SPRUIU1C July 2020 – February 2024 DRA821U , DRA821U-Q1
The Top of Receive FIFO Rule sets PFC triggers on the associated receive port for all priorities. This rule requires no configuration and is enabled when PFC flow control is enabled. The Top of Receive FIFO rule triggers when a received packet cannot be transferred to an intended destination port due to insufficient space in that destination port transmit FIFO. The receive FIFO exists to accept flow control runout (especially for 802.3 flow control). The trigger condition for this rule is cleared when the receive FIFO is emptied after runout. Preferable PFC configurations preclude this rule from triggering. If a lower priority packet triggers this rule then higher priority packets could be in the runout after the lower priority packet(s), which would increase the switch latency of the higher priority packets. This rule ensures that there is no packet loss on any priority in any configuration. However, PFC configuration should ensure that this rule is triggered only by high priority packets or preferably not at all. The [27] TORF bit in the CPSW_PN_MAC_STATUS_REG_k register indicates that this rule has triggered. The CPSW_PN_MAC_STATUS_REG_k[26-24] TORF_PRI field indicates the lowest priority that has triggered this rule since the last clearing of the status register fields. More aggressive shutdown of lower priorities in the other rules is preferable to having this rule trigger.