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

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Jul 25, 2022 at 04:12:12PM +0800, Chengwen Feng wrote:
> 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 | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/examples/dma/dmafwd.c b/examples/dma/dmafwd.c
> index 67b5a9b22b..e3fe226dff 100644
> --- a/examples/dma/dmafwd.c
> +++ b/examples/dma/dmafwd.c
> @@ -408,7 +408,7 @@ 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 && copy_mode != COPY_MODE_DMA_NUM)
>  			continue;
>  
>  		port_statistics.rx[rx_config->rxtx_port] += nb_rx;

With this change, we would work through the all the receive packet
processing code, and calling all it's functions, just witha packet count of
zero. I therefore wonder if it would be cleaner to do the dma_dequeue
immediately here on receiving zero, and then jumping to handle those
dequeued packets. Something like the diff below.

/Bruce

@@ -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);