31.3. Switch IP Functional Description
Autoconsume minimizes backpressure during switching. Turn on Autoconsume inputs if the switch inputs connect directly or indirectly to other IPs that are sensitive to backpressure, such as the video connectivity IPs. If the switch inputs connect directly or indirectly to sources such as a frame buffer, the frame buffer can tolerate backpressure and so you can turn off Autoconsume inputs to ensure that outputs switch promptly.
In this example, input 0 (IP0) drives output 0 (OP0) and input 1 (IP1) drives both outputs 1 and 2 (OP1, OP2). 4k60 video drives both inputs but with the HDMI start-of-fields occurring before the DisplayPort start-of-fields.
The figure shows a new configuration with IP0 driving OP1 and IP1 driving OP0 , but keeping the OP2 from IP1 connection open. The Switch IP makes the new connections at the earliest opportunity after a write to the COMMIT register.
The figure shows the IP receiving a write to the COMMIT register for the new configuration at a time marked by the dotted line.
OP0 is still completing the HDMI frame from IP0. OP0 must therefore create backpressure for this new connection request to IP1, which consequently creates backpressure to the DisplayPort input on completing its current field. This backpressure has the undesirable side effect of starving OP2 of video and stalling it. Incidentally, OP1 is changing its input source and so also stalls until the next HDMI field becomes available from IP0.
To prevent OP2 from starving, you must turn on Autoconsume in the GUI, so OP2 can continue uninterrupted while its current field completes.
Turn on Autoconsume inputs so that the IP defers the new connections until the end of the current HDMI field. Until that time OP0 and OP1 consume the IP1 DisplayPort input so the IP does not create backpressure for IP1 and new connections do not affect OP2.
Broken field behavior
The switch has no sensitivity to broken fields. The IP performs switches on broken fields in the same way as unbroken fields.
Metapacket and auxiliary control packet support for full variants
For full variants, the switch supports all packet types and routes all the packets received on its inputs to the appropriate outputs. The IP defers the switch until it sees an end of field packet on an input, so that all packets between the image information packet and the end of field packet keep together. For more information, refer to the Intel FPGA Streaming Video Protocol specification.
Pairs of image information and end-of-field packets mark one field of video. Any auxiliary control packets before the current field’s end-of-field packet are associated with that field. In addition, any auxiliary control packets that occur after the end-of-field packet of the preceding field are also associated with the current field.