DRM-KMS(7) Direct Rendering Manager DRM-KMS(7)NAMEdrm-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 explicitely 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-depedent 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 http://bugs.freedesktop.org
under the "Mesa" product, with "Other" or "libdrm" as the component.
SEE ALSOdrm(7), drm-memory(7), drmModeGetResources(3), drmModeGetConnector(3),
drmModeGetEncoder(3), drmModeGetCrtc(3), drmModeSetCrtc(3),
drmModeGetFB(3), drmModeAddFB(3), drmModeAddFB2(3), drmModeRmFB(3),
drmModePageFlip(3), drmModeGetPlaneResources(3), drmModeGetPlane(3),
drmModeSetPlane(3), drmModeSetCursor(3), drmModeMoveCursor(3),
drmSetMaster(3), drmAvailable(3), drmCheckModesettingSupported(3),
drmOpen(3)libdrm September 2012 DRM-KMS(7)
