[dpdk-dev] [RFC 8/9] net/e1000: optimize rx queue descriptor count

[Olivier Matz]

Use a binary search algorithm to find the first empty DD bit. The
ring-empty and ring-full cases are managed separately as they are more
likely to happen.

PR=52423
Signed-off-by: Olivier Matz <olivier.matz at 6wind.com>
Acked-by: Ivan Boule <ivan.boule at 6wind.com>
---
 drivers/net/e1000/em_rxtx.c | 55 +++++++++++++++++++++++++++++++++------------
 1 file changed, 41 insertions(+), 14 deletions(-)

diff --git a/drivers/net/e1000/em_rxtx.c b/drivers/net/e1000/em_rxtx.c
index c1c724b..a469fd7 100644
--- a/drivers/net/e1000/em_rxtx.c
+++ b/drivers/net/e1000/em_rxtx.c
@@ -1385,24 +1385,51 @@ eth_em_rx_queue_setup(struct rte_eth_dev *dev,
 uint32_t
 eth_em_rx_queue_count(struct rte_eth_dev *dev, uint16_t rx_queue_id)
 {
-#define EM_RXQ_SCAN_INTERVAL 4
-	volatile struct e1000_rx_desc *rxdp;
+	volatile uint8_t *status;
 	struct em_rx_queue *rxq;
-	uint32_t desc = 0;
+	uint32_t offset, interval, resolution;
+	int32_t idx;
 
 	rxq = dev->data->rx_queues[rx_queue_id];
-	rxdp = &(rxq->rx_ring[rxq->rx_tail]);
-
-	while ((desc < rxq->nb_rx_desc) &&
-		(rxdp->status & E1000_RXD_STAT_DD)) {
-		desc += EM_RXQ_SCAN_INTERVAL;
-		rxdp += EM_RXQ_SCAN_INTERVAL;
-		if (rxq->rx_tail + desc >= rxq->nb_rx_desc)
-			rxdp = &(rxq->rx_ring[rxq->rx_tail +
-				desc - rxq->nb_rx_desc]);
-	}
 
-	return desc;
+	/* check if ring empty */
+	idx = rxq->rx_tail;
+	status = &rxq->rx_ring[idx].status;
+	if (!(*status & E1000_RXD_STAT_DD))
+		return 0;
+
+	/* decrease the precision if ring is large */
+	if (rxq->nb_rx_desc <= 256)
+		resolution = 4;
+	else
+		resolution = 16;
+
+	/* check if ring full */
+	idx = rxq->rx_tail - rxq->nb_rx_hold - resolution;
+	if (idx < 0)
+		idx += rxq->nb_rx_desc;
+	status = &rxq->rx_ring[idx].status;
+	if (*status & E1000_RXD_STAT_DD)
+		return rxq->nb_rx_desc;
+
+	/* use a binary search */
+	interval = (rxq->nb_rx_desc - rxq->nb_rx_hold) >> 1;
+	offset = interval;
+
+	do {
+		idx = rxq->rx_tail + offset;
+		if (idx >= rxq->nb_rx_desc)
+			idx -= rxq->nb_rx_desc;
+
+		interval >>= 1;
+		status = &rxq->rx_ring[idx].status;
+		if (*status & E1000_RXD_STAT_DD)
+			offset += interval;
+		else
+			offset -= interval;
+	} while (interval >= resolution);
+
+	return offset;
 }
 
 int
-- 
2.8.1