1DRM-KMS(7)                 Direct Rendering Manager                 DRM-KMS(7)


6       drm-kms - Kernel Mode-Setting


9       #include <xf86drm.h>
11       #include <xf86drmMode.h>


14       Each DRM device provides access to manage which monitors and displays
15       are currently used and what frames to be displayed. This task is called
16       Kernel Mode-Setting (KMS). Historically, this was done in user-space
17       and called User-space Mode-Setting (UMS). Almost all open-source
18       drivers now provide the KMS kernel API to do this in the kernel,
19       however, many non-open-source binary drivers from different vendors
20       still do not support this. You can use drmModeSettingSupported(3) to
21       check whether your driver supports this. To understand how KMS works,
22       we need to introduce 5 objects: CRTCs, Planes, Encoders, Connectors and
23       Framebuffers.
25       CRTCs
26           A CRTC short for CRT Controller is an abstraction representing a
27           part of the chip that contains a pointer to a scanout buffer.
28           Therefore, the number of CRTCs available determines how many
29           independent scanout buffers can be active at any given time. The
30           CRTC structure contains several fields to support this: a pointer
31           to some video memory (abstracted as a frame-buffer object), a list
32           of driven connectors, a display mode and an (x, y) offset into the
33           video memory to support panning or configurations where one piece
34           of video memory spans multiple CRTCs. A CRTC is the central point
35           where configuration of displays happens. You select which objects
36           to use, which modes and which parameters and then configure each
37           CRTC via drmModeCrtcSet(3) to drive the display devices.
39       Planes
40           A plane respresents an image source that can be blended with or
41           overlayed on top of a CRTC during the scanout process. Planes are
42           associated with a frame-buffer to crop a portion of the image
43           memory (source) and optionally scale it to a destination size. The
44           result is then blended with or overlayed on top of a CRTC. Planes
45           are not provided by all hardware and the number of available planes
46           is limited. If planes are not available or if not enough planes are
47           available, the user should fall back to normal software blending
48           (via GPU or CPU).
50       Encoders
51           An encoder takes pixel data from a CRTC and converts it to a format
52           suitable for any attached connectors. On some devices, it may be
53           possible to have a CRTC send data to more than one encoder. In that
54           case, both encoders would receive data from the same scanout
55           buffer, resulting in a cloned display configuration across the
56           connectors attached to each encoder.
58       Connectors
59           A connector is the final destination of pixel-data on a device, and
60           usually connects directly to an external display device like a
61           monitor or laptop panel. A connector can only be attached to one
62           encoder at a time. The connector is also the structure where
63           information about the attached display is kept, so it contains
64           fields for display data, EDID data, DPMS and connection status, and
65           information about modes supported on the attached displays.
67       Framebuffers
68           Framebuffers are abstract memory objects that provide a source of
69           pixel data to scanout to a CRTC. Applications explicitly request
70           the creation of framebuffers and can control their behavior.
71           Framebuffers rely on the underneath memory manager for low-level
72           memory operations. When creating a framebuffer, applications pass a
73           memory handle through the API which is used as backing storage. The
74           framebuffer itself is only an abstract object with no data. It just
75           refers to memory buffers that must be created with the drm-
76           memory(7) API.
78   Mode-Setting
79       Before mode-setting can be performed, an application needs to call
80       drmSetMaster(3) to become DRM-Master. It then has exclusive access to
81       the KMS API. A call to drmModeGetResources(3) returns a list of CRTCs,
82       Connectors, Encoders and Planes.
84       Normal procedure now includes: First, you select which connectors you
85       want to use. Users are mostly interested in which monitor or
86       display-panel is active so you need to make sure to arrange them in the
87       correct logical order and select the correct ones to use. For each
88       connector, you need to find a CRTC to drive this connector. If you want
89       to clone output to two or more connectors, you may use a single CRTC
90       for all cloned connectors (if the hardware supports this). To find a
91       suitable CRTC, you need to iterate over the list of encoders that are
92       available for each connector. Each encoder contains a list of CRTCs
93       that it can work with and you simply select one of these CRTCs. If you
94       later program the CRTC to control a connector, it automatically selects
95       the best encoder. However, this procedure is needed so your CRTC has at
96       least one working encoder for the selected connector. See the Examples
97       section below for more information.
99       All valid modes for a connector can be retrieved with a call to
100       drmModeGetConnector(3) You need to select the mode you want to use and
101       save it. The first mode in the list is the default mode with the
102       highest resolution possible and often a suitable choice.
104       After you have a working connector+CRTC+mode combination, you need to
105       create a framebuffer that is used for scanout. Memory buffer allocation
106       is driver-depedent and described in drm-memory(7). You need to create a
107       buffer big enough for your selected mode. Now you can create a
108       framebuffer object that uses your memory-buffer as scanout buffer. You
109       can do this with drmModeAddFB(3) and drmModeAddFB2(3).
111       As a last step, you want to program your CRTC to drive your selected
112       connector. You can do this with a call to drmModeSetCrtc(3).
114   Page-Flipping
115       A call to drmModeSetCrtc(3) is executed immediately and forces the CRTC
116       to use the new scanout buffer. If you want smooth-transitions without
117       tearing, you probably use double-buffering. You need to create one
118       framebuffer object for each buffer you use. You can then call
119       drmModeSetCrtc(3) on the next buffer to flip. If you want to
120       synchronize your flips with vertical-blanks, you can use
121       drmModePageFlip(3) which schedules your page-flip for the next vblank.
123   Planes
124       Planes are controlled independently from CRTCs. That is, a call to
125       drmModeSetCrtc(3) does not affect planes. Instead, you need to call
126       drmModeSetPlane(3) to configure a plane. This requires the plane ID, a
127       CRTC, a framebuffer and offsets into the plane-framebuffer and the
128       CRTC-framebuffer. The CRTC then blends the content from the plane over
129       the CRTC framebuffer buffer during scanout. As this does not involve
130       any software-blending, it is way faster than traditional blending.
131       However, plane resources are limited. See drmModeGetPlaneResources(3)
132       for more information.
134   Cursors
135       Similar to planes, many hardware also supports cursors. A cursor is a
136       very small buffer with an image that is blended over the CRTC
137       framebuffer. You can set a different cursor for each CRTC with
138       drmModeSetCursor(3) and move it on the screen with
139       drmModeMoveCursor(3). This allows to move the cursor on the screen
140       without rerendering. If no hardware cursors are supported, you need to
141       rerender for each frame the cursor is moved.


