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diff --git a/Documentation/gpu/vkms.rst b/Documentation/gpu/vkms.rst new file mode 100644 index 000000000000..1e79e62a6bc4 --- /dev/null +++ b/Documentation/gpu/vkms.rst @@ -0,0 +1,287 @@ +.. _vkms: + +========================================== + drm/vkms Virtual Kernel Modesetting +========================================== + +.. kernel-doc:: drivers/gpu/drm/vkms/vkms_drv.c + :doc: vkms (Virtual Kernel Modesetting) + +Setup +===== + +The VKMS driver can be setup with the following steps: + +To check if VKMS is loaded, run:: + + lsmod | grep vkms + +This should list the VKMS driver. If no output is obtained, then +you need to enable and/or load the VKMS driver. +Ensure that the VKMS driver has been set as a loadable module in your +kernel config file. Do:: + + make nconfig + + Go to `Device Drivers> Graphics support` + + Enable `Virtual KMS (EXPERIMENTAL)` + +Compile and build the kernel for the changes to get reflected. +Now, to load the driver, use:: + + sudo modprobe vkms + +On running the lsmod command now, the VKMS driver will appear listed. +You can also observe the driver being loaded in the dmesg logs. + +The VKMS driver has optional features to simulate different kinds of hardware, +which are exposed as module options. You can use the `modinfo` command +to see the module options for vkms:: + + modinfo vkms + +Module options are helpful when testing, and enabling modules +can be done while loading vkms. For example, to load vkms with cursor enabled, +use:: + + sudo modprobe vkms enable_cursor=1 + +To disable the driver, use :: + + sudo modprobe -r vkms + +Configuring With Configfs +========================= + +It is possible to create and configure multiple VKMS instances via configfs. + +Start by mounting configfs and loading VKMS:: + + sudo mount -t configfs none /config + sudo modprobe vkms + +Once VKMS is loaded, ``/config/vkms`` is created automatically. Each directory +under ``/config/vkms`` represents a VKMS instance, create a new one:: + + sudo mkdir /config/vkms/my-vkms + +By default, the instance is disabled:: + + cat /config/vkms/my-vkms/enabled + 0 + +And directories are created for each configurable item of the display pipeline:: + + tree /config/vkms/my-vkms + ├── connectors + ├── crtcs + ├── enabled + ├── encoders + └── planes + +To add items to the display pipeline, create one or more directories under the +available paths. + +Start by creating one or more planes:: + + sudo mkdir /config/vkms/my-vkms/planes/plane0 + +Planes have 1 configurable attribute: + +- type: Plane type: 0 overlay, 1 primary, 2 cursor (same values as those + exposed by the "type" property of a plane) + +Continue by creating one or more CRTCs:: + + sudo mkdir /config/vkms/my-vkms/crtcs/crtc0 + +CRTCs have 1 configurable attribute: + +- writeback: Enable or disable writeback connector support by writing 1 or 0 + +Next, create one or more encoders:: + + sudo mkdir /config/vkms/my-vkms/encoders/encoder0 + +Last but not least, create one or more connectors:: + + sudo mkdir /config/vkms/my-vkms/connectors/connector0 + +Connectors have 1 configurable attribute: + +- status: Connection status: 1 connected, 2 disconnected, 3 unknown (same values + as those exposed by the "status" property of a connector) + +To finish the configuration, link the different pipeline items:: + + sudo ln -s /config/vkms/my-vkms/crtcs/crtc0 /config/vkms/my-vkms/planes/plane0/possible_crtcs + sudo ln -s /config/vkms/my-vkms/crtcs/crtc0 /config/vkms/my-vkms/encoders/encoder0/possible_crtcs + sudo ln -s /config/vkms/my-vkms/encoders/encoder0 /config/vkms/my-vkms/connectors/connector0/possible_encoders + +Since at least one primary plane is required, make sure to set the right type:: + + echo "1" | sudo tee /config/vkms/my-vkms/planes/plane0/type + +Once you are done configuring the VKMS instance, enable it:: + + echo "1" | sudo tee /config/vkms/my-vkms/enabled + +Finally, you can remove the VKMS instance disabling it:: + + echo "0" | sudo tee /config/vkms/my-vkms/enabled + +And removing the top level directory and its subdirectories:: + + sudo rm /config/vkms/my-vkms/planes/*/possible_crtcs/* + sudo rm /config/vkms/my-vkms/encoders/*/possible_crtcs/* + sudo rm /config/vkms/my-vkms/connectors/*/possible_encoders/* + sudo rmdir /config/vkms/my-vkms/planes/* + sudo rmdir /config/vkms/my-vkms/crtcs/* + sudo rmdir /config/vkms/my-vkms/encoders/* + sudo rmdir /config/vkms/my-vkms/connectors/* + sudo rmdir /config/vkms/my-vkms + +Testing With IGT +================ + +The IGT GPU Tools is a test suite used specifically for debugging and +development of the DRM drivers. +The IGT Tools can be installed from +`here <https://gitlab.freedesktop.org/drm/igt-gpu-tools>`_ . + +The tests need to be run without a