[sonic-swss]:enable unconfiguring PFC on last TC on a port (#1962)

* enable unconfiguring PFC on last TC on a port
Signed-off-by: Alpesh S Patel <alpesh@cisco.com>

orchagent/qosorch.cpp

@@ -1451,7 +1451,13 @@ task_process_status QosOrch::handlePortQosMapTable(Consumer& consumer)
             SWSS_LOG_INFO("Applied %s to port %s", it->second.first.c_str(), port_name.c_str());
         }
 
-        if (pfc_enable)
+        sai_uint8_t old_pfc_enable = 0;
+        if (!gPortsOrch->getPortPfc(port.m_port_id, &old_pfc_enable)) 
+        {
+            SWSS_LOG_ERROR("Failed to retrieve PFC bits on port %s", port_name.c_str());
+        }
+
+        if (pfc_enable || old_pfc_enable)
         {
             if (!gPortsOrch->setPortPfc(port.m_port_id, pfc_enable))
             {

tests/test_pfc.py

@@ -101,7 +101,34 @@ def test_PfcAsymmetric(self, dvs, testlog):
         pfc = getPortAttr(dvs, port_oid, 'SAI_PORT_ATTR_PRIORITY_FLOW_CONTROL')
         assert pfc == pfc_tx
 
+    def test_PfcUnconfig(self, dvs, testlog):
 
+        port_name = 'Ethernet0'
+        pfc_queues = [ 3, 4 ]
+
+        # Configure default PFC
+        setPortPfc(dvs, port_name, pfc_queues)
+
+        # Get SAI object ID for the interface
+        port_oid = getPortOid(dvs, port_name)
+
+        # Verify default PFC is set to configured value
+        pfc = getPortAttr(dvs, port_oid, 'SAI_PORT_ATTR_PRIORITY_FLOW_CONTROL')
+        assert pfc == getBitMaskStr(pfc_queues)
+
+        # Configure PFC on single TC
+        pfc_queues = [ 3 ]
+        setPortPfc(dvs, port_name, pfc_queues)
+        # Verify default PFC is set to configured value
+        pfc = getPortAttr(dvs, port_oid, 'SAI_PORT_ATTR_PRIORITY_FLOW_CONTROL')
+        assert pfc == getBitMaskStr(pfc_queues)
+
+        # Disable PFC on last TC
+        pfc_queues = [ ]
+        setPortPfc(dvs, port_name, pfc_queues)
+        # Verify default PFC is set to configured value
+        pfc = getPortAttr(dvs, port_oid, 'SAI_PORT_ATTR_PRIORITY_FLOW_CONTROL')
+        assert pfc == getBitMaskStr(pfc_queues)
 
 # Add Dummy always-pass test at end as workaroud
 # for issue when Flaky fail on final test it invokes module tear-down before retrying