1=============
2DRM Internals
3=============
4
5This chapter documents DRM internals relevant to driver authors and
6developers working to add support for the latest features to existing
7drivers.
8
9First, we go over some typical driver initialization requirements, like
10setting up command buffers, creating an initial output configuration,
11and initializing core services. Subsequent sections cover core internals
12in more detail, providing implementation notes and examples.
13
14The DRM layer provides several services to graphics drivers, many of
15them driven by the application interfaces it provides through libdrm,
16the library that wraps most of the DRM ioctls. These include vblank
17event handling, memory management, output management, framebuffer
18management, command submission & fencing, suspend/resume support, and
19DMA services.
20
21Driver Initialization
22=====================
23
24At the core of every DRM driver is a :c:type:`struct drm_driver
25<drm_driver>` structure. Drivers typically statically initialize
26a drm_driver structure, and then pass it to
27drm_dev_alloc() to allocate a device instance. After the
28device instance is fully initialized it can be registered (which makes
29it accessible from userspace) using drm_dev_register().
30
31The :c:type:`struct drm_driver <drm_driver>` structure
32contains static information that describes the driver and features it
33supports, and pointers to methods that the DRM core will call to
34implement the DRM API. We will first go through the :c:type:`struct
35drm_driver <drm_driver>` static information fields, and will
36then describe individual operations in details as they get used in later
37sections.
38
39Driver Information
40------------------
41
42Major, Minor and Patchlevel
43~~~~~~~~~~~~~~~~~~~~~~~~~~~
44
45int major; int minor; int patchlevel;
46The DRM core identifies driver versions by a major, minor and patch
47level triplet. The information is printed to the kernel log at
48initialization time and passed to userspace through the
49DRM_IOCTL_VERSION ioctl.
50
51The major and minor numbers are also used to verify the requested driver
52API version passed to DRM_IOCTL_SET_VERSION. When the driver API
53changes between minor versions, applications can call
54DRM_IOCTL_SET_VERSION to select a specific version of the API. If the
55requested major isn't equal to the driver major, or the requested minor
56is larger than the driver minor, the DRM_IOCTL_SET_VERSION call will
57return an error. Otherwise the driver's set_version() method will be
58called with the requested version.
59
60Name, Description and Date
61~~~~~~~~~~~~~~~~~~~~~~~~~~
62
63char \*name; char \*desc; char \*date;
64The driver name is printed to the kernel log at initialization time,
65used for IRQ registration and passed to userspace through
66DRM_IOCTL_VERSION.
67
68The driver description is a purely informative string passed to
69userspace through the DRM_IOCTL_VERSION ioctl and otherwise unused by
70the kernel.
71
72The driver date, formatted as YYYYMMDD, is meant to identify the date of
73the latest modification to the driver. However, as most drivers fail to
74update it, its value is mostly useless. The DRM core prints it to the
75kernel log at initialization time and passes it to userspace through the
76DRM_IOCTL_VERSION ioctl.
77
78Module Initialization
79---------------------
80
81.. kernel-doc:: include/drm/drm_module.h
82   :doc: overview
83
84Managing Ownership of the Framebuffer Aperture
85----------------------------------------------
86
87.. kernel-doc:: drivers/gpu/drm/drm_aperture.c
88   :doc: overview
89
90.. kernel-doc:: include/drm/drm_aperture.h
91   :internal:
92
93.. kernel-doc:: drivers/gpu/drm/drm_aperture.c
94   :export:
95
96Device Instance and Driver Handling
97-----------------------------------
98
99.. kernel-doc:: drivers/gpu/drm/drm_drv.c
100   :doc: driver instance overview
101
102.. kernel-doc:: include/drm/drm_device.h
103   :internal:
104
105.. kernel-doc:: include/drm/drm_drv.h
106   :internal:
107
108.. kernel-doc:: drivers/gpu/drm/drm_drv.c
109   :export:
110
111Driver Load
112-----------
113
114Component Helper Usage
115~~~~~~~~~~~~~~~~~~~~~~
116
117.. kernel-doc:: drivers/gpu/drm/drm_drv.c
118   :doc: component helper usage recommendations
119
120Memory Manager Initialization
121~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
122
123Every DRM driver requires a memory manager which must be initialized at
124load time. DRM currently contains two memory managers, the Translation
125Table Manager (TTM) and the Graphics Execution Manager (GEM). This
126document describes the use of the GEM memory manager only. See ? for
127details.
