Ubuntu Manpage: drm-kms - Kernel Mode-Setting

Provided by: libdrm-dev_2.4.113-2~ubuntu0.22.04.1_amd64

NAME

       drm-kms - Kernel Mode-Setting

SYNOPSIS

       #include <xf86drm.h>

       #include <xf86drmMode.h>

DESCRIPTION

Each DRM device provides access to manage which monitors and displays are currently used
and what frames to be displayed. This task is called Kernel Mode-Setting (KMS).
Historically, this was done in user-space and called User-space Mode-Setting (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 drmModeSettingSupported(3) to check whether your driver supports this. To
understand how KMS works, we need to introduce 5 objects: CRTCs, Planes, Encoders,
Connectors and Framebuffers.

CRTCs A CRTC short for CRT Controller 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
drmModeCrtcSet(3) to drive the display devices.

Planes A plane 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).

Encoders
An encoder 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 cloned display configuration across the
connectors attached to each encoder.

Connectors
A connector 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, EDID data, DPMS and connection status, and information
about modes supported on the attached displays.

Framebuffers
Framebuffers 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 drm-memory(7) API.

Mode-Setting
Before mode-setting can be performed, an application needs to call drmSetMaster(3) to
become DRM-Master. It then has exclusive access to the KMS API. A call to
drmModeGetResources(3) returns a list of CRTCs, Connectors, Encoders and Planes.

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 Examples section below for more information.

All valid modes for a connector can be retrieved with a call to drmModeGetConnector(3) 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.

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-dependent and described in
drm-memory(7). 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 drmModeAddFB(3) and drmModeAddFB2(3).

As a last step, you want to program your CRTC to drive your selected connector. You can
do this with a call to drmModeSetCrtc(3).

Page-Flipping
A call to drmModeSetCrtc(3) 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 drmModeSetCrtc(3) on the next buffer to flip. If you want to synchronize
your flips with vertical-blanks, you can use drmModePageFlip(3) which schedules your
page-flip for the next vblank.

Planes
Planes are controlled independently from CRTCs. That is, a call to drmModeSetCrtc(3) does
not affect planes. Instead, you need to call drmModeSetPlane(3) 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
drmModeGetPlaneResources(3) for more information.

Cursors
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 drmModeSetCursor(3) and move it on the screen with
drmModeMoveCursor(3). 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.

EXAMPLES

       Some examples of how basic mode-setting can be done. See the man-page of each DRM function
       for more information.

   CRTC/Encoder Selection
       If you retrieved all  display  configuration  information  via  drmModeGetResources(3)  as
       drmModeRes  *res,  selected a connector from the list in res->connectors and retrieved the
       connector-information as  drmModeConnector  *conn  via  drmModeGetConnector(3)  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  drmOpen(3))  as  fd,  a  pointer  to  the
       retrieved  resources as res and a pointer to the selected connector as conn. It returns an
       integer smaller than 0 on failure, otherwise, a valid CRTC id is returned.

          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;
          }

REPORTING BUGS

       Bugs in this manual should be reported to https://gitlab.freedesktop.org/mesa/drm/-/issues

NAME

SYNOPSIS

DESCRIPTION

EXAMPLES

REPORTING BUGS

SEE ALSO