1.. _vkms:
2
3==========================================
4 drm/vkms Virtual Kernel Modesetting
5==========================================
6
7.. kernel-doc:: drivers/gpu/drm/vkms/vkms_drv.c
8   :doc: vkms (Virtual Kernel Modesetting)
9
10Setup
11=====
12
13The VKMS driver can be setup with the following steps:
14
15To check if VKMS is loaded, run::
16
17  lsmod | grep vkms
18
19This should list the VKMS driver. If no output is obtained, then
20you need to enable and/or load the VKMS driver.
21Ensure that the VKMS driver has been set as a loadable module in your
22kernel config file. Do::
23
24  make nconfig
25
26  Go to `Device Drivers> Graphics support`
27
28  Enable `Virtual KMS (EXPERIMENTAL)`
29
30Compile and build the kernel for the changes to get reflected.
31Now, to load the driver, use::
32
33  sudo modprobe vkms
34
35On running the lsmod command now, the VKMS driver will appear listed.
36You can also observe the driver being loaded in the dmesg logs.
37
38The VKMS driver has optional features to simulate different kinds of hardware,
39which are exposed as module options. You can use the `modinfo` command
40to see the module options for vkms::
41
42  modinfo vkms
43
44Module options are helpful when testing, and enabling modules
45can be done while loading vkms. For example, to load vkms with cursor enabled,
46use::
47
48  sudo modprobe vkms enable_cursor=1
49
50To disable the driver, use ::
51
52  sudo modprobe -r vkms
53
54Testing With IGT
55================
56
57The IGT GPU Tools is a test suite used specifically for debugging and
58development of the DRM drivers.
59The IGT Tools can be installed from
60`here <https://gitlab.freedesktop.org/drm/igt-gpu-tools>`_ .
61
62The tests need to be run without a compositor, so you need to switch to text
63only mode. You can do this by::
64
65  sudo systemctl isolate multi-user.target
66
67To return to graphical mode, do::
68
69  sudo systemctl isolate graphical.target
70
71Once you are in text only mode, you can run tests using the --device switch
72or IGT_DEVICE variable to specify the device filter for the driver we want
73to test. IGT_DEVICE can also be used with the run-test.sh script to run the
74tests for a specific driver::
75
76  sudo ./build/tests/<name of test> --device "sys:/sys/devices/platform/vkms"
77  sudo IGT_DEVICE="sys:/sys/devices/platform/vkms" ./build/tests/<name of test>
78  sudo IGT_DEVICE="sys:/sys/devices/platform/vkms" ./scripts/run-tests.sh -t <name of test>
79
80For example, to test the functionality of the writeback library,
81we can run the kms_writeback test::
82
83  sudo ./build/tests/kms_writeback --device "sys:/sys/devices/platform/vkms"
84  sudo IGT_DEVICE="sys:/sys/devices/platform/vkms" ./build/tests/kms_writeback
85  sudo IGT_DEVICE="sys:/sys/devices/platform/vkms" ./scripts/run-tests.sh -t kms_writeback
86
87You can also run subtests if you do not want to run the entire test::
88
89  sudo ./build/tests/kms_flip --run-subtest basic-plain-flip --device "sys:/sys/devices/platform/vkms"
90  sudo IGT_DEVICE="sys:/sys/devices/platform/vkms" ./build/tests/kms_flip --run-subtest basic-plain-flip
91
92TODO
93====
94
95If you want to do any of the items listed below, please share your interest
96with VKMS maintainers.
97
98IGT better support
99------------------
100
101Debugging:
102
103- kms_plane: some test cases are failing due to timeout on capturing CRC;
104
105- kms_flip: when running test cases in sequence, some successful individual
106  test cases are failing randomly; when individually, some successful test
107  cases display in the log the following error::
108
109  [drm:vkms_prepare_fb [vkms]] ERROR vmap failed: -4
110
111Virtual hardware (vblank-less) mode:
112
113- VKMS already has support for vblanks simulated via hrtimers, which can be
114  tested with kms_flip test; in some way, we can say that VKMS already mimics
115  the real hardware vblank. However, we also have virtual hardware that does
116  not support vblank interrupt and completes page_flip events right away; in
117  this case, compositor developers may end up creating a busy loop on virtual
118  hardware. It would be useful to support Virtual Hardware behavior in VKMS
119  because this can help compositor developers to test their features in
120  multiple scenarios.
121
122Add Plane Features
123------------------
124
125There's lots of plane features we could add support for:
126
127- Multiple overlay planes. [Good to get started]
128
129- Clearing primary plane: clear primary plane before plane composition (at the
130  start) for correctness of pixel blend ops. It also guarantees alpha channel
131  is cleared in the target buffer for stable crc. [Good to get started]
132
133- ARGB format on primary plane: blend the primary plane into background with
134  translucent alpha.
135
136- Support when the primary plane isn't exactly matching the output size: blend
137  the primary plane into the black background.
138
139- Full alpha blending on all planes.
140
141- Rotation, scaling.
142
143- Additional buffer formats, especially YUV formats for video like NV12.
144  Low/high bpp RGB formats would also be interesting.
145
146- Async updates (currently only possible on cursor plane using the legacy
147  cursor api).
148
149For all of these, we also want to review the igt test coverage and make sure
150all relevant igt testcases work on vkms. They are good options for internship
151project.
152
153Runtime Configuration
154---------------------
155
156We want to be able to reconfigure vkms instance without having to reload the
157module. Use/Test-cases:
158
159- Hotplug/hotremove connectors on the fly (to be able to test DP MST handling
160  of compositors).
161
162- Configure planes/crtcs/connectors (we'd need some code to have more than 1 of
163  them first).
164
165- Change output configuration: Plug/unplug screens, change EDID, allow changing
166  the refresh rate.
167
168The currently proposed solution is to expose vkms configuration through
169configfs. All existing module options should be supported through configfs
170too.
171
172Writeback support
173-----------------
174
175- The writeback and CRC capture operations share the use of composer_enabled
176  boolean to ensure vblanks. Probably, when these operations work together,
177  composer_enabled needs to refcounting the composer state to proper work.
178  [Good to get started]
179
180- Add support for cloned writeback outputs and related test cases using a
181  cloned output in the IGT kms_writeback.
182
183- As a v4l device. This is useful for debugging compositors on special vkms
184  configurations, so that developers see what's really going on.
185
186Output Features
187---------------
188
189- Variable refresh rate/freesync support. This probably needs prime buffer
190  sharing support, so that we can use vgem fences to simulate rendering in
191  testing. Also needs support to specify the EDID.
192
193- Add support for link status, so that compositors can validate their runtime
194  fallbacks when e.g. a Display Port link goes bad.
195
196CRC API Improvements
197--------------------
198
199- Optimize CRC computation ``compute_crc()`` and plane blending ``blend()``
200
201Atomic Check using eBPF
202-----------------------
203
204Atomic drivers have lots of restrictions which are not exposed to userspace in
205any explicit form through e.g. possible property values. Userspace can only
206inquiry about these limits through the atomic IOCTL, possibly using the
207TEST_ONLY flag. Trying to add configurable code for all these limits, to allow
208compositors to be tested against them, would be rather futile exercise. Instead
209we could add support for eBPF to validate any kind of atomic state, and
210implement a library of different restrictions.
211
212This needs a bunch of features (plane compositing, multiple outputs, ...)
213enabled already to make sense.
214