3_Developer_Guides/5_General_setup: Simple docs for video camera streaming

This mostly targets rpi4 as a camera and display are rather easy to get
by, but also vivid testing module which can also be used from h3ulcb.

Bug-AGL: SPEC-4170

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Id76c2a2bf5c1156de70c906c0534c36b36af30c4
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/27134
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>

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+---
+title: Generic devices setup
+---
+
+## Camera setup on RPi4
+
+This assumes that you'll be using the Raspberry Pi Camera Module, which can
+easily hooked on the board using the LVDS connector. Further more information
+about how to connect it, could be found at
+[Installing a Raspberry Pi camera](https://www.raspberrypi.com/documentation/accessories/camera.html#installing-a-raspberry-pi-camera)
+
+With the camera installed, you'll need to enable it by editing /boot/config.txt
+file (if you're booting an AGL image over a sd-card it will be there by
+default, otherwise -- in case of doing netboot, you'll have to create
+manually) and add the following entry:
+
+
+       ## start_x
+       ##     Set to "1" to enable the camera module.
+       ##
+       ##     Enabling the camera requires gpu_mem option to be specified with a value
+       ##     of at least 128.
+       ##
+       ##     Default 0
+       ##
+       start_x=1
+
+And reboot your device. Afterwards, after logging it, make sure that you have
+the /dev/video0 device created. You could also use v4l2-ctl to verify
+that is indeed usable as a capture device:
+
+       $ v4l2-ctl -d /dev/video0 --all
+
+In order to test out video playback, use the following gstreamer pipeline:
+
+       $ gst-launch-1.0 v4l2src device=/dev/video0 ! \
+                                               video/x-raw,width=640,height=480 ! waylandsink fullscreen=true
+
+Alternatively, using [camera-gstreamer](https://gerrit.automotivelinux.org/gerrit/gitweb?p=apps/camera-gstreamer.git;a=summary)
+application could might be another possibility, but you'll have to build it
+yourself and add it in the image.
+
+This includes an example on how to create a xdg-toplevel surface and create a
+gstreamer pipeline, instead of relaying on waylandsink to create one for you,
+in a programmatic fashion.
+
+## Display setup on RPi4
+
+This assumes that you'll be using the Raspberry Pi 7'' display. Installation
+can be found at [Rpi Display page](https://www.raspberrypi.com/documentation/accessories/display.html).
+
+If booting over the network, the dtb should already contain the ft5406 dtb,
+while booting over sd-card the following show be in the /boot/config.txt
+file:
+
+       dtoverlay=rpi-ft5406-overlay
+
+Once the board boots up, you should get a rainbow like effect and afterwards
+booting up would display debug scrolling over. You'll need to adjust the
+orientation as the 800x480 is in portrait.  Edit /etc/xdg/weston/weston.ini and
+add a new output entry, like the following:
+
+       [output]
+       name=DSI-1
+       transform=90
+
+Note that for the Qt platform, the homescreen application, together with the
+other demo applications, is tailored specifically for a 1080p display and it
+will display incorrectly on such a smaller display.
+
+## Testing out video camera without a real device
+
+While the above requires having a real video camera device, one can use out
+the [vivid module](https://docs.kernel.org/admin-guide/media/vivid.html?highlight=vivid#the-virtual-video-test-driver-vivid)
+to try out your custom application or just testing out camera functionality in AGL.
+
+You should normally have the module present, not loaded, for either **rpi4** or for
+**h3ulcb** boards. Load the module, like in the following command:
+
+       modprobe vivid allocators=0x1
+
+Have a look at the kernel ring buffer to see what capture devices are created
+and use as source:
+
+       $ gst-launch-1.0 v4l2src device=/dev/videoXX ! \
+                               video/x-raw,width=640,height=480 ! waylandsink fullscreen=true
+
+making sure to replace /dev/videoXX with correct capture device. You can check
+that easily using the v4l2-ctl mentioned above.
+
+Note that using camera-gstreamer will attempt to find the first available
+capture device, so if you happen to have to another one before the one created
+by this module it might not work. You can bypass that, by just making a symlink
+from /dev/videoXX to /dev/video0 to make first available capture device.