Is there a way to force the downstream HDMI Sink to reset and re-establish a link similar to how the HDP can force the HDMI Source to retrain?
There is nothing in the specification that directly addresses this issue, however if you disable the TMDS clock that should have the same effect.
The sink will stop if there is no clock being forwarded. Also, for the RX side to force a retrain, the HDP toggle is 100ms. By using these together you can force the HDMI Sink to retrain.
There is a function in the Video PHY Controller software driver that allows the user to disable the TMDS clock by disabling the OBUF.
You can then add a delay for 100ms, and then re-enable the TMDS clock.
This was tested with a KC705 board and the inrevium HDMI2.0 FMC Card (TB-FMCH-HDMI4K) running the HDMI TX Subsystem, and using the QD 780D as the HDMI Sink.
The below steps are sufficient to cause the sink (QD 780D) to retrain to the incoming video.
A reset of the HDMI TX or VPHY are not required as it will stop the clock and then restart it.
Note: Another solution would be to completely reset the HDMI TX.
This could be done by simply reprogramming the reference clock which will trigger the Video PHY and the HDMI TX Subsystem to completely reset, which will also reset the link.