1 =========================
2 Kernel Mode Setting (KMS)
3 =========================
5 Drivers must initialize the mode setting core by calling
6 :c:func:`drm_mode_config_init()` on the DRM device. The function
7 initializes the :c:type:`struct drm_device <drm_device>`
8 mode_config field and never fails. Once done, mode configuration must
9 be setup by initializing the following fields.
11 - int min_width, min_height; int max_width, max_height;
12 Minimum and maximum width and height of the frame buffers in pixel
15 - struct drm_mode_config_funcs \*funcs;
16 Mode setting functions.
21 .. kernel-doc:: include/drm/drm_crtc.h
27 .. kernel-doc:: drivers/gpu/drm/drm_crtc.c
30 Atomic Mode Setting Function Reference
31 ======================================
33 .. kernel-doc:: drivers/gpu/drm/drm_atomic.c
36 .. kernel-doc:: include/drm/drm_atomic.h
39 Frame Buffer Abstraction
40 ========================
42 .. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c
45 Frame Buffer Functions Reference
46 --------------------------------
48 .. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c
51 .. kernel-doc:: include/drm/drm_framebuffer.h
57 .. kernel-doc:: drivers/gpu/drm/drm_fourcc.c
63 The KMS API doesn't standardize backing storage object creation and
64 leaves it to driver-specific ioctls. Furthermore actually creating a
65 buffer object even for GEM-based drivers is done through a
66 driver-specific ioctl - GEM only has a common userspace interface for
67 sharing and destroying objects. While not an issue for full-fledged
68 graphics stacks that include device-specific userspace components (in
69 libdrm for instance), this limit makes DRM-based early boot graphics
70 unnecessarily complex.
72 Dumb objects partly alleviate the problem by providing a standard API to
73 create dumb buffers suitable for scanout, which can then be used to
74 create KMS frame buffers.
76 To support dumb objects drivers must implement the dumb_create,
77 dumb_destroy and dumb_map_offset operations.
79 - int (\*dumb_create)(struct drm_file \*file_priv, struct
80 drm_device \*dev, struct drm_mode_create_dumb \*args);
81 The dumb_create operation creates a driver object (GEM or TTM
82 handle) suitable for scanout based on the width, height and depth
83 from the struct :c:type:`struct drm_mode_create_dumb
84 <drm_mode_create_dumb>` argument. It fills the argument's
85 handle, pitch and size fields with a handle for the newly created
86 object and its line pitch and size in bytes.
88 - int (\*dumb_destroy)(struct drm_file \*file_priv, struct
89 drm_device \*dev, uint32_t handle);
90 The dumb_destroy operation destroys a dumb object created by
93 - int (\*dumb_map_offset)(struct drm_file \*file_priv, struct
94 drm_device \*dev, uint32_t handle, uint64_t \*offset);
95 The dumb_map_offset operation associates an mmap fake offset with
96 the object given by the handle and returns it. Drivers must use the
97 :c:func:`drm_gem_create_mmap_offset()` function to associate
98 the fake offset as described in ?.
100 Note that dumb objects may not be used for gpu acceleration, as has been
101 attempted on some ARM embedded platforms. Such drivers really must have
102 a hardware-specific ioctl to allocate suitable buffer objects.
104 Display Modes Function Reference
105 ================================
107 .. kernel-doc:: include/drm/drm_modes.h
110 .. kernel-doc:: drivers/gpu/drm/drm_modes.c
113 Connector Abstraction
114 =====================
116 .. kernel-doc:: drivers/gpu/drm/drm_connector.c
119 Connector Functions Reference
120 -----------------------------
122 .. kernel-doc:: include/drm/drm_connector.h
125 .. kernel-doc:: drivers/gpu/drm/drm_connector.c
131 .. kernel-doc:: include/drm/drm_encoder.h
134 .. kernel-doc:: drivers/gpu/drm/drm_encoder.c
137 KMS Initialization and Cleanup
138 ==============================
140 A KMS device is abstracted and exposed as a set of planes, CRTCs,
141 encoders and connectors. KMS drivers must thus create and initialize all
142 those objects at load time after initializing mode setting.
