"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

  Written 2012 by David Herrmann <dh.herrmann@googlemail.com>

  Dedicated to the Public Domain

-->

<refentry id="drm-kms">

  <refentryinfo>

    <title>Direct Rendering Manager</title>

    <productname>libdrm</productname>

    <date>September 2012</date>

    <authorgroup>

      <author>

        <contrib>Developer</contrib>

        <firstname>David</firstname>

        <surname>Herrmann</surname>

        <email>dh.herrmann@googlemail.com</email>

      </author>

    </authorgroup>

  </refentryinfo>

  <refmeta>

    <refentrytitle>drm-kms</refentrytitle>

    <manvolnum>7</manvolnum>

  </refmeta>

  <refnamediv>

    <refname>drm-kms</refname>

    <refpurpose>Kernel Mode-Setting</refpurpose>

  </refnamediv>

  <refsynopsisdiv>

    <funcsynopsis>

      <funcsynopsisinfo>#include <xf86drm.h></funcsynopsisinfo>

      <funcsynopsisinfo>#include <xf86drmMode.h></funcsynopsisinfo>

    </funcsynopsis>

  </refsynopsisdiv>

  <refsect1>

    <title>Description</title>

    <para>Each DRM device provides access to manage which monitors and displays

          are currently used and what frames to be displayed. This task is

          called <emphasis>Kernel Mode-Setting</emphasis> (KMS). Historically,

          this was done in user-space and called 

          <emphasis>User-space Mode-Setting</emphasis> (UMS). Almost all

          open-source drivers now provide the KMS kernel API to do this in the

          kernel, however, many non-open-source binary drivers from different

          vendors still do not support this. You can use

          <citerefentry><refentrytitle>drmModeSettingSupported</refentrytitle><manvolnum>3</manvolnum></citerefentry>

          to check whether your driver supports this. To understand how KMS

          works, we need to introduce 5 objects: <emphasis>CRTCs</emphasis>,

          <emphasis>Planes</emphasis>, <emphasis>Encoders</emphasis>,

          <emphasis>Connectors</emphasis> and

          <emphasis>Framebuffers</emphasis>.

      <variablelist>

        <varlistentry>

          <term>CRTCs</term>

          <listitem>

            <para>A <emphasis>CRTC</emphasis> short for

                  <emphasis>CRT Controller</emphasis> 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 list of driven connectors, 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 CRTCs. A CRTC is the central point where

                  configuration of displays happens. You select which objects to

                  use, which modes and which parameters and then configure each

                  CRTC via

                  <citerefentry><refentrytitle>drmModeCrtcSet</refentrytitle><manvolnum>3</manvolnum></citerefentry>

                  to drive the display devices.</para>

          </listitem>

        </varlistentry>

        <varlistentry>

          <term>Planes</term>

          <listitem>

            <para>A <emphasis>plane</emphasis> respresents an image source that

                  can be blended with or overlayed on top of a CRTC during the

                  scanout process. Planes are associated with a frame-buffer to

                  crop a portion of the image memory (source) and optionally

                  scale it to a destination size. The result is then blended

                  with or overlayed on top of a CRTC. Planes are not provided by

                  all hardware and the number of available planes is limited. If

                  planes are not available or if not enough planes are

                  available, the user should fall back to normal software

                  blending (via GPU or CPU).</para>

          </listitem>

        </varlistentry>

        <varlistentry>

          <term>Encoders</term>

          <listitem>

            <para>An <emphasis>encoder</emphasis> takes pixel data from a CRTC

                  and converts it to a format suitable for any attached

                  connectors. On some devices, it may be possible to have a CRTC

                  send data to more than one encoder. In that case, both

                  encoders would receive data from the same scanout buffer,

                  resulting in a <emphasis>cloned</emphasis> display

                  configuration across the connectors attached to each

                  encoder.</para>

          </listitem>

        </varlistentry>

        <varlistentry>

          <term>Connectors</term>

          <listitem>

            <para>A <emphasis>connector</emphasis> is the final destination of

                  pixel-data on a device, and usually connects directly to an

                  external display device like a monitor or laptop panel. A

                  connector can only be attached to one encoder at a time. The

                  connector is also the structure where information about the

                  attached display is kept, so it contains fields for display

                  data, <emphasis>EDID</emphasis> data,

                  <emphasis>DPMS</emphasis> and

                  <emphasis>connection status</emphasis>, and information about

                  modes supported on the attached displays.</para>

          </listitem>

        </varlistentry>

        <varlistentry>

          <term>Framebuffers</term>

          <listitem>

            <para><emphasis>Framebuffers</emphasis> are abstract memory objects

                  that provide a source of pixel data to scanout to a CRTC.

