Editing Direct Rendering Manager (section)

==== KMS device model ====
KMS models and manages the output devices as a series of abstract hardware blocks commonly found on the display output pipeline of a [[display controller]]. These blocks are:{{r|drmbook-kms}}

* '''CRTCs''': each CRTC (from [[Cathode-ray tube|CRT]] Controller{{r|X wiki videocards}}{{r|Paalanen 2014}}) represents a scanout engine of the display controller, pointing to a ''scanout buffer'' ([[framebuffer]]).{{r|drmbook-kms}} The purpose of a CRTC is to read the pixel data currently in the scanout buffer and generate from it the [[Video signal|video mode timing signal]] with the help of a [[Phase-locked loop|PLL circuit]].{{r|Deucher 2010}} The number of CRTCs available determines how many independent output devices the hardware can handle at the same time, so in order to use ''[[Multi-monitor|multi-head]]'' configurations at least one CRTC per display device is required.{{r|drmbook-kms}} Two&mdash;or more&mdash;CRTCs can also work in ''clone mode'' if they scan out from the same framebuffer to send the same image to several output devices.{{r|Deucher 2010}}{{r|X wiki videocards}}
* '''Connectors''': a connector represents where the display controller sends the video signal from a scanout operation to be displayed. Usually, the KMS concept of a connector corresponds to a physical connector ([[VGA connector|VGA]], [[Digital Visual Interface|DVI]], [[FPD-Link]], [[HDMI]], [[DisplayPort]], [[S-Video]], ...) in the hardware where an output device ([[Computer monitor|monitor]], [[laptop]] panel, ...) is permanently or can temporarily be attached. Information related to the current physically attached output device&mdash;such as connection status, [[Extended Display Identification Data|EDID]] data, [[VESA Display Power Management Signaling|DPMS]] status or supported video modes&mdash;is also stored within the connector.{{r|drmbook-kms}}
* '''Encoders''': the display controller must encode the video mode timing signal from the CRTC using a format suitable for the intended connector.{{r|drmbook-kms}} An encoder represents the hardware block able to do one of these encodings. Examples of encodings&mdash;for digital outputs&mdash;are [[Transition-minimized differential signaling|TMDS]] and [[Low-voltage differential signaling|LVDS]]; for analog outputs such as [[Video Graphics Array|VGA]] and TV out, specific [[Digital-to-analog converter|DAC]] blocks are generally used. A connector can only receive the signal from one encoder at a time,{{r|drmbook-kms}} and each type of connector only supports some encodings. There also might be additional physical restrictions by which not every CRTC is connected to every available encoder, limiting the possible combinations of CRTC-encoder-connector.
* '''Planes''': a plane is not a hardware block but a memory object containing a buffer from which a scanout engine (a CRTC) is fed. The plane that holds the [[framebuffer]] is called the ''primary plane'', and each CRTC must have one associated,{{r|drmbook-kms}} since it is the source for the CRTC to determine the video mode&mdash;display resolution (width and height), pixel size, pixel format, refresh rate, etc. A CRTC might have also ''cursor planes'' associated to it if the display controller supports hardware cursor overlays, or ''secondary planes'' if it's able to scan out from additional [[hardware overlay]]s and compose or blend "on the fly" the final image sent to the output device.{{r|Paalanen 2014}}