summaryrefslogtreecommitdiffstats
path: root/Documentation/gpu/drm-kms.rst
blob: 5dee6b8a4c12511f16bb9e540ec7e1eb854e88cb (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
=========================
Kernel Mode Setting (KMS)
=========================

Drivers must initialize the mode setting core by calling
:c:func:`drm_mode_config_init()` on the DRM device. The function
initializes the :c:type:`struct drm_device <drm_device>`
mode_config field and never fails. Once done, mode configuration must
be setup by initializing the following fields.

-  int min_width, min_height; int max_width, max_height;
   Minimum and maximum width and height of the frame buffers in pixel
   units.

-  struct drm_mode_config_funcs \*funcs;
   Mode setting functions.

Overview
========

.. kernel-render:: DOT
   :alt: KMS Display Pipeline
   :caption: KMS Display Pipeline Overview

   digraph "KMS" {
      node [shape=box]

      subgraph cluster_static {
          style=dashed
          label="Static Objects"

          node [bgcolor=grey style=filled]
          "drm_plane A" -> "drm_crtc"
          "drm_plane B" -> "drm_crtc"
          "drm_crtc" -> "drm_encoder A"
          "drm_crtc" -> "drm_encoder B"
      }

      subgraph cluster_user_created {
          style=dashed
          label="Userspace-Created"

          node [shape=oval]
          "drm_framebuffer 1" -> "drm_plane A"
          "drm_framebuffer 2" -> "drm_plane B"
      }

      subgraph cluster_connector {
          style=dashed
          label="Hotpluggable"

          "drm_encoder A" -> "drm_connector A"
          "drm_encoder B" -> "drm_connector B"
      }
   }

The basic object structure KMS presents to userspace is fairly simple.
Framebuffers (represented by :c:type:`struct drm_framebuffer <drm_framebuffer>`,
see `Frame Buffer Abstraction`_) feed into planes. Planes are represented by
:c:type:`struct drm_plane <drm_plane>`, see `Plane Abstraction`_ for more
details. One or more (or even no) planes feed their pixel data into a CRTC
(represented by :c:type:`struct drm_crtc <drm_crtc>`, see `CRTC Abstraction`_)
for blending. The precise blending step is explained in more detail in `Plane
Composition Properties`_ and related chapters.

For the output routing the first step is encoders (represented by
:c:type:`struct drm_encoder <drm_encoder>`, see `Encoder Abstraction`_). Those
are really just internal artifacts of the helper libraries used to implement KMS
drivers. Besides that they make it unecessarily more complicated for userspace
to figure out which connections between a CRTC and a connector are possible, and
what kind of cloning is supported, they serve no purpose in the userspace API.
Unfortunately encoders have been exposed to userspace, hence can't remove them
at this point.  Futhermore the exposed restrictions are often wrongly set by
drivers, and in many cases not powerful enough to express the real restrictions.
A CRTC can be connected to multiple encoders, and for an active CRTC there must
be at least one encoder.

The final, and real, endpoint in the display chain is the connector (represented
by :c:type:`struct drm_connector <drm_connector>`, see `Connector
Abstraction`_). Connectors can have different possible encoders, but the kernel
driver selects which encoder to use for each connector. The use case is DVI,
which could switch between an analog and a digital encoder. Encoders can also
drive multiple different connectors. There is exactly one active connector for
every active encoder.

Internally the output pipeline is a bit more complex and matches today's
hardware more closely:

.. kernel-render:: DOT
   :alt: KMS Output Pipeline
   :caption: KMS Output Pipeline

   digraph "Output Pipeline" {
      node [shape=box]

      subgraph {
          "drm_crtc" [bgcolor=grey style=filled]
      }

      subgraph cluster_internal {
          style=dashed
          label="Internal Pipeline"
          {
              node [bgcolor=grey style=filled]
              "drm_encoder A";
              "drm_encoder B";
              "drm_encoder C";
          }

          {
              node [bgcolor=grey style=filled]
              "drm_encoder B" -> "drm_bridge B"
              "drm_encoder C" -> "drm_bridge C1"
              "drm_bridge C1" -> "drm_bridge C2";
          }
      }