                  Applications explicitly request the creation of framebuffers

                  and can control their behavior. Framebuffers rely on the

                  underneath memory manager for low-level memory operations.

                  When creating a framebuffer, applications pass a memory handle

                  through the API which is used as backing storage. The

                  framebuffer itself is only an abstract object with no data. It

                  just refers to memory buffers that must be created with the

                  <citerefentry><refentrytitle>drm-memory</refentrytitle><manvolnum>7</manvolnum></citerefentry>

                  API.</para>

          </listitem>

        </varlistentry>

      </variablelist>

    </para>

    <refsect2>

      <title>Mode-Setting</title>

      <para>Before mode-setting can be performed, an application needs to call

            <citerefentry><refentrytitle>drmSetMaster</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            to become <emphasis>DRM-Master</emphasis>. It then has exclusive

            access to the KMS API. A call to

            <citerefentry><refentrytitle>drmModeGetResources</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            returns a list of <emphasis>CRTCs</emphasis>,

            <emphasis>Connectors</emphasis>, <emphasis>Encoders</emphasis> and

            <emphasis>Planes</emphasis>.</para>

      <para>Normal procedure now includes: First, you select which connectors

            you want to use. Users are mostly interested in which monitor or

            display-panel is active so you need to make sure to arrange them in

            the correct logical order and select the correct ones to use. For

            each connector, you need to find a CRTC to drive this connector. If

            you want to clone output to two or more connectors, you may use a

            single CRTC for all cloned connectors (if the hardware supports

            this). To find a suitable CRTC, you need to iterate over the list of

            encoders that are available for each connector. Each encoder

            contains a list of CRTCs that it can work with and you simply select

            one of these CRTCs. If you later program the CRTC to control a

            connector, it automatically selects the best encoder. However, this

            procedure is needed so your CRTC has at least one working encoder

            for the selected connector. See the <emphasis>Examples</emphasis>

            section below for more information.</para>

      <para>All valid modes for a connector can be retrieved with a call to

            <citerefentry><refentrytitle>drmModeGetConnector</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            You need to select the mode you want to use and save it. The first

            mode in the list is the default mode with the highest resolution

            possible and often a suitable choice.</para>

      <para>After you have a working connector+CRTC+mode combination, you need

            to create a framebuffer that is used for scanout. Memory buffer

            allocation is driver-depedent and described in

            <citerefentry><refentrytitle>drm-memory</refentrytitle><manvolnum>7</manvolnum></citerefentry>.

            You need to create a buffer big enough for your selected mode. Now

            you can create a framebuffer object that uses your memory-buffer as

            scanout buffer. You can do this with

            <citerefentry><refentrytitle>drmModeAddFB</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            and

            <citerefentry><refentrytitle>drmModeAddFB2</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

      <para>As a last step, you want to program your CRTC to drive your selected

            connector. You can do this with a call to

            <citerefentry><refentrytitle>drmModeSetCrtc</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

    </refsect2>

    <refsect2>

      <title>Page-Flipping</title>

      <para>A call to

            <citerefentry><refentrytitle>drmModeSetCrtc</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            is executed immediately and forces the CRTC to use the new scanout

            buffer. If you want smooth-transitions without tearing, you probably

            use double-buffering. You need to create one framebuffer object for

            each buffer you use. You can then call

            <citerefentry><refentrytitle>drmModeSetCrtc</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            on the next buffer to flip. If you want to synchronize your flips

            with <emphasis>vertical-blanks</emphasis>, you can use

            <citerefentry><refentrytitle>drmModePageFlip</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            which schedules your page-flip for the next

            <emphasis>vblank</emphasis>.</para>

    </refsect2>

    <refsect2>

      <title>Planes</title>

      <para>Planes are controlled independently from CRTCs. That is, a call to

            <citerefentry><refentrytitle>drmModeSetCrtc</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            does not affect planes. Instead, you need to call