128
129Miscellaneous Device Configuration
130~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
131
132Another task that may be necessary for PCI devices during configuration
133is mapping the video BIOS. On many devices, the VBIOS describes device
134configuration, LCD panel timings (if any), and contains flags indicating
135device state. Mapping the BIOS can be done using the pci_map_rom()
136call, a convenience function that takes care of mapping the actual ROM,
137whether it has been shadowed into memory (typically at address 0xc0000)
138or exists on the PCI device in the ROM BAR. Note that after the ROM has
139been mapped and any necessary information has been extracted, it should
140be unmapped; on many devices, the ROM address decoder is shared with
141other BARs, so leaving it mapped could cause undesired behaviour like
142hangs or memory corruption.
143
144Managed Resources
145-----------------
146
147.. kernel-doc:: drivers/gpu/drm/drm_managed.c
148   :doc: managed resources
149
150.. kernel-doc:: drivers/gpu/drm/drm_managed.c
151   :export:
152
153.. kernel-doc:: include/drm/drm_managed.h
154   :internal:
155
156Open/Close, File Operations and IOCTLs
157======================================
158
159.. _drm_driver_fops:
160
161File Operations
162---------------
163
164.. kernel-doc:: drivers/gpu/drm/drm_file.c
165   :doc: file operations
166
167.. kernel-doc:: include/drm/drm_file.h
168   :internal:
169
170.. kernel-doc:: drivers/gpu/drm/drm_file.c
171   :export:
172
173Misc Utilities
174==============
175
176Printer
177-------
178
179.. kernel-doc:: include/drm/drm_print.h
180   :doc: print
181
182.. kernel-doc:: include/drm/drm_print.h
183   :internal:
184
185.. kernel-doc:: drivers/gpu/drm/drm_print.c
186   :export:
187
188Utilities
189---------
190
191.. kernel-doc:: include/drm/drm_util.h
192   :doc: drm utils
193
194.. kernel-doc:: include/drm/drm_util.h
195   :internal:
196
197
198Unit testing
199============
200
201KUnit
202-----
203
204KUnit (Kernel unit testing framework) provides a common framework for unit tests
205within the Linux kernel.
206
207This section covers the specifics for the DRM subsystem. For general information
208about KUnit, please refer to Documentation/dev-tools/kunit/start.rst.
209
210How to run the tests?
211~~~~~~~~~~~~~~~~~~~~~
212
213In order to facilitate running the test suite, a configuration file is present
214in ``drivers/gpu/drm/tests/.kunitconfig``. It can be used by ``kunit.py`` as
215follows:
216
217.. code-block:: bash
218
219	$ ./tools/testing/kunit/kunit.py run --kunitconfig=drivers/gpu/drm/tests \
220		--kconfig_add CONFIG_VIRTIO_UML=y \
221		--kconfig_add CONFIG_UML_PCI_OVER_VIRTIO=y
222
223.. note::
224	The configuration included in ``.kunitconfig`` should be as generic as
225	possible.
226	``CONFIG_VIRTIO_UML`` and ``CONFIG_UML_PCI_OVER_VIRTIO`` are not
227	included in it because they are only required for User Mode Linux.
228
229
230Legacy Support Code
231===================
232
233The section very briefly covers some of the old legacy support code
234which is only used by old DRM drivers which have done a so-called
235shadow-attach to the underlying device instead of registering as a real
236driver. This also includes some of the old generic buffer management and
237command submission code. Do not use any of this in new and modern
238drivers.
239
240Legacy Suspend/Resume
241---------------------
242
243The DRM core provides some suspend/resume code, but drivers wanting full
244suspend/resume support should provide save() and restore() functions.
245These are called at suspend, hibernate, or resume time, and should
246perform any state save or restore required by your device across suspend
247or hibernate states.
248
249int (\*suspend) (struct drm_device \*, pm_message_t state); int
250(\*resume) (struct drm_device \*);
251Those are legacy suspend and resume methods which *only* work with the
252legacy shadow-attach driver registration functions. New driver should
253use the power management interface provided by their bus type (usually
254through the :c:type:`struct device_driver <device_driver>`
255dev_pm_ops) and set these methods to NULL.
256
257Legacy DMA Services
258-------------------
259
260This should cover how DMA mapping etc. is supported by the core. These
261functions are deprecated and should not be used.
262