[dpdk-dev] [PATCH 19.11 V3 04/12] net/e1000: fix Tx descriptor status api (em)

[Thierry Herbelot]

From: Olivier Matz <olivier.matz at 6wind.com>

The Tx descriptor status api was not behaving as expected. This API is
used to inspect the content of the descriptors in the Tx ring to
determine the length of the Tx queue.

Since the software advances the tail pointer and the hardware advances
the head pointer, the Tx queue is located before txq->tx_tail in the
ring. Therefore, a call to rte_eth_tx_descriptor_status(..., offset=20)
should inspect the 20th descriptor before the tail, not after.

As before, we still need to take care about only checking descriptors
that have the RS bit.

Additionally, we can avoid an access to the ring if offset is greater or
equal to nb_tx_desc - nb_tx_free.

Fixes: b9082317cab3 ("net/e1000: implement descriptor status API (em)")
Cc: stable at dpdk.org

Signed-off-by: Olivier Matz <olivier.matz at 6wind.com>
---
 drivers/net/e1000/em_rxtx.c | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/drivers/net/e1000/em_rxtx.c b/drivers/net/e1000/em_rxtx.c
index 5925e490641b..3061998c7768 100644
--- a/drivers/net/e1000/em_rxtx.c
+++ b/drivers/net/e1000/em_rxtx.c
@@ -152,6 +152,7 @@ struct em_tx_queue {
 	uint64_t               tx_ring_phys_addr; /**< TX ring DMA address. */
 	struct em_tx_entry    *sw_ring; /**< virtual address of SW ring. */
 	volatile uint32_t      *tdt_reg_addr; /**< Address of TDT register. */
+	volatile uint32_t      *tdh_reg_addr; /**< Address of TDH register. */
 	uint16_t               nb_tx_desc;    /**< number of TX descriptors. */
 	uint16_t               tx_tail;  /**< Current value of TDT register. */
 	/**< Start freeing TX buffers if there are less free descriptors than
@@ -1304,6 +1305,7 @@ eth_em_tx_queue_setup(struct rte_eth_dev *dev,
 	txq->port_id = dev->data->port_id;
 
 	txq->tdt_reg_addr = E1000_PCI_REG_ADDR(hw, E1000_TDT(queue_idx));
+	txq->tdh_reg_addr = E1000_PCI_REG_ADDR(hw, E1000_TDH(queue_idx));
 	txq->tx_ring_phys_addr = tz->iova;
 	txq->tx_ring = (struct e1000_data_desc *) tz->addr;
 
@@ -1557,22 +1559,33 @@ eth_em_tx_descriptor_status(void *tx_queue, uint16_t offset)
 {
 	struct em_tx_queue *txq = tx_queue;
 	volatile uint8_t *status;
-	uint32_t desc;
+	int32_t desc, dd;
 
 	if (unlikely(offset >= txq->nb_tx_desc))
 		return -EINVAL;
+	if (offset >= txq->nb_tx_desc - txq->nb_tx_free)
+		return RTE_ETH_TX_DESC_DONE;
 
-	desc = txq->tx_tail + offset;
-	/* go to next desc that has the RS bit */
-	desc = ((desc + txq->tx_rs_thresh - 1) / txq->tx_rs_thresh) *
-		txq->tx_rs_thresh;
-	if (desc >= txq->nb_tx_desc) {
-		desc -= txq->nb_tx_desc;
-		if (desc >= txq->nb_tx_desc)
-			desc -= txq->nb_tx_desc;
+	desc = txq->tx_tail - offset - 1;
+	if (desc < 0)
+		desc += txq->nb_tx_desc;
+
+	/* offset is too small, no other way than reading PCI reg */
+	if (unlikely(offset < txq->tx_rs_thresh)) {
+		int16_t tx_head, queue_size;
+		tx_head = e1000_read_addr(txq->tdh_reg_addr);
+		queue_size = txq->tx_tail - tx_head;
+		if (queue_size < 0)
+			queue_size += txq->nb_tx_desc;
+		return queue_size > offset ? RTE_ETH_TX_DESC_FULL :
+			RTE_ETH_TX_DESC_DONE;
 	}
 
-	status = &txq->tx_ring[desc].upper.fields.status;
+	/* index of the dd bit to look at */
+	dd = (desc / txq->tx_rs_thresh + 1) * txq->tx_rs_thresh - 1;
+	dd = txq->sw_ring[dd].last_id;
+
+	status = &txq->tx_ring[dd].upper.fields.status;
 	if (*status & E1000_TXD_STAT_DD)
 		return RTE_ETH_TX_DESC_DONE;
 
-- 
2.11.0