compositor, so you need to switch to text +only mode. You can do this by:: + + sudo systemctl isolate multi-user.target + +To return to graphical mode, do:: + + sudo systemctl isolate graphical.target + +Once you are in text only mode, you can run tests using the IGT_FORCE_DRIVER +variable to specify the device filter for the driver we want to test. +IGT_FORCE_DRIVER can also be used with the run-tests.sh script to run the +tests for a specific driver:: + + sudo IGT_FORCE_DRIVER="vkms" ./build/tests/<name of test> + sudo IGT_FORCE_DRIVER="vkms" ./scripts/run-tests.sh -t <name of test> + +For example, to test the functionality of the writeback library, +we can run the kms_writeback test:: + + sudo IGT_FORCE_DRIVER="vkms" ./build/tests/kms_writeback + sudo IGT_FORCE_DRIVER="vkms" ./scripts/run-tests.sh -t kms_writeback + +You can also run subtests if you do not want to run the entire test:: + + sudo IGT_FORCE_DRIVER="vkms" ./build/tests/kms_flip --run-subtest basic-plain-flip + +Testing With KUnit +================== + +KUnit (Kernel unit testing framework) provides a common framework for unit tests +within the Linux kernel. +More information in ../dev-tools/kunit/index.rst . + +To run the VKMS KUnit tests:: + + tools/testing/kunit/kunit.py run --kunitconfig=drivers/gpu/drm/vkms/tests + +TODO +==== + +If you want to do any of the items listed below, please share your interest +with VKMS maintainers. + +IGT better support +------------------ + +Debugging: + +- kms_plane: some test cases are failing due to timeout on capturing CRC; + +Virtual hardware (vblank-less) mode: + +- VKMS already has support for vblanks simulated via hrtimers, which can be + tested with kms_flip test; in some way, we can say that VKMS already mimics + the real hardware vblank. However, we also have virtual hardware that does + not support vblank interrupt and completes page_flip events right away; in + this case, compositor developers may end up creating a busy loop on virtual + hardware. It would be useful to support Virtual Hardware behavior in VKMS + because this can help compositor developers to test their features in + multiple scenarios. + +Add Plane Features +------------------ + +There's lots of plane features we could add support for: + +- Add background color KMS property[Good to get started]. + +- Scaling. + +- Additional buffer formats. Low/high bpp RGB formats would be interesting + [Good to get started]. + +- Async updates (currently only possible on cursor plane using the legacy + cursor api). + +For all of these, we also want to review the igt test coverage and make sure +all relevant igt testcases work on vkms. They are good options for internship +project. + +Runtime Configuration +--------------------- + +We want to be able to reconfigure vkms instance without having to reload the +module through configfs. Use/Test-cases: + +- Hotplug/hotremove connectors on the fly (to be able to test DP MST handling + of compositors). + +- Change output configuration: Plug/unplug screens, change EDID, allow changing + the refresh rate. + +Writeback support +----------------- + +- The writeback and CRC capture operations share the use of composer_enabled + boolean to ensure vblanks. Probably, when these operations work together, + composer_enabled needs to refcounting the composer state to proper work. + [Good to get started] + +- Add support for cloned writeback outputs and related test cases using a + cloned output in the IGT kms_writeback. + +- As a v4l device. This is useful for debugging compositors on special vkms + configurations, so that developers see what's really going on. + +Output Features +--------------- + +- Variable refresh rate/freesync support. This probably needs prime buffer + sharing support, so that we can use vgem fences to simulate rendering in + testing. Also needs support to specify the EDID. + +- Add support for link status, so that compositors can validate their runtime + fallbacks when e.g. a Display Port link goes bad. + +CRC API Improvements +-------------------- + +- Optimize CRC computation ``compute_crc()`` and plane blending ``blend()`` + +Atomic Check using eBPF +----------------------- + +Atomic drivers have lots of restrictions which are not exposed to userspace in +any explicit form through e.g. possible property values. Userspace can only +inquiry about these limits through the atomic IOCTL, possibly using the +TEST_ONLY flag. Trying to add configurable code for all these limits, to allow +compositors to be tested against them, would be rather futile exercise. Instead +we could add support for eBPF to validate any kind of atomic state, and +implement a library of different restrictions. + +This needs a bunch of features (plane compositing, multiple outputs, ...) +enabled already to make sense. |