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
105Virtual hardware (vblank-less) mode:
106
107- VKMS already has support for vblanks simulated via hrtimers, which can be
108  tested with kms_flip test; in some way, we can say that VKMS already mimics
109  the real hardware vblank. However, we also have virtual hardware that does
110  not support vblank interrupt and completes page_flip events right away; in
111  this case, compositor developers may end up creating a busy loop on virtual
112  hardware. It would be useful to support Virtual Hardware behavior in VKMS
113  because this can help compositor developers to test their features in
114  multiple scenarios.
115
116Add Plane Features
117------------------
118
119There's lots of plane features we could add support for:
120
121- Add background color KMS property[Good to get started].
122
123- Scaling.
124
125- Additional buffer formats, especially YUV formats for video like NV12.
126  Low/high bpp RGB formats would also be interesting.
127
128- Async updates (currently only possible on cursor plane using the legacy
129  cursor api).
130
131For all of these, we also want to review the igt test coverage and make sure
132all relevant igt testcases work on vkms. They are good options for internship
133project.
134
135Runtime Configuration
136---------------------
137
138We want to be able to reconfigure vkms instance without having to reload the
139module. Use/Test-cases:
140
141- Hotplug/hotremove connectors on the fly (to be able to test DP MST handling
142  of compositors).
143
144- Configure planes/crtcs/connectors (we'd need some code to have more than 1 of
145  them first).
146
147- Change output configuration: Plug/unplug screens, change EDID, allow changing
148  the refresh rate.
149
150The currently proposed solution is to expose vkms configuration through
151configfs. All existing module options should be supported through configfs
152too.
153
154Writeback support
155-----------------
156
157- The writeback and CRC capture operations share the use of composer_enabled
158  boolean to ensure vblanks. Probably, when these operations work together,
159  composer_enabled needs to refcounting the composer state to proper work.
160  [Good to get started]
161
162- Add support for cloned writeback outputs and related test cases using a
163  cloned output in the IGT kms_writeback.
164
165- As a v4l device. This is useful for debugging compositors on special vkms
166  configurations, so that developers see what's really going on.
167
168Output Features
169---------------
170
171- Variable refresh rate/freesync support. This probably needs prime buffer
172  sharing support, so that we can use vgem fences to simulate rendering in
173  testing. Also needs support to specify the EDID.
174
175- Add support for link status, so that compositors can validate their runtime
176  fallbacks when e.g. a Display Port link goes bad.
177
178CRC API Improvements
179--------------------
180
181- Optimize CRC computation ``compute_crc()`` and plane blending ``blend()``
182
183Atomic Check using eBPF
184-----------------------
185
186Atomic drivers have lots of restrictions which are not exposed to userspace in
187any explicit form through e.g. possible property values. Userspace can only
188inquiry about these limits through the atomic IOCTL, possibly using the
189TEST_ONLY flag. Trying to add configurable code for all these limits, to allow
190compositors to be tested against them, would be rather futile exercise. Instead
191we could add support for eBPF to validate any kind of atomic state, and
192implement a library of different restrictions.
193
194This needs a bunch of features (plane compositing, multiple outputs, ...)
195enabled already to make sense.
196