144 CRTCs (:c:type:`struct drm_crtc <drm_crtc>`)
145 --------------------------------------------
147 A CRTC is an abstraction representing a part of the chip that contains a
148 pointer to a scanout buffer. Therefore, the number of CRTCs available
149 determines how many independent scanout buffers can be active at any
150 given time. The CRTC structure contains several fields to support this:
151 a pointer to some video memory (abstracted as a frame buffer object), a
152 display mode, and an (x, y) offset into the video memory to support
153 panning or configurations where one piece of video memory spans multiple
159 A KMS device must create and register at least one struct
160 :c:type:`struct drm_crtc <drm_crtc>` instance. The instance is
161 allocated and zeroed by the driver, possibly as part of a larger
162 structure, and registered with a call to :c:func:`drm_crtc_init()`
163 with a pointer to CRTC functions.
165 Planes (:c:type:`struct drm_plane <drm_plane>`)
166 -----------------------------------------------
168 A plane represents an image source that can be blended with or overlayed
169 on top of a CRTC during the scanout process. Planes are associated with
170 a frame buffer to crop a portion of the image memory (source) and
171 optionally scale it to a destination size. The result is then blended
172 with or overlayed on top of a CRTC.
174 The DRM core recognizes three types of planes:
176 - DRM_PLANE_TYPE_PRIMARY represents a "main" plane for a CRTC.
177 Primary planes are the planes operated upon by CRTC modesetting and
178 flipping operations described in the page_flip hook in
179 :c:type:`struct drm_crtc_funcs <drm_crtc_funcs>`.
180 - DRM_PLANE_TYPE_CURSOR represents a "cursor" plane for a CRTC.
181 Cursor planes are the planes operated upon by the
182 DRM_IOCTL_MODE_CURSOR and DRM_IOCTL_MODE_CURSOR2 ioctls.
183 - DRM_PLANE_TYPE_OVERLAY represents all non-primary, non-cursor
184 planes. Some drivers refer to these types of planes as "sprites"
187 For compatibility with legacy userspace, only overlay planes are made
188 available to userspace by default. Userspace clients may set the
189 DRM_CLIENT_CAP_UNIVERSAL_PLANES client capability bit to indicate
190 that they wish to receive a universal plane list containing all plane
196 To create a plane, a KMS drivers allocates and zeroes an instances of
197 :c:type:`struct drm_plane <drm_plane>` (possibly as part of a
198 larger structure) and registers it with a call to
199 :c:func:`drm_universal_plane_init()`. The function takes a
200 bitmask of the CRTCs that can be associated with the plane, a pointer to
201 the plane functions, a list of format supported formats, and the type of
202 plane (primary, cursor, or overlay) being initialized.
204 Cursor and overlay planes are optional. All drivers should provide one
205 primary plane per CRTC (although this requirement may change in the
206 future); drivers that do not wish to provide special handling for
207 primary planes may make use of the helper functions described in ? to
208 create and register a primary plane with standard capabilities.
210 Encoders (:c:type:`struct drm_encoder <drm_encoder>`)
211 -----------------------------------------------------
213 An encoder takes pixel data from a CRTC and converts it to a format
214 suitable for any attached connectors. On some devices, it may be
215 possible to have a CRTC send data to more than one encoder. In that
216 case, both encoders would receive data from the same scanout buffer,
217 resulting in a "cloned" display configuration across the connectors
218 attached to each encoder.
220 Encoder Initialization
221 ~~~~~~~~~~~~~~~~~~~~~~
223 As for CRTCs, a KMS driver must create, initialize and register at least
224 one :c:type:`struct drm_encoder <drm_encoder>` instance. The
225 instance is allocated and zeroed by the driver, possibly as part of a
228 Drivers must initialize the :c:type:`struct drm_encoder
229 <drm_encoder>` possible_crtcs and possible_clones fields before
230 registering the encoder. Both fields are bitmasks of respectively the
231 CRTCs that the encoder can be connected to, and sibling encoders
232 candidate for cloning.
234 After being initialized, the encoder must be registered with a call to
235 :c:func:`drm_encoder_init()`. The function takes a pointer to the
236 encoder functions and an encoder type. Supported types are
238 - DRM_MODE_ENCODER_DAC for VGA and analog on DVI-I/DVI-A
239 - DRM_MODE_ENCODER_TMDS for DVI, HDMI and (embedded) DisplayPort
240 - DRM_MODE_ENCODER_LVDS for display panels
241 - DRM_MODE_ENCODER_TVDAC for TV output (Composite, S-Video,
243 - DRM_MODE_ENCODER_VIRTUAL for virtual machine displays
245 Encoders must be attached to a CRTC to be used. DRM drivers leave
246 encoders unattached at initialization time. Applications (or the fbdev
247 compatibility layer when implemented) are responsible for attaching the
248 encoders they want to use to a CRTC.
