[PATCH v2] examples/dma: support DMA dequeue when no packet received

[Chengwen Feng <fengchengwen@huawei.com>]

Currently the example using DMA in asynchronous mode, which are:
	nb_rx = rte_eth_rx_burst();
	if (nb_rx == 0)
		continue;
	...
	dma_enqueue(); // enqueue the received packets copy request
	nb_cpl = dma_dequeue(); // get copy completed packets
	...

There are no waiting inside dma_dequeue(), and this is why it's called
asynchronus. If there are no packet received, it won't call
dma_dequeue(), but some packets may still in the DMA queue which
enqueued in last cycle. As a result, when the traffic is stopped, the
sent packets and received packets are unbalanced from the perspective
of the traffic generator.

The patch supports DMA dequeue when no packet received, it helps to
judge the test result by comparing the sent packets with the received
packets on traffic generator sides.

Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
---
 examples/dma/dmafwd.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/examples/dma/dmafwd.c b/examples/dma/dmafwd.c
index 67b5a9b22b..9fc46f5255 100644
--- a/examples/dma/dmafwd.c
+++ b/examples/dma/dmafwd.c
@@ -408,8 +408,13 @@ dma_rx_port(struct rxtx_port_config *rx_config)
 		nb_rx = rte_eth_rx_burst(rx_config->rxtx_port, i,
 			pkts_burst, MAX_PKT_BURST);
 
-		if (nb_rx == 0)
+		if (nb_rx == 0) {
+			if (copy_mode == COPY_MODE_DMA_NUM &&
+				(nb_rx = dma_dequeue(pkts_burst, pkts_burst_copy,
+					MAX_PKT_BURST, rx_config->dmadev_ids[i])) > 0)
+				goto handle_tx;
 			continue;
+		}
 
 		port_statistics.rx[rx_config->rxtx_port] += nb_rx;
 
@@ -450,6 +455,7 @@ dma_rx_port(struct rxtx_port_config *rx_config)
 					pkts_burst_copy[j]);
 		}
 
+handle_tx:
 		rte_mempool_put_bulk(dma_pktmbuf_pool,
 			(void *)pkts_burst, nb_rx);
 
-- 
2.33.0