Kernel Mode Setting (KMS)

Drivers must initialize the mode setting core by calling drmm_mode_config_init() on the DRM device. The function initializes the struct 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.

Overview

KMS Display Pipeline

KMS Display Pipeline Overview

The basic object structure KMS presents to userspace is fairly simple. Framebuffers (represented by struct drm_framebuffer, see Frame Buffer Abstraction) feed into planes. Planes are represented by struct drm_plane, see Plane Abstraction for more details. One or more (or even no) planes feed their pixel data into a CRTC (represented by struct 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 struct 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 struct 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:

KMS Output Pipeline

KMS Output Pipeline

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 Bridges (represented by struct 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 struct drm_panel, see Panel Helper Reference. Panels do not have a fixed binding point, but are generally linked to the driver private structure that embeds struct 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

Error

kernel-doc missing

Error

kernel-doc missing

Modeset Base Object Abstraction

Mode Objects and Properties

Mode Objects and Properties

The base structure for all KMS objects is struct 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 struct 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().

Error

kernel-doc missing

Error

kernel-doc missing

Atomic Mode Setting

Mode Objects and Properties

Mode Objects and Properties

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: struct drm_plane_state for planes, struct drm_crtc_state for CRTCs and struct 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, see struct drm_private_state.

  • An atomic update is assembled and validated as an entirely free-standing pile of structures within the 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 struct 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 struct drm_crtc_commit.

Read on in this chapter, and also in Atomic Modeset Helper Functions Reference for more detailed coverage of specific topics.

Handling Driver Private State

Error

kernel-doc missing

Atomic Mode Setting Function Reference

Error

kernel-doc missing

Error

kernel-doc missing

Atomic Mode Setting IOCTL and UAPI Functions

Error

kernel-doc missing

Error

kernel-doc missing

CRTC Abstraction

Error

kernel-doc missing

CRTC Functions Reference

Error

kernel-doc missing

Error

kernel-doc missing

Frame Buffer Abstraction

Error

kernel-doc missing

Frame Buffer Functions Reference

Error

kernel-doc missing

Error

kernel-doc missing

DRM Format Handling

Error

kernel-doc missing

Format Functions Reference

Error

kernel-doc missing

Error

kernel-doc missing

Dumb Buffer Objects

Error

kernel-doc missing

Plane Abstraction

Error

kernel-doc missing

Plane Functions Reference

Error

kernel-doc missing

Error

kernel-doc missing

Display Modes Function Reference

Error

kernel-doc missing

Error

kernel-doc missing

Connector Abstraction

Error

kernel-doc missing

Connector Functions Reference

Error

kernel-doc missing

Error

kernel-doc missing

Writeback Connectors

Error

kernel-doc missing

Error

kernel-doc missing

Error

kernel-doc missing

Encoder Abstraction

Error

kernel-doc missing

Encoder Functions Reference

Error

kernel-doc missing

Error

kernel-doc missing

KMS Locking

Error

kernel-doc missing

Error

kernel-doc missing

Error

kernel-doc missing

KMS Properties

Property Types and Blob Property Support

Error

kernel-doc missing

Error

kernel-doc missing

Error

kernel-doc missing

Standard Connector Properties

Error

kernel-doc missing

HDMI Specific Connector Properties

Error

kernel-doc missing

Standard CRTC Properties

Error

kernel-doc missing

Plane Composition Properties

Error

kernel-doc missing

Error

kernel-doc missing

FB_DAMAGE_CLIPS

Error

kernel-doc missing

Error

kernel-doc missing

Error

kernel-doc missing

Color Management Properties

Error

kernel-doc missing

Error

kernel-doc missing

Error

kernel-doc missing

Tile Group Property

Error

kernel-doc missing

Explicit Fencing Properties

Error

kernel-doc missing

Variable Refresh Properties

Error

kernel-doc missing

Existing KMS 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.