253 The DRM core manages its objects' lifetime. When an object is not needed
254 anymore the core calls its destroy function, which must clean up and
255 free every resource allocated for the object. Every
256 :c:func:`drm_\*_init()` call must be matched with a corresponding
257 :c:func:`drm_\*_cleanup()` call to cleanup CRTCs
258 (:c:func:`drm_crtc_cleanup()`), planes
259 (:c:func:`drm_plane_cleanup()`), encoders
260 (:c:func:`drm_encoder_cleanup()`) and connectors
261 (:c:func:`drm_connector_cleanup()`). Furthermore, connectors that
262 have been added to sysfs must be removed by a call to
263 :c:func:`drm_connector_unregister()` before calling
264 :c:func:`drm_connector_cleanup()`.
266 Connectors state change detection must be cleanup up with a call to
267 :c:func:`drm_kms_helper_poll_fini()`.
269 Output discovery and initialization example
270 -------------------------------------------
274 void intel_crt_init(struct drm_device *dev)
276 struct drm_connector *connector;
277 struct intel_output *intel_output;
279 intel_output = kzalloc(sizeof(struct intel_output), GFP_KERNEL);
283 connector = &intel_output->base;
284 drm_connector_init(dev, &intel_output->base,
285 &intel_crt_connector_funcs, DRM_MODE_CONNECTOR_VGA);
287 drm_encoder_init(dev, &intel_output->enc, &intel_crt_enc_funcs,
288 DRM_MODE_ENCODER_DAC);
290 drm_mode_connector_attach_encoder(&intel_output->base,
293 /* Set up the DDC bus. */
294 intel_output->ddc_bus = intel_i2c_create(dev, GPIOA, "CRTDDC_A");
295 if (!intel_output->ddc_bus) {
296 dev_printk(KERN_ERR, &dev->pdev->dev, "DDC bus registration "
301 intel_output->type = INTEL_OUTPUT_ANALOG;
302 connector->interlace_allowed = 0;
303 connector->doublescan_allowed = 0;
305 drm_encoder_helper_add(&intel_output->enc, &intel_crt_helper_funcs);
306 drm_connector_helper_add(connector, &intel_crt_connector_helper_funcs);
308 drm_connector_register(connector);
311 In the example above (taken from the i915 driver), a CRTC, connector and
312 encoder combination is created. A device-specific i2c bus is also
313 created for fetching EDID data and performing monitor detection. Once
314 the process is complete, the new connector is registered with sysfs to
315 make its properties available to applications.
320 .. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c
323 .. kernel-doc:: include/drm/drm_modeset_lock.h
326 .. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c
332 Drivers may need to expose additional parameters to applications than
333 those described in the previous sections. KMS supports attaching
334 properties to CRTCs, connectors and planes and offers a userspace API to
335 list, get and set the property values.
337 Properties are identified by a name that uniquely defines the property
338 purpose, and store an associated value. For all property types except
339 blob properties the value is a 64-bit unsigned integer.
341 KMS differentiates between properties and property instances. Drivers
342 first create properties and then create and associate individual
343 instances of those properties to objects. A property can be instantiated
344 multiple times and associated with different objects. Values are stored
345 in property instances, and all other property information are stored in
346 the property and shared between all instances of the property.
348 Every property is created with a type that influences how the KMS core
349 handles the property. Supported property types are
352 Range properties report their minimum and maximum admissible values.
353 The KMS core verifies that values set by application fit in that
357 Enumerated properties take a numerical value that ranges from 0 to
358 the number of enumerated values defined by the property minus one,
359 and associate a free-formed string name to each value. Applications
360 can retrieve the list of defined value-name pairs and use the
361 numerical value to get and set property instance values.
363 DRM_MODE_PROP_BITMASK
364 Bitmask properties are enumeration properties that additionally
365 restrict all enumerated values to the 0..63 range. Bitmask property
366 instance values combine one or more of the enumerated bits defined
370 Blob properties store a binary blob without any format restriction.
371 The binary blobs are created as KMS standalone objects, and blob
372 property instance values store the ID of their associated blob
375 Blob properties are only used for the connector EDID property and
376 cannot be created by drivers.
378 To create a property drivers call one of the following functions
379 depending on the property type. All property creation functions take
380 property flags and name, as well as type-specific arguments.
382 - struct drm_property \*drm_property_create_range(struct
383 drm_device \*dev, int flags, const char \*name, uint64_t min,
385 Create a range property with the given minimum and maximum values.
387 - struct drm_property \*drm_property_create_enum(struct drm_device
388 \*dev, int flags, const char \*name, const struct
389 drm_prop_enum_list \*props, int num_values);
390 Create an enumerated property. The ``props`` argument points to an
391 array of ``num_values`` value-name pairs.
