video: fill readme

Complete the README file for the video crate
with the help information for the CLI, and a
working example to run and test the device.

Signed-off-by: Albert Esteve <[email protected]>

Author: aesteve-rh

crates/video/README.md

@@ -1,11 +1,152 @@
 # vhost-user-video
 
-## Design
+## Synopsis
+    vhost-user-video --socket-path <SOCKET_PATH> --backend <BACKEND>
 
-## Usage
+## Description
+    A virtio-video device using the vhost-user protocol.
+
+## Arguments
+
+```text
+    <BACKEND>
+        Video backend to be used [possible values: null, v4l2-decoder]
+```
+
+## Options
+
+```text
+    -s, --socket-path <SOCKET_PATH>
+        Unix socket to which a hypervisor connects to and sets up the control path with the device
+
+    -d, --v4l2-device <V4L2_DEVICE>
+        Path to the video device file [default: /dev/video0]
+
+    -b, --backend <BACKEND>
+        Video backend to be used [possible values: null, v4l2-decoder]
+
+    -h, --help
+        Print help
+
+    -V, --version
+        Print version
+```
+
+## Limitations
+
+Currently this crate only supports v4l2 stateful decoder devices, and
+the intention is it will be used with Arm SoCs that implement stateful
+decode/encode devices such as Qcom Venus, RPi, MediaTek etc.
+
+Support for VAAPI or decoding via libavcodec or similar
+libraries is not implemented, but this could be added in the future
+through different video backends.
+
+## Features
+
+This crate is a work-in-progress. Also, the specification for this device is
+still a work-in-progress, so it requires and out-of-tree kernel on the
+guest. Currently, the vmm translates from virtio-video
+[v3](http://archive.lwn.net:8080/linux-media/6557912.4vTCxPXJkl@os-lin-dmo/T/)
+protocol and writes to a 
+[v4l2 mem2mem stateful decoder device](https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/dev-decoder.html).
+The v3 of the specification was chosen as there is a
+virtio-video Linux frontend driver implementation available for testing.
+
+The primary goal so far is to enable development of virtio-video
+frontend driver using purely open source software. Using
+[vicodec](https://lwn.net/Articles/760650/)
+v4l2 stateful decoder on the host for testing allows a pure
+virtual environment for development and testing.
 
 ## Working example
 
+In this section we provide with some example commands to run the daemon
+and decode a video using vicodec.
+
+Guest Linux kernel modules:
+
+```text
+CONFIG_MEDIA_SUPPORT=y
+CONFIG_MEDIA_TEST_SUPPORT=y
+CONFIG_V4L_TEST_DRIVERS=y
+CONFIG_VIRTIO_VIDEO=y
+CONFIG_GDB_SCRIPTS=y
+CONFIG_DRM_VIRTIO_GPU=y
+```
+
+
+Host kernel modules:
+
+```text
+CONFIG_MEDIA_SUPPORT=y
+CONFIG_MEDIA_TEST_SUPPORT=y
+CONFIG_V4L_TEST_DRIVERS=y
+CONFIG_VIDEO_VICODEC=y
+```
+
+The daemon should be started first (video3 typically is the stateful video):
+
+```text
+host# vhost-user-video --socket-path=/tmp/video.sock --v4l2-device=/dev/video3 --backend=v4l2-decoder
+```
+
+The QEMU invocation needs to create a chardev socket the device can
+use to communicate as well as share the guests memory over a memfd.
+
+```text
+host# qemu-system								                            \
+    -device vhost-user-video-pci,chardev=video,id=video                     \
+    -chardev socket,path=/tmp/video.sock,id=video                           \
+    -m 4096 		        					                            \
+    -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on	\
+    -numa node,memdev=mem							                        \
+    ...
+```
+
+After booting, the device should be available at /dev/video0:
+
+```text
+guest# v4l2-ctl -d/dev/video0 --info
+Driver Info:
+        Driver name      : virtio-video
+        Card type        : 
+        Bus info         : virtio:stateful-decoder
+        Driver version   : 6.1.0
+        Capabilities     : 0x84204000
+                Video Memory-to-Memory Multiplanar
+                Streaming
+                Extended Pix Format
+                Device Capabilities
+        Device Caps      : 0x04204000
+                Video Memory-to-Memory Multiplanar
+                Streaming
+                Extended Pix Format
+```
+
+Example v4l2-ctl decode command:
+
+```text
+guest# v4l2-ctl -d0 -x width=640,height=480 -v width=640,height=480,pixelformat=YU12 \
+    --stream-mmap --stream-out-mmap --stream-from test_640_480-420P.fwht            \
+    --stream-to out-test-640-480.YU12
+```
+
+Play the raw decoded video with ffplay or mplayer:
+
+```text
+guest# ffplay -loglevel warning -v info -f rawvideo -pixel_format yuv420p \
+    -video_size "640x480" ./out-test-640-480.YU12
+guest# mplayer -demuxer rawvideo -rawvideo \
+    format=i420:w=640:h=480:fps=25 out-test-640-480.YU12
+```
+
+Enable v4l2 debug in virtio-video driver:
+
+```text
+# echo 0x1f > /sys/class/video4linux/videoX/dev_debug
+```
+
 ## License
 
 This project is licensed under either of