Owner Module/Drivers

Group

Property Name

Type

Property Values

Object attached

Description/Restrictions

DVI-I

“subconnector”

ENUM

{ “Unknown”, “DVI-D”, “DVI-A” }

Connector

TBD

“select subconnector”

ENUM

{ “Automatic”, “DVI-D”, “DVI-A” }

Connector

TBD

TV

“subconnector”

ENUM

{ “Unknown”, “Composite”, “SVIDEO”, “Component”, “SCART” }

Connector

TBD

“select subconnector”

ENUM

{ “Automatic”, “Composite”, “SVIDEO”, “Component”, “SCART” }

Connector

TBD

“mode”

ENUM

{ “NTSC_M”, “NTSC_J”, “NTSC_443”, “PAL_B” } etc.

Connector

TBD

“left margin”

RANGE

Min=0, Max=100

Connector

TBD

“right margin”

RANGE

Min=0, Max=100

Connector

TBD

“top margin”

RANGE

Min=0, Max=100

Connector

TBD

“bottom margin”

RANGE

Min=0, Max=100

Connector

TBD

“brightness”

RANGE

Min=0, Max=100

Connector

TBD

“contrast”

RANGE

Min=0, Max=100

Connector

TBD

“flicker reduction”

RANGE

Min=0, Max=100

Connector

TBD

“overscan”

RANGE

Min=0, Max=100

Connector

TBD

“saturation”

RANGE

Min=0, Max=100

Connector

TBD

“hue”

RANGE

Min=0, Max=100

Connector

TBD

Virtual GPU

“suggested X”

RANGE

Min=0, Max=0xffffffff

Connector

property to suggest an X offset for a connector

“suggested Y”

RANGE

Min=0, Max=0xffffffff

Connector

property to suggest an Y offset for a connector

Optional

“aspect ratio”

ENUM

{ “None”, “4:3”, “16:9” }

Connector

TDB

i915

Generic

“Broadcast RGB”

ENUM

{ “Automatic”, “Full”, “Limited 16:235” }

Connector

When this property is set to Limited 16:235 and CTM is set, the hardware will be programmed with the result of the multiplication of CTM by the limited range matrix to ensure the pixels normaly in the range 0..1.0 are remapped to the range 16/255..235/255.

“audio”

ENUM

{ “force-dvi”, “off”, “auto”, “on” }

Connector

TBD

SDVO-TV

“mode”

ENUM

{ “NTSC_M”, “NTSC_J”, “NTSC_443”, “PAL_B” } etc.

Connector

TBD

“left_margin”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“right_margin”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“top_margin”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“bottom_margin”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“hpos”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“vpos”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“contrast”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“saturation”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“hue”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“sharpness”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“flicker_filter”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“flicker_filter_adaptive”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“flicker_filter_2d”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“tv_chroma_filter”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“tv_luma_filter”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“dot_crawl”

RANGE

Min=0, Max=1

Connector

TBD

SDVO-TV/LVDS

“brightness”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

CDV gma-500

Generic

“Broadcast RGB”

ENUM

{ “Full”, “Limited 16:235” }

Connector

TBD

“Broadcast RGB”

ENUM

{ “off”, “auto”, “on” }

Connector

TBD

Poulsbo

Generic

“backlight”

RANGE

Min=0, Max=100

Connector

TBD

SDVO-TV

“mode”

ENUM

{ “NTSC_M”, “NTSC_J”, “NTSC_443”, “PAL_B” } etc.

Connector

TBD

“left_margin”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“right_margin”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“top_margin”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“bottom_margin”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“hpos”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“vpos”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“contrast”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“saturation”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“hue”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“sharpness”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“flicker_filter”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“flicker_filter_adaptive”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“flicker_filter_2d”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“tv_chroma_filter”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“tv_luma_filter”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

“dot_crawl”

RANGE

Min=0, Max=1

Connector

TBD

SDVO-TV/LVDS

“brightness”

RANGE

Min=0, Max= SDVO dependent

Connector

TBD

armada

CRTC

“CSC_YUV”

ENUM

{ “Auto” , “CCIR601”, “CCIR709” }

CRTC

TBD

“CSC_RGB”

ENUM

{ “Auto”, “Computer system”, “Studio” }

CRTC

TBD

Overlay

“colorkey”

RANGE

Min=0, Max=0xffffff

Plane

TBD

“colorkey_min”