393 - struct drm_property \*drm_property_create_bitmask(struct
394 drm_device \*dev, int flags, const char \*name, const struct
395 drm_prop_enum_list \*props, int num_values);
396 Create a bitmask property. The ``props`` argument points to an array
397 of ``num_values`` value-name pairs.
399 Properties can additionally be created as immutable, in which case they
400 will be read-only for applications but can be modified by the driver. To
401 create an immutable property drivers must set the
402 DRM_MODE_PROP_IMMUTABLE flag at property creation time.
404 When no array of value-name pairs is readily available at property
405 creation time for enumerated or range properties, drivers can create the
406 property using the :c:func:`drm_property_create()` function and
407 manually add enumeration value-name pairs by calling the
408 :c:func:`drm_property_add_enum()` function. Care must be taken to
409 properly specify the property type through the ``flags`` argument.
411 After creating properties drivers can attach property instances to CRTC,
412 connector and plane objects by calling the
413 :c:func:`drm_object_attach_property()`. The function takes a
414 pointer to the target object, a pointer to the previously created
415 property and an initial instance value.
417 Blending and Z-Position properties
418 ----------------------------------
420 .. kernel-doc:: drivers/gpu/drm/drm_blend.c
423 Existing KMS Properties
424 -----------------------
426 The following table gives description of drm properties exposed by
427 various modules/drivers.
431 :file: kms-properties.csv
436 Vertical blanking plays a major role in graphics rendering. To achieve
437 tear-free display, users must synchronize page flips and/or rendering to
438 vertical blanking. The DRM API offers ioctls to perform page flips
439 synchronized to vertical blanking and wait for vertical blanking.
441 The DRM core handles most of the vertical blanking management logic,
442 which involves filtering out spurious interrupts, keeping race-free
443 blanking counters, coping with counter wrap-around and resets and
444 keeping use counts. It relies on the driver to generate vertical
445 blanking interrupts and optionally provide a hardware vertical blanking
446 counter. Drivers must implement the following operations.
448 - int (\*enable_vblank) (struct drm_device \*dev, int crtc); void
449 (\*disable_vblank) (struct drm_device \*dev, int crtc);
450 Enable or disable vertical blanking interrupts for the given CRTC.
452 - u32 (\*get_vblank_counter) (struct drm_device \*dev, int crtc);
453 Retrieve the value of the vertical blanking counter for the given
454 CRTC. If the hardware maintains a vertical blanking counter its value
455 should be returned. Otherwise drivers can use the
456 :c:func:`drm_vblank_count()` helper function to handle this
459 Drivers must initialize the vertical blanking handling core with a call
460 to :c:func:`drm_vblank_init()` in their load operation.
462 Vertical blanking interrupts can be enabled by the DRM core or by
463 drivers themselves (for instance to handle page flipping operations).
464 The DRM core maintains a vertical blanking use count to ensure that the
465 interrupts are not disabled while a user still needs them. To increment
466 the use count, drivers call :c:func:`drm_vblank_get()`. Upon
467 return vertical blanking interrupts are guaranteed to be enabled.
469 To decrement the use count drivers call
470 :c:func:`drm_vblank_put()`. Only when the use count drops to zero
471 will the DRM core disable the vertical blanking interrupts after a delay
472 by scheduling a timer. The delay is accessible through the
473 vblankoffdelay module parameter or the ``drm_vblank_offdelay`` global
474 variable and expressed in milliseconds. Its default value is 5000 ms.
475 Zero means never disable, and a negative value means disable
476 immediately. Drivers may override the behaviour by setting the
477 :c:type:`struct drm_device <drm_device>`
478 vblank_disable_immediate flag, which when set causes vblank interrupts
479 to be disabled immediately regardless of the drm_vblank_offdelay
480 value. The flag should only be set if there's a properly working
481 hardware vblank counter present.
483 When a vertical blanking interrupt occurs drivers only need to call the
484 :c:func:`drm_handle_vblank()` function to account for the
487 Resources allocated by :c:func:`drm_vblank_init()` must be freed
488 with a call to :c:func:`drm_vblank_cleanup()` in the driver unload
491 Vertical Blanking and Interrupt Handling Functions Reference
492 ------------------------------------------------------------
494 .. kernel-doc:: drivers/gpu/drm/drm_irq.c
497 .. kernel-doc:: include/drm/drm_irq.h