      "drm_crtc" -> "drm_encoder A"
      "drm_crtc" -> "drm_encoder B"
      "drm_crtc" -> "drm_encoder C"


      subgraph cluster_output {
          style=dashed
          label="Outputs"

          "drm_encoder A" -> "drm_connector A";
          "drm_bridge B" -> "drm_connector B";
          "drm_bridge C2" -> "drm_connector C";

          "drm_panel"
      }
   }

Internally two additional helper objects come into play. First, to be able to
share code for encoders (sometimes on the same SoC, sometimes off-chip) one or
more :ref:`drm_bridges` (represented by :c:type:`struct drm_bridge
<drm_bridge>`) can be linked to an encoder. This link is static and cannot be
changed, which means the cross-bar (if there is any) needs to be mapped between
the CRTC and any encoders. Often for drivers with bridges there's no code left
at the encoder level. Atomic drivers can leave out all the encoder callbacks to
essentially only leave a dummy routing object behind, which is needed for
backwards compatibility since encoders are exposed to userspace.

The second object is for panels, represented by :c:type:`struct drm_panel
<drm_panel>`, see :ref:`drm_panel_helper`. Panels do not have a fixed binding
point, but are generally linked to the driver private structure that embeds
:c:type:`struct drm_connector <drm_connector>`.

Note that currently the bridge chaining and interactions with connectors and
panels are still in-flux and not really fully sorted out yet.

KMS Core Structures and Functions
=================================

.. kernel-doc:: include/drm/drm_mode_config.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_mode_config.c
   :export:

Modeset Base Object Abstraction
===============================

.. kernel-render:: DOT
   :alt: Mode Objects and Properties
   :caption: Mode Objects and Properties

   digraph {
      node [shape=box]

      "drm_property A" -> "drm_mode_object A"
      "drm_property A" -> "drm_mode_object B"
      "drm_property B" -> "drm_mode_object A"
   }

The base structure for all KMS objects is :c:type:`struct drm_mode_object
<drm_mode_object>`. One of the base services it provides is tracking properties,
which are especially important for the atomic IOCTL (see `Atomic Mode
Setting`_). The somewhat surprising part here is that properties are not
directly instantiated on each object, but free-standing mode objects themselves,
represented by :c:type:`struct drm_property <drm_property>`, which only specify
the type and value range of a property. Any given property can be attached
multiple times to different objects using :c:func:`drm_object_attach_property()
<drm_object_attach_property>`.

.. kernel-doc:: include/drm/drm_mode_object.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_mode_object.c
   :export:

Atomic Mode Setting
===================


.. kernel-render:: DOT
   :alt: Mode Objects and Properties
   :caption: Mode Objects and Properties

   digraph {
      node [shape=box]

      subgraph cluster_state {
          style=dashed
          label="Free-standing state"

          "drm_atomic_state" -> "duplicated drm_plane_state A"
          "drm_atomic_state" -> "duplicated drm_plane_state B"
          "drm_atomic_state" -> "duplicated drm_crtc_state"
          "drm_atomic_state" -> "duplicated drm_connector_state"
          "drm_atomic_state" -> "duplicated driver private state"
      }

      subgraph cluster_current {
          style=dashed
          label="Current state"

          "drm_device" -> "drm_plane A"
          "drm_device" -> "drm_plane B"
          "drm_device" -> "drm_crtc"
          "drm_device" -> "drm_connector"
          "drm_device" -> "driver private object"

          "drm_plane A" -> "drm_plane_state A"
          "drm_plane B" -> "drm_plane_state B"
          "drm_crtc" -> "drm_crtc_state"
          "drm_connector" -> "drm_connector_state"
          "driver private object" -> "driver private state"
      }

      "drm_atomic_state" -> "drm_device" [label="atomic_commit"]
      "duplicated drm_plane_state A" -> "drm_device"[style=invis]
   }

Atomic provides transactional modeset (including planes) updates, but a
bit differently from the usual transactional approach of try-commit and
rollback:

- Firstly, no hardware changes are allowed when the commit would fail. This
  allows us to implement the DRM_MODE_ATOMIC_TEST_ONLY mode, which allows
  userspace to explore whether certain configurations would work or not.

