IGT test must be submitted where reasonable.
For historical reasons, non-standard, driver-specific properties exist. If a KMS driver wants to add support for one of those properties, the requirements for new properties apply where possible. Additionally, the documented behavior must match the de facto semantics of the existing property to ensure compatibility. Developers of the driver that first added the property should help with those tasks and must ACK the documented behavior if possible.
.. kernel-doc:: drivers/gpu/drm/drm_property.c :doc: overview
.. kernel-doc:: include/drm/drm_property.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_property.c :export:
.. _standard_connector_properties:
.. kernel-doc:: drivers/gpu/drm/drm_connector.c :doc: standard connector properties
.. kernel-doc:: drivers/gpu/drm/drm_connector.c :doc: HDMI connector properties
.. kernel-doc:: drivers/gpu/drm/drm_connector.c :doc: Analog TV Connector Properties
.. kernel-doc:: drivers/gpu/drm/drm_crtc.c :doc: standard CRTC properties
.. kernel-doc:: drivers/gpu/drm/drm_plane.c :doc: standard plane properties
.. _plane_composition_properties:
.. kernel-doc:: drivers/gpu/drm/drm_blend.c :doc: overview
.. _damage_tracking_properties:
.. kernel-doc:: drivers/gpu/drm/drm_plane.c :doc: damage tracking
.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c :doc: overview
.. kernel-doc:: drivers/gpu/drm/drm_connector.c :doc: Tile group
.. kernel-doc:: drivers/gpu/drm/drm_atomic_uapi.c :doc: explicit fencing properties
.. kernel-doc:: drivers/gpu/drm/drm_connector.c :doc: Variable refresh properties
.. kernel-doc:: drivers/gpu/drm/drm_plane.c :doc: hotspot properties
The following table gives description of drm properties exposed by various modules/drivers. Because this table is very unwieldy, do not add any new properties here. Instead document them in a section above.
.. csv-table:: :header-rows: 1 :file: kms-properties.csv
.. kernel-doc:: drivers/gpu/drm/drm_vblank.c :doc: vblank handling
.. kernel-doc:: include/drm/drm_vblank.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_vblank.c :export:
.. kernel-doc:: drivers/gpu/drm/drm_vblank_work.c :doc: vblank works
.. kernel-doc:: include/drm/drm_vblank_work.h :internal:
.. kernel-doc:: drivers/gpu/drm/drm_vblank_work.c :export: