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

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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().

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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

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Atomic Mode Setting Function Reference

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Atomic Mode Setting IOCTL and UAPI Functions

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CRTC Abstraction

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CRTC Functions Reference

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Frame Buffer Abstraction

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Frame Buffer Functions Reference

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DRM Format Handling

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Format Functions Reference

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Dumb Buffer Objects

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Plane Abstraction

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Plane Functions Reference

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Display Modes Function Reference

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Connector Abstraction

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Connector Functions Reference

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Writeback Connectors

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Encoder Abstraction

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Encoder Functions Reference

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KMS Locking

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KMS Properties

Property Types and Blob Property Support

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Standard Connector Properties

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HDMI Specific Connector Properties

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Standard CRTC Properties

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Plane Composition Properties

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FB_DAMAGE_CLIPS

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Color Management Properties

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Tile Group Property

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Explicit Fencing Properties

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Variable Refresh Properties

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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

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Vertical Blanking and Interrupt Handling Functions Reference

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Vertical Blank Work

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Vertical Blank Work Functions Reference

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