rfnoc: Update the MTU forwarding property for some blocks

Note that the default MTU forwarding policy is ONE_TO_ONE, therefore,
it is only strictly necessary to modify the MTU forwarding policy for
blocks that route data in a different manner. However, it may be nice to
explicitly state the forwarding policy for the benefit of the reader.

The following blocks had their policies updated:
- addsub: ONE_TO_FAN
- duc: ONE_TO_ONE
- dmafifo: ONE_TO_ONE
- null block: DROP
- replay block: DROP
- split stream: ONE_TO_FAN
- switchboard: ONE_TO_FAN

1 files changed, 3 insertions, 0 deletions

diff --git a/host/lib/rfnoc/null_block_control.cpp b/host/lib/rfnoc/null_block_control.cpp
index b51dbb906..1c32ba8b9 100644
--- a/host/lib/rfnoc/null_block_control.cpp
+++ b/host/lib/rfnoc/null_block_control.cpp
@@ -40,6 +40,9 @@ class null_block_control_impl : public null_block_control
 public:
     RFNOC_BLOCK_CONSTRUCTOR(null_block_control)
     {
+        // This block doesn't pass through packets, so the MTU can differ on
+        // input and output.
+        set_mtu_forwarding_policy(forwarding_policy_t::DROP);
         uint32_t initial_state = regs().peek32(REG_CTRL_STATUS);
         _streaming             = initial_state & 0x2;
         _nipc                  = (initial_state >> 24) & 0xFF;