RANGE

Min=0, Max=0xffffff

Plane

TBD

“colorkey_max”

RANGE

Min=0, Max=0xffffff

Plane

TBD

“colorkey_val”

RANGE

Min=0, Max=0xffffff

Plane

TBD

“colorkey_alpha”

RANGE

Min=0, Max=0xffffff

Plane

TBD

“colorkey_mode”

ENUM

{ “disabled”, “Y component”, “U component” , “V component”, “RGB”, “R component”, “G component”, “B component” }

Plane

TBD

“brightness”

RANGE

Min=0, Max=256 + 255

Plane

TBD

“contrast”

RANGE

Min=0, Max=0x7fff

Plane

TBD

“saturation”

RANGE

Min=0, Max=0x7fff

Plane

TBD

exynos

CRTC

“mode”

ENUM

{ “normal”, “blank” }

CRTC

TBD

i2c/ch7006_drv

Generic

“scale”

RANGE

Min=0, Max=2

Connector

TBD

TV

“mode”

ENUM

{ “PAL”, “PAL-M”,”PAL-N”}, ”PAL-Nc” , “PAL-60”, “NTSC-M”, “NTSC-J” }

Connector

TBD

nouveau

NV10 Overlay

“colorkey”

RANGE

Min=0, Max=0x01ffffff

Plane

TBD

“contrast”

RANGE

Min=0, Max=8192-1

Plane

TBD

“brightness”

RANGE

Min=0, Max=1024

Plane

TBD

“hue”

RANGE

Min=0, Max=359

Plane

TBD

“saturation”

RANGE

Min=0, Max=8192-1

Plane

TBD

“iturbt_709”

RANGE

Min=0, Max=1

Plane

TBD

Nv04 Overlay

“colorkey”

RANGE

Min=0, Max=0x01ffffff

Plane

TBD

“brightness”

RANGE

Min=0, Max=1024

Plane

TBD

Display

“dithering mode”

ENUM

{ “auto”, “off”, “on” }

Connector

TBD

“dithering depth”

ENUM

{ “auto”, “off”, “on”, “static 2x2”, “dynamic 2x2”, “temporal” }

Connector

TBD

“underscan”

ENUM

{ “auto”, “6 bpc”, “8 bpc” }

Connector

TBD

“underscan hborder”

RANGE

Min=0, Max=128

Connector

TBD

“underscan vborder”

RANGE

Min=0, Max=128

Connector

TBD

“vibrant hue”

RANGE

Min=0, Max=180

Connector

TBD

“color vibrance”

RANGE

Min=0, Max=200

Connector

TBD

omap

Generic

“zorder”

RANGE

Min=0, Max=3

CRTC, Plane

TBD

qxl

Generic

“hotplug_mode_update”

RANGE

Min=0, Max=1

Connector

TBD

radeon

DVI-I

“coherent”

RANGE

Min=0, Max=1

Connector

TBD

DAC enable load detect

“load detection”

RANGE

Min=0, Max=1

Connector

TBD

TV Standard

“tv standard”

ENUM

{ “ntsc”, “pal”, “pal-m”, “pal-60”, “ntsc-j” , “scart-pal”, “pal-cn”, “secam” }

Connector

TBD

legacy TMDS PLL detect

“tmds_pll”

ENUM

{ “driver”, “bios” }

TBD

Underscan

“underscan”

ENUM

{ “off”, “on”, “auto” }

Connector

TBD

“underscan hborder”

RANGE

Min=0, Max=128

Connector

TBD

“underscan vborder”

RANGE

Min=0, Max=128

Connector

TBD

Audio

“audio”

ENUM

{ “off”, “on”, “auto” }

Connector

TBD

FMT Dithering

“dither”

ENUM

{ “off”, “on” }

Connector

TBD

“colorkey”

RANGE

Min=0, Max=0x01ffffff

Plane

TBD

Vertical Blanking

Error

kernel-doc missing

Vertical Blanking and Interrupt Handling Functions Reference

Error

kernel-doc missing

Error

kernel-doc missing

Vertical Blank Work

Error

kernel-doc missing

Vertical Blank Work Functions Reference

Error

kernel-doc missing

Error

kernel-doc missing