documentation: media: vivid: Update documentation on vivid loopback support

Modify section "Video and Sliced VBI Looping" in Documentation to explain
the vivid loopback support for video across multiple vivid instances.
Previous documentation is out-of-date as it was explaining looping in a
single vivid instance only.

Also, in "Some Future Improvements" the item "Add support to loop
from a specific output to a specific input across vivid instances"
can be dropped since that's now implemented.

Signed-off-by: Dorcas Anono Litunya <anonolitunya@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

Documentation/admin-guide/media/vivid.rst

@@ -1135,27 +1135,33 @@ Metadata Capture Controls
         if set, then the generated metadata stream contains Source Clock information.
 
 
-Video and Sliced VBI Looping
-----------------------------
+Video, Sliced VBI and HDMI CEC Looping
+--------------------------------------
 
-The vivid driver supports looping of video output to video input, and VBI
-output to VBI input. For video/VBI looping this emulates as if a cable was
-hooked up between the output and input connector. So video and VBI looping
-is only supported between S-Video and HDMI inputs and outputs.
-VBI is only valid for S-Video as it makes no sense for HDMI.
+Video Looping functionality is supported for devices created by the same
+vivid driver instance, as well as across multiple instances of the vivid driver.
+The vivid driver supports looping of video and Sliced VBI data between an S-Video output
+and an S-Video input. It also supports looping of video and HDMI CEC data between an
+HDMI output and an HDMI input.
 
-Looping is currently supported only between devices created by the same
-vivid driver instance.
+To enable looping, set the 'HDMI/S-Video XXX-N Is Connected To' control(s) to select
+whether an input uses the Test Pattern Generator, or is disconnected, or is connected
+to an output. An input can be connected to an output from any vivid instance.
+The inputs and outputs are numbered XXX-N where XXX is the vivid instance number
+(see module option n_devs). If there is only one vivid instance (the default), then
+XXX will be 000. And N is the Nth S-Video/HDMI input or output of that instance.
+If vivid is loaded without module options, then you can connect the S-Video 000-0 input
+to the S-Video 000-0 output, or the HDMI 000-0 input to the HDMI 000-0 output.
+This is the equivalent of connecting or disconnecting a cable between an input and an
+output in a physical device.
 
-The way to enable video/VBI looping is currently fairly crude. A 'Loop Video'
-control is available in the "Vivid" control class of the video
-capture and VBI capture devices. When checked the video looping will be enabled.
-Once enabled any video S-Video or HDMI input will show a static test pattern
-until the video output has started. At that time the video output will be
-looped to the video input provided that:
+If an 'HDMI/S-Video XXX-N Is Connected To' control selected an output, then the video
+output will be looped to the video input provided that:
 
-- the input type matches the output type. So the HDMI input cannot receive
-  video from the S-Video output.
+- the currently selected input matches the input indicated by the control name.
+
+- in the vivid instance of the output connector, the currently selected output matches
+  the output indicated by the control's value.
 
 - the video resolution of the video input must match that of the video output.
   So it is not possible to loop a 50 Hz (720x576) S-Video output to a 60 Hz
@@ -1182,6 +1188,8 @@ looped to the video input provided that:
   "DV Timings Signal Mode" for the HDMI input should be configured so that a
   valid signal is passed to the video input.
 
+If any condition is not valid, then the 'Noise' test pattern is shown.
+
 The framerates do not have to match, although this might change in the future.
 
 By default you will see the OSD text superimposed on top of the looped video.
@@ -1344,8 +1352,6 @@ Just as a reminder and in no particular order:
 - Add ARGB888 overlay support: better testing of the alpha channel
 - Improve pixel aspect support in the tpg code by passing a real v4l2_fract
 - Use per-queue locks and/or per-device locks to improve throughput
-- Add support to loop from a specific output to a specific input across
-  vivid instances
 - The SDR radio should use the same 'frequencies' for stations as the normal
   radio receiver, and give back noise if the frequency doesn't match up with
   a station frequency