blob: 307284125d7aa964522f85ea6fb34247e67c9de2 [file] [log] [blame]
Kernel Mode Setting (KMS)
Drivers must initialize the mode setting core by calling
:c:func:`drm_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
- 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 {
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 {
node [shape=oval]
"drm_framebuffer 1" -> "drm_plane A"
"drm_framebuffer 2" -> "drm_plane B"
subgraph cluster_connector {
"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. 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 unecessarily 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. 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, 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 {
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 {
"drm_encoder A" -> "drm_connector A";
"drm_bridge B" -> "drm_connector B";
"drm_bridge C2" -> "drm_connector C";
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.
KMS Core Structures and Functions
.. kernel-doc:: include/drm/drm_mode_config.h
.. kernel-doc:: drivers/gpu/drm/drm_mode_config.c
Modeset 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 :c:func:`drm_object_attach_property()
.. kernel-doc:: include/drm/drm_mode_object.h
.. kernel-doc:: drivers/gpu/drm/drm_mode_object.c
Atomic Mode Setting
.. kernel-render:: DOT
:alt: Mode Objects and Properties
:caption: Mode Objects and Properties
digraph {
node [shape=box]
subgraph cluster_state {
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 {
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
- 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
.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
CRTC Abstraction
.. kernel-doc:: drivers/gpu/drm/drm_crtc.c
:doc: overview
CRTC Functions Reference
.. kernel-doc:: include/drm/drm_crtc.h
.. kernel-doc:: drivers/gpu/drm/drm_crtc.c
Frame Buffer Abstraction
.. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c
:doc: overview
Frame Buffer Functions Reference
.. kernel-doc:: include/drm/drm_framebuffer.h
.. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c
DRM Format Handling
.. kernel-doc:: include/drm/drm_fourcc.h
.. kernel-doc:: drivers/gpu/drm/drm_fourcc.c
Dumb Buffer Objects
.. kernel-doc:: drivers/gpu/drm/drm_dumb_buffers.c
:doc: overview
Plane Abstraction
.. kernel-doc:: drivers/gpu/drm/drm_plane.c
:doc: overview
Plane Functions Reference
.. kernel-doc:: include/drm/drm_plane.h
.. kernel-doc:: drivers/gpu/drm/drm_plane.c
Display Modes Function Reference
.. kernel-doc:: include/drm/drm_modes.h
.. kernel-doc:: drivers/gpu/drm/drm_modes.c
Connector Abstraction
.. kernel-doc:: drivers/gpu/drm/drm_connector.c
:doc: overview
Connector Functions Reference
.. kernel-doc:: include/drm/drm_connector.h
.. kernel-doc:: drivers/gpu/drm/drm_connector.c
Encoder Abstraction
.. kernel-doc:: drivers/gpu/drm/drm_encoder.c
:doc: overview
Encoder Functions Reference
.. kernel-doc:: include/drm/drm_encoder.h
.. kernel-doc:: drivers/gpu/drm/drm_encoder.c
KMS Initialization and Cleanup
A KMS device is abstracted and exposed as a set of planes, CRTCs,
encoders and connectors. KMS drivers must thus create and initialize all
those objects at load time after initializing mode setting.
CRTCs (:c:type:`struct drm_crtc <drm_crtc>`)
A CRTC is an abstraction representing a part of the chip that contains a
pointer to a scanout buffer. Therefore, the number of CRTCs available
determines how many independent scanout buffers can be active at any
given time. The CRTC structure contains several fields to support this:
a pointer to some video memory (abstracted as a frame buffer object), a
display mode, and an (x, y) offset into the video memory to support
panning or configurations where one piece of video memory spans multiple
CRTC Initialization
A KMS device must create and register at least one struct
:c:type:`struct drm_crtc <drm_crtc>` instance. The instance is
allocated and zeroed by the driver, possibly as part of a larger
structure, and registered with a call to :c:func:`drm_crtc_init()`
with a pointer to CRTC functions.
The DRM core manages its objects' lifetime. When an object is not needed
anymore the core calls its destroy function, which must clean up and
free every resource allocated for the object. Every
:c:func:`drm_\*_init()` call must be matched with a corresponding
:c:func:`drm_\*_cleanup()` call to cleanup CRTCs
(:c:func:`drm_crtc_cleanup()`), planes
(:c:func:`drm_plane_cleanup()`), encoders
(:c:func:`drm_encoder_cleanup()`) and connectors
(:c:func:`drm_connector_cleanup()`). Furthermore, connectors that
have been added to sysfs must be removed by a call to
:c:func:`drm_connector_unregister()` before calling
Connectors state change detection must be cleanup up with a call to
Output discovery and initialization example
.. code-block:: c
void intel_crt_init(struct drm_device *dev)
struct drm_connector *connector;
struct intel_output *intel_output;
intel_output = kzalloc(sizeof(struct intel_output), GFP_KERNEL);
if (!intel_output)
connector = &intel_output->base;
drm_connector_init(dev, &intel_output->base,
&intel_crt_connector_funcs, DRM_MODE_CONNECTOR_VGA);
drm_encoder_init(dev, &intel_output->enc, &intel_crt_enc_funcs,
/* Set up the DDC bus. */
intel_output->ddc_bus = intel_i2c_create(dev, GPIOA, "CRTDDC_A");
if (!intel_output->ddc_bus) {
dev_printk(KERN_ERR, &dev->pdev->dev, "DDC bus registration "
intel_output->type = INTEL_OUTPUT_ANALOG;
connector->interlace_allowed = 0;
connector->doublescan_allowed = 0;
drm_encoder_helper_add(&intel_output->enc, &intel_crt_helper_funcs);
drm_connector_helper_add(connector, &intel_crt_connector_helper_funcs);
In the example above (taken from the i915 driver), a CRTC, connector and
encoder combination is created. A device-specific i2c bus is also
created for fetching EDID data and performing monitor detection. Once
the process is complete, the new connector is registered with sysfs to
make its properties available to applications.
KMS Locking
.. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c
:doc: kms locking
.. kernel-doc:: include/drm/drm_modeset_lock.h
.. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c
KMS Properties
Property Types and Blob Property Support
.. kernel-doc:: drivers/gpu/drm/drm_property.c
:doc: overview
.. kernel-doc:: include/drm/drm_property.h
.. kernel-doc:: drivers/gpu/drm/drm_property.c
Standard Connector Properties
.. kernel-doc:: drivers/gpu/drm/drm_connector.c
:doc: standard connector properties
Plane Composition Properties
.. kernel-doc:: drivers/gpu/drm/drm_blend.c
:doc: overview
.. kernel-doc:: drivers/gpu/drm/drm_blend.c
Color Management Properties
.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
:doc: overview
.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
Tile Group Property
.. kernel-doc:: drivers/gpu/drm/drm_connector.c
:doc: Tile group
Explicit Fencing Properties
.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
:doc: explicit fencing properties
Existing KMS Properties
The following table gives description of drm properties exposed by
various modules/drivers.
.. csv-table::
:header-rows: 1
:file: kms-properties.csv
Vertical Blanking
.. kernel-doc:: drivers/gpu/drm/drm_vblank.c
:doc: vblank handling
Vertical Blanking and Interrupt Handling Functions Reference
.. kernel-doc:: include/drm/drm_vblank.h
.. kernel-doc:: drivers/gpu/drm/drm_vblank.c