144       Some examples of how basic mode-setting can be done. See the man-page
145       of each DRM function for more information.
147   CRTC/Encoder Selection
148       If you retrieved all display configuration information via
149       drmModeGetResources(3) as drmModeRes *res, selected a connector from
150       the list in res->connectors and retrieved the connector-information as
151       drmModeConnector *conn via drmModeGetConnector(3) then this example
152       shows, how you can find a suitable CRTC id to drive this connector.
153       This function takes a file-descriptor to the DRM device (see
154       drmOpen(3)) as fd, a pointer to the retrieved resources as res and a
155       pointer to the selected connector as conn. It returns an integer
156       smaller than 0 on failure, otherwise, a valid CRTC id is returned.
158           static int modeset_find_crtc(int fd, drmModeRes *res, drmModeConnector *conn)
159           {
160                drmModeEncoder *enc;
161                unsigned int i, j;
163                /* iterate all encoders of this connector */
164                for (i = 0; i < conn->count_encoders; ++i) {
165                     enc = drmModeGetEncoder(fd, conn->encoders[i]);
166                     if (!enc) {
167                          /* cannot retrieve encoder, ignoring... */
168                          continue;
169                     }
171                     /* iterate all global CRTCs */
172                     for (j = 0; j < res->count_crtcs; ++j) {
173                          /* check whether this CRTC works with the encoder */
174                          if (!(enc->possible_crtcs & (1 << j)))
175                               continue;
178                          /* Here you need to check that no other connector
179                           * currently uses the CRTC with id "crtc". If you intend
180                           * to drive one connector only, then you can skip this
181                           * step. Otherwise, simply scan your list of configured
182                           * connectors and CRTCs whether this CRTC is already
183                           * used. If it is, then simply continue the search here. */
184                          if (res->crtcs[j] "is unused") {
185                               drmModeFreeEncoder(enc);
186                               return res->crtcs[j];
187                          }
188                     }
190                     drmModeFreeEncoder(enc);
191                }
193                /* cannot find a suitable CRTC */
194                return -ENOENT;
195           }


198       Bugs in this manual should be reported to
199       https://bugs.freedesktop.org/enter_bug.cgi?product=DRI&component=libdrm
200       under the "DRI" product, component "libdrm"


203       drm(7), drm-memory(7), drmModeGetResources(3), drmModeGetConnector(3),
204       drmModeGetEncoder(3), drmModeGetCrtc(3), drmModeSetCrtc(3),
205       drmModeGetFB(3), drmModeAddFB(3), drmModeAddFB2(3), drmModeRmFB(3),
206       drmModePageFlip(3), drmModeGetPlaneResources(3), drmModeGetPlane(3),
207       drmModeSetPlane(3), drmModeSetCursor(3), drmModeMoveCursor(3),
208       drmSetMaster(3), drmAvailable(3), drmCheckModesettingSupported(3),
209       drmOpen(3)
213libdrm                          September 2012                      DRM-KMS(7)