            <citerefentry><refentrytitle>drmModeSetPlane</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            to configure a plane. This requires the plane ID, a CRTC, a

            framebuffer and offsets into the plane-framebuffer and the

            CRTC-framebuffer. The CRTC then blends the content from the plane

            over the CRTC framebuffer buffer during scanout. As this does not

            involve any software-blending, it is way faster than traditional

            blending. However, plane resources are limited. See

            <citerefentry><refentrytitle>drmModeGetPlaneResources</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            for more information.</para>

    </refsect2>

    <refsect2>

      <title>Cursors</title>

      <para>Similar to planes, many hardware also supports cursors. A cursor is

            a very small buffer with an image that is blended over the CRTC

            framebuffer. You can set a different cursor for each CRTC with

            <citerefentry><refentrytitle>drmModeSetCursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            and move it on the screen with

            <citerefentry><refentrytitle>drmModeMoveCursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>.

            This allows to move the cursor on the screen without rerendering. If

            no hardware cursors are supported, you need to rerender for each

            frame the cursor is moved.</para>

    </refsect2>

  </refsect1>

  <refsect1>

    <title>Examples</title>

    <para>Some examples of how basic mode-setting can be done. See the man-page

          of each DRM function for more information.</para>

    <refsect2>

      <title>CRTC/Encoder Selection</title>

      <para>If you retrieved all display configuration information via

            <citerefentry><refentrytitle>drmModeGetResources</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            as <structname>drmModeRes</structname> *<varname>res</varname>,

            selected a connector from the list in

            <varname>res</varname>-><structfield>connectors</structfield>

            and retrieved the connector-information as

            <structname>drmModeConnector</structname> *<varname>conn</varname>

            via

            <citerefentry><refentrytitle>drmModeGetConnector</refentrytitle><manvolnum>3</manvolnum></citerefentry>

            then this example shows, how you can find a suitable CRTC id to

            drive this connector. This function takes a file-descriptor to the

            DRM device (see

            <citerefentry><refentrytitle>drmOpen</refentrytitle><manvolnum>3</manvolnum></citerefentry>)

            as <varname>fd</varname>, a pointer to the retrieved resources as

            <varname>res</varname> and a pointer to the selected connector as

            <varname>conn</varname>. It returns an integer smaller than 0 on

            failure, otherwise, a valid CRTC id is returned.</para>

<programlisting>

static int modeset_find_crtc(int fd, drmModeRes *res, drmModeConnector *conn)

{

	drmModeEncoder *enc;

	unsigned int i, j;

	/* iterate all encoders of this connector */

	for (i = 0; i < conn->count_encoders; ++i) {

		enc = drmModeGetEncoder(fd, conn->encoders[i]);

		if (!enc) {

			/* cannot retrieve encoder, ignoring... */

			continue;

		}

		/* iterate all global CRTCs */

		for (j = 0; j < res->count_crtcs; ++j) {

			/* check whether this CRTC works with the encoder */

			if (!(enc->possible_crtcs & (1 << j)))

				continue;

			/* Here you need to check that no other connector

			 * currently uses the CRTC with id "crtc". If you intend

			 * to drive one connector only, then you can skip this

			 * step. Otherwise, simply scan your list of configured

			 * connectors and CRTCs whether this CRTC is already

			 * used. If it is, then simply continue the search here. */

			if (res->crtcs[j] "is unused") {

				drmModeFreeEncoder(enc);

				return res->crtcs[j];

			}

		}

		drmModeFreeEncoder(enc);

	}

	/* cannot find a suitable CRTC */

	return -ENOENT;

}

</programlisting>

    </refsect2>

  </refsect1>

  <refsect1>

    <title>Reporting Bugs</title>

    <para>Bugs in this manual should be reported to

      https://gitlab.freedesktop.org/mesa/drm/-/issues</para>

  </refsect1>

  <refsect1>

    <title>See Also</title>

    <para>

      <citerefentry><refentrytitle>drm</refentrytitle><manvolnum>7</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drm-memory</refentrytitle><manvolnum>7</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeGetResources</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeGetConnector</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeGetEncoder</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeGetCrtc</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeSetCrtc</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeGetFB</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeAddFB</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeAddFB2</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeRmFB</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModePageFlip</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeGetPlaneResources</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeGetPlane</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeSetPlane</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeSetCursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmModeMoveCursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmSetMaster</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmAvailable</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmCheckModesettingSupported</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>drmOpen</refentrytitle><manvolnum>3</manvolnum></citerefentry>

    </para>

  </refsect1>

</refentry>