- This would still allow setting and rollback of just the software state,
  simplifying conversion of existing drivers. But auditing drivers for
  correctness of the atomic_check code becomes really hard with that: Rolling
  back changes in data structures all over the place is hard to get right.

- Lastly, for backwards compatibility and to support all use-cases, atomic
  updates need to be incremental and be able to execute in parallel. Hardware
  doesn't always allow it, but where possible plane updates on different CRTCs
  should not interfere, and not get stalled due to output routing changing on
  different CRTCs.

Taken all together there's two consequences for the atomic design:

- The overall state is split up into per-object state structures:
  :c:type:`struct drm_plane_state <drm_plane_state>` for planes, :c:type:`struct
  drm_crtc_state <drm_crtc_state>` for CRTCs and :c:type:`struct
  drm_connector_state <drm_connector_state>` for connectors. These are the only
  objects with userspace-visible and settable state. For internal state drivers
  can subclass these structures through embeddeding, or add entirely new state
  structures for their globally shared hardware functions.

- An atomic update is assembled and validated as an entirely free-standing pile
  of structures within the :c:type:`drm_atomic_state <drm_atomic_state>`
  container. Driver private state structures are also tracked in the same
  structure; see the next chapter.  Only when a state is committed is it applied
  to the driver and modeset objects. This way rolling back an update boils down
  to releasing memory and unreferencing objects like framebuffers.

Read on in this chapter, and also in :ref:`drm_atomic_helper` for more detailed
coverage of specific topics.

Handling Driver Private State
-----------------------------

.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
   :doc: handling driver private state

Atomic Mode Setting Function Reference
--------------------------------------

.. kernel-doc:: include/drm/drm_atomic.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
   :export:

.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
   :internal:

CRTC Abstraction
================

.. kernel-doc:: drivers/gpu/drm/drm_crtc.c
   :doc: overview

CRTC Functions Reference
--------------------------------

.. kernel-doc:: include/drm/drm_crtc.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_crtc.c
   :export:

Frame Buffer Abstraction
========================

.. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c
   :doc: overview

Frame Buffer Functions Reference
--------------------------------

.. kernel-doc:: include/drm/drm_framebuffer.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_framebuffer.c
   :export:

DRM Format Handling
===================

.. kernel-doc:: include/drm/drm_fourcc.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_fourcc.c
   :export:

Dumb Buffer Objects
===================

.. kernel-doc:: drivers/gpu/drm/drm_dumb_buffers.c
   :doc: overview

Plane Abstraction
=================

.. kernel-doc:: drivers/gpu/drm/drm_plane.c
   :doc: overview

Plane Functions Reference
-------------------------

.. kernel-doc:: include/drm/drm_plane.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_plane.c
   :export:

Display Modes Function Reference
================================

.. kernel-doc:: include/drm/drm_modes.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_modes.c
   :export:

Connector Abstraction
=====================

.. kernel-doc:: drivers/gpu/drm/drm_connector.c
   :doc: overview

Connector Functions Reference
-----------------------------

.. kernel-doc:: include/drm/drm_connector.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_connector.c
   :export:

Writeback Connectors
--------------------

.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
  :doc: overview

.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
  :export:

Encoder Abstraction
===================

.. kernel-doc:: drivers/gpu/drm/drm_encoder.c
   :doc: overview

Encoder Functions Reference
---------------------------

.. kernel-doc:: include/drm/drm_encoder.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_encoder.c
   :export:

KMS Initialization and Cleanup
==============================

A KMS device is abstracted and exposed as a set of planes, CRTCs,
encoders and connectors. KMS drivers must thus create and initialize all
those objects at load time after initializing mode setting.

CRTCs (:c:type:`struct drm_crtc <drm_crtc>`)
--------------------------------------------

A CRTC 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
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.

