RFNoC: Disable FC ACK packets for lossless links

FC ACK packets are unnecessary on lossless links and degrade overall
performance.  This change disables those packets on all lossless links.

Signed-off-by: Michael West <michael.west@ettus.com>

3 files changed, 16 insertions, 8 deletions

diff --git a/host/lib/rfnoc/graph_impl.cpp b/host/lib/rfnoc/graph_impl.cpp
index 989bcb0c4..326e9205d 100644
--- a/host/lib/rfnoc/graph_impl.cpp
+++ b/host/lib/rfnoc/graph_impl.cpp
@@ -120,17 +120,17 @@ void graph_impl::connect(const block_id_t& src_block,
                 % dst->get_block_id().get() % (dst->get_fifo_size(dst_block_port) / 1024)
                 % (pkt_size / 1024) % src->get_block_id().get()));
     }
+    const bool same_xbar = sid.get_src_addr() == sid.get_dst_addr();
     src->configure_flow_control_out(true, /* enable output */
+        same_xbar, // Lossless link if on same crossbar
         buf_size_bytes,
         0, /* no packet limit. We need to revisit this at some point. */
         src_block_port);
     // On the same crossbar, use lots of FC packets
-    size_t bytes_per_response =
-        buf_size_bytes / uhd::rfnoc::DEFAULT_FC_XBAR_RESPONSE_FREQ;
     // Over the network, use less or we'd flood the transport
-    if (sid.get_src_addr() != sid.get_dst_addr()) {
-        bytes_per_response = buf_size_bytes / uhd::rfnoc::DEFAULT_FC_TX_RESPONSE_FREQ;
-    }
+    const size_t bytes_per_response =
+        same_xbar ? buf_size_bytes / uhd::rfnoc::DEFAULT_FC_XBAR_RESPONSE_FREQ
+                  : buf_size_bytes / uhd::rfnoc::DEFAULT_FC_TX_RESPONSE_FREQ;
     UHD_ASSERT_THROW(bytes_per_response != 0);
     dst->configure_flow_control_in(bytes_per_response, dst_block_port);
 
@@ -189,7 +189,12 @@ void graph_impl::connect_src(const block_id_t& src_block,
                 % (buf_size_dst_bytes / 1024) % (pkt_size / 1024)
                 % src->get_block_id().get()));
     }
-    src->configure_flow_control_out(buf_size_pkts, src_block_port);
+
+    src->configure_flow_control_out(true, /* enable output */
+        (dst_sid.get_src_addr() == dst_sid.get_dst_addr()),
+        buf_size_dst_bytes,
+        0, /* no packet limit. We need to revisit this at some point. */
+        src_block_port);
 }
 
 void graph_impl::connect_sink(
diff --git a/host/lib/rfnoc/source_block_ctrl_base.cpp b/host/lib/rfnoc/source_block_ctrl_base.cpp
index 6c41fae14..656ab26af 100644
--- a/host/lib/rfnoc/source_block_ctrl_base.cpp
+++ b/host/lib/rfnoc/source_block_ctrl_base.cpp
@@ -86,6 +86,7 @@ void source_block_ctrl_base::set_destination(
 }
 
 void source_block_ctrl_base::configure_flow_control_out(const bool enable_fc_output,
+    const bool lossless_link,
     const size_t buf_size_bytes,
     const size_t pkt_limit,
     const size_t block_port,
@@ -124,8 +125,9 @@ void source_block_ctrl_base::configure_flow_control_out(const bool enable_fc_out
     // count based flow control
     const bool enable_byte_fc    = (buf_size_bytes != 0);
     const bool enable_pkt_cnt_fc = (pkt_limit != 0);
-    const uint32_t config        = (enable_fc_output ? 1 : 0) | (enable_byte_fc << 1)
-                            | (enable_pkt_cnt_fc << 2);
+    const uint32_t config = (enable_fc_output ? 1 : 0) | ((enable_byte_fc ? 1 : 0) << 1)
+                            | ((enable_pkt_cnt_fc ? 1 : 0) << 2)
+                            | ((lossless_link ? 1 : 0) << 3);
 
     // Resize the FC window.
     // Precondition: No data can be buffered upstream.
diff --git a/host/lib/usrp/device3/device3_io_impl.cpp b/host/lib/usrp/device3/device3_io_impl.cpp
index 2bc59f1a8..ce8ff2cbf 100644
--- a/host/lib/usrp/device3/device3_io_impl.cpp
+++ b/host/lib/usrp/device3/device3_io_impl.cpp
@@ -357,6 +357,7 @@ rx_streamer::sptr device3_impl::get_rx_stream(const stream_args_t& args_)
         UHD_RX_STREAMER_LOG() << "Flow Control Window = " << (fc_window)
                               << ", Flow Control Handler Window = " << fc_handle_window;
         blk_ctrl->configure_flow_control_out(true,
+            xport.lossless,
             fc_window,
             rx_hints.cast<size_t>("recv_pkt_limit",
                 0), // On rfnoc-devel, update e300_impl::get_rx_hints() to set this to 32