[PATCH v3 19/36] net/intel: avoid writing the final pkt descriptor twice

Bruce Richardson
2026-01-30T11:44:29Z

In the scalar datapath, there is a loop to handle multi-segment, and
multi-descriptor packets on Tx. After that loop, the end-of-packet bit
was written to the descriptor separately, meaning that for each
single-descriptor packet there were two writes to the second quad-word -
basically 3 x 64-bit writes rather than just 2. Adjusting the code to
compute the EOP bit inside the loop saves that extra write per packet
and so improves performance.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
drivers/net/intel/common/tx_scalar_fns.h | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/net/intel/common/tx_scalar_fns.h b/drivers/net/intel/common/tx_scalar_fns.h
index 5e301699be..bd8053f58c 100644
--- a/drivers/net/intel/common/tx_scalar_fns.h
+++ b/drivers/net/intel/common/tx_scalar_fns.h
@@ -377,6 +377,10 @@ ci_xmit_pkts(struct ci_tx_queue *txq,
txn = &sw_ring[txe->next_id];
}

+ /* fill the last descriptor with End of Packet (EOP) bit */
+ if (m_seg->next == NULL)
+ td_cmd |= CI_TX_DESC_CMD_EOP;
+
txd->buffer_addr = rte_cpu_to_le_64(buf_dma_addr);
txd->cmd_type_offset_bsz = rte_cpu_to_le_64(CI_TX_DESC_DTYPE_DATA |
((uint64_t)td_cmd << CI_TXD_QW1_CMD_S) |
@@ -389,21 +393,17 @@ ci_xmit_pkts(struct ci_tx_queue *txq,
txe = txn;
m_seg = m_seg->next;
} while (m_seg);
-
- /* fill the last descriptor with End of Packet (EOP) bit */
- td_cmd |= CI_TX_DESC_CMD_EOP;
txq->nb_tx_used = (uint16_t)(txq->nb_tx_used + nb_used);
txq->nb_tx_free = (uint16_t)(txq->nb_tx_free - nb_used);

/* set RS bit on the last descriptor of one packet */
if (txq->nb_tx_used >= txq->tx_rs_thresh) {
- td_cmd |= CI_TX_DESC_CMD_RS;
+ txd->cmd_type_offset_bsz |=
+ rte_cpu_to_le_64(CI_TX_DESC_CMD_RS << CI_TXD_QW1_CMD_S);

/* Update txq RS bit counters */
txq->nb_tx_used = 0;
}
- txd->cmd_type_offset_bsz |=
- rte_cpu_to_le_64(((uint64_t)td_cmd) << CI_TXD_QW1_CMD_S);

if (ts_fns != NULL)
ts_id = ts_fns->write_ts_desc(txq, tx_pkt, tx_id, ts_id);
--
2.51.0