CRTC Initialization
~~~~~~~~~~~~~~~~~~~

A KMS device must create and register at least one struct
:c:type:`struct drm_crtc <drm_crtc>` instance. The instance is
allocated and zeroed by the driver, possibly as part of a larger
structure, and registered with a call to :c:func:`drm_crtc_init()`
with a pointer to CRTC functions.


Cleanup
-------

The DRM core manages its objects' lifetime. When an object is not needed
anymore the core calls its destroy function, which must clean up and
free every resource allocated for the object. Every
:c:func:`drm_\*_init()` call must be matched with a corresponding
:c:func:`drm_\*_cleanup()` call to cleanup CRTCs
(:c:func:`drm_crtc_cleanup()`), planes
(:c:func:`drm_plane_cleanup()`), encoders
(:c:func:`drm_encoder_cleanup()`) and connectors
(:c:func:`drm_connector_cleanup()`). Furthermore, connectors that
have been added to sysfs must be removed by a call to
:c:func:`drm_connector_unregister()` before calling
:c:func:`drm_connector_cleanup()`.

Connectors state change detection must be cleanup up with a call to
:c:func:`drm_kms_helper_poll_fini()`.

Output discovery and initialization example
-------------------------------------------

.. code-block:: c

    void intel_crt_init(struct drm_device *dev)
    {
        struct drm_connector *connector;
        struct intel_output *intel_output;

        intel_output = kzalloc(sizeof(struct intel_output), GFP_KERNEL);
        if (!intel_output)
            return;

        connector = &intel_output->base;
        drm_connector_init(dev, &intel_output->base,
                   &intel_crt_connector_funcs, DRM_MODE_CONNECTOR_VGA);

        drm_encoder_init(dev, &intel_output->enc, &intel_crt_enc_funcs,
                 DRM_MODE_ENCODER_DAC);

        drm_connector_attach_encoder(&intel_output->base,
                          &intel_output->enc);

        /* Set up the DDC bus. */
        intel_output->ddc_bus = intel_i2c_create(dev, GPIOA, "CRTDDC_A");
        if (!intel_output->ddc_bus) {
            dev_printk(KERN_ERR, &dev->pdev->dev, "DDC bus registration "
                   "failed.\n");
            return;
        }

        intel_output->type = INTEL_OUTPUT_ANALOG;
        connector->interlace_allowed = 0;
        connector->doublescan_allowed = 0;

        drm_encoder_helper_add(&intel_output->enc, &intel_crt_helper_funcs);
        drm_connector_helper_add(connector, &intel_crt_connector_helper_funcs);

        drm_connector_register(connector);
    }

In the example above (taken from the i915 driver), a CRTC, connector and
encoder combination is created. A device-specific i2c bus is also
created for fetching EDID data and performing monitor detection. Once
the process is complete, the new connector is registered with sysfs to
make its properties available to applications.

KMS Locking
===========

.. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c
   :doc: kms locking

.. kernel-doc:: include/drm/drm_modeset_lock.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c
   :export:

KMS Properties
==============

Property Types and Blob Property Support
----------------------------------------

.. kernel-doc:: drivers/gpu/drm/drm_property.c
   :doc: overview

.. kernel-doc:: include/drm/drm_property.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_property.c
   :export:

Standard Connector Properties
-----------------------------

.. kernel-doc:: drivers/gpu/drm/drm_connector.c
   :doc: standard connector properties

HDMI Specific Connector Properties
----------------------------------

.. kernel-doc:: drivers/gpu/drm/drm_connector.c
   :doc: HDMI connector properties

Plane Composition Properties
----------------------------

.. kernel-doc:: drivers/gpu/drm/drm_blend.c
   :doc: overview

.. kernel-doc:: drivers/gpu/drm/drm_blend.c
   :export:

Color Management Properties
---------------------------

.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
   :doc: overview

.. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
   :export:

Tile Group Property
-------------------

.. kernel-doc:: drivers/gpu/drm/drm_connector.c
   :doc: Tile group

Explicit Fencing Properties
---------------------------

.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
   :doc: explicit fencing properties

Existing KMS Properties
-----------------------

The following table gives description of drm properties exposed by various
modules/drivers. Because this table is very unwieldy, do not add any new
properties here. Instead document them in a section above.

.. csv-table::
   :header-rows: 1
   :file: kms-properties.csv

Vertical Blanking
=================

.. kernel-doc:: drivers/gpu/drm/drm_vblank.c
   :doc: vblank handling

Vertical Blanking and Interrupt Handling Functions Reference
------------------------------------------------------------

.. kernel-doc:: include/drm/drm_vblank.h
   :internal:

.. kernel-doc:: drivers/gpu/drm/drm_vblank.c
   :export: