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

host/lib/rfnoc/addsub_block_control.cpp

@@ -37,6 +37,9 @@ public:
         // opposite side from which they are received.
         set_prop_forwarding_policy(forwarding_policy_t::ONE_TO_FAN);
         set_action_forwarding_policy(forwarding_policy_t::ONE_TO_FAN);
+        // MTU is also fanned out the same way as we produce output packets the
+        // same size as input packets.
+        set_mtu_forwarding_policy(forwarding_policy_t::ONE_TO_FAN);
 
         // Register the block's edge type properties.
         register_property(&_prop_type_in_a);

host/lib/rfnoc/ddc_block_control.cpp

@@ -74,6 +74,8 @@ public:
                                             << " halfbands and "
                                                "max CIC decimation "
                                             << _cic_max_decim);
+        // This line is not strictly necessary, as ONE_TO_ONE is the default.
+        // We set it make it explicit how this block works.
         set_mtu_forwarding_policy(forwarding_policy_t::ONE_TO_ONE);
         // Load list of valid decimation values
         std::set<size_t> decims{1}; // 1 is always a valid decimation

host/lib/rfnoc/dmafifo_block_control.cpp

@@ -30,6 +30,9 @@ public:
         UHD_ASSERT_THROW(get_num_input_ports() == get_num_output_ports());
         set_action_forwarding_policy(forwarding_policy_t::ONE_TO_ONE);
         set_prop_forwarding_policy(forwarding_policy_t::ONE_TO_ONE);
+        // This line is not strictly necessary, as ONE_TO_ONE is the default.
+        // We set it make it explicit how this block works.
+        set_mtu_forwarding_policy(forwarding_policy_t::ONE_TO_ONE);
         // Now init DMA/DRAM control
         _fifo_cores.reserve(get_num_input_ports());
         for (size_t i = 0; i < get_num_input_ports(); i++) {

host/lib/rfnoc/duc_block_control.cpp

@@ -72,6 +72,10 @@ public:
                                             << " halfbands and "
                                                "max CIC interpolation "
                                             << _cic_max_interp);
+        // This line is not strictly necessary, as ONE_TO_ONE is the default.
+        // We set it make it explicit how this block works. Output packets have
+        // the same size as the input packet.
+        set_mtu_forwarding_policy(forwarding_policy_t::ONE_TO_ONE);
         // Load list of valid interpolation values
         std::set<size_t> interps{1}; // 1 is always a valid interpolation
         for (size_t hb = 0; hb < _num_halfbands; hb++) {

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;

host/lib/rfnoc/replay_block_control.cpp

@@ -96,6 +96,8 @@ public:
         // the graph).
         set_prop_forwarding_policy(forwarding_policy_t::DROP);
         set_action_forwarding_policy(forwarding_policy_t::DROP);
+        // Same for MTU
+        set_mtu_forwarding_policy(forwarding_policy_t::DROP);
 
         // Initialize record properties
         _record_type.reserve(_num_input_ports);

host/lib/rfnoc/split_stream_block_control.cpp

@@ -41,6 +41,10 @@ public:
         // the split stream block.
         set_prop_forwarding_policy(forwarding_policy_t::USE_MAP);
         set_action_forwarding_policy(forwarding_policy_t::USE_MAP);
+        // MTU forwarding doesn't allow for USE_MAP, but we assume that packets
+        // coming in may potentially go to any output port. We thus fan out the
+        // MTU propagation.
+        set_mtu_forwarding_policy(forwarding_policy_t::ONE_TO_FAN);
 
         // Property propagation scheme (X --> Y means 'Properties received on
         // X propagate to Y'):

host/lib/rfnoc/switchboard_block_control.cpp

@@ -36,6 +36,10 @@ public:
         // Configure property propagation and action forwarding behavior.
         set_prop_forwarding_policy(forwarding_policy_t::USE_MAP);
         set_action_forwarding_policy(forwarding_policy_t::USE_MAP);
+        // MTU forwarding doesn't allow for USE_MAP, but we assume that packets
+        // coming in may potentially go to any output port. We thus fan out the
+        // MTU propagation.
+        set_mtu_forwarding_policy(forwarding_policy_t::ONE_TO_FAN);
 
         _update_forwarding_map();
     }