stmmac: first frame prep at the end of xmit routine

This patch is to fill the first descriptor just before granting
the DMA engine so at the end of the xmit.
The patch takes care about the algorithm adopted to mitigate the
interrupts, then it fixes the last segment in case of no fragments.
Moreover, this new implementation does not pass any "ter" field when
prepare the descriptors because this is not necessary.
The patch also details the memory barrier in the xmit.

As final results, this patch guarantees the same performances
but fixing a case if small datagram are sent. In fact, this
kind of test is impacted if no coalesce is done.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 66 insertions, 48 deletions

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 2e4c10a..90b2612 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -1942,12 +1942,12 @@ static int stmmac_release(struct net_device *dev)
 static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct stmmac_priv *priv = netdev_priv(dev);
-	int entry;
+	unsigned int nopaged_len = skb_headlen(skb);
 	int i, csum_insertion = 0, is_jumbo = 0;
 	int nfrags = skb_shinfo(skb)->nr_frags;
+	unsigned int entry, first_entry;
 	struct dma_desc *desc, *first;
-	unsigned int nopaged_len = skb_headlen(skb);
-	unsigned int enh_desc = priv->plat->enh_desc;
+	unsigned int enh_desc;
 
 	spin_lock(&priv->tx_lock);
 
@@ -1965,34 +1965,25 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		stmmac_disable_eee_mode(priv);
 
 	entry = priv->cur_tx;
-
+	first_entry = entry;
 
 	csum_insertion = (skb->ip_summed == CHECKSUM_PARTIAL);
 
-	if (priv->extend_desc)
+	if (likely(priv->extend_desc))
 		desc = (struct dma_desc *)(priv->dma_etx + entry);
 	else
 		desc = priv->dma_tx + entry;
 
 	first = desc;
 
+	priv->tx_skbuff[first_entry] = skb;
+
+	enh_desc = priv->plat->enh_desc;
 	/* To program the descriptors according to the size of the frame */
 	if (enh_desc)
 		is_jumbo = priv->hw->mode->is_jumbo_frm(skb->len, enh_desc);
 
-	if (likely(!is_jumbo)) {
-		desc->des2 = dma_map_single(priv->device, skb->data,
-					    nopaged_len, DMA_TO_DEVICE);
-		if (dma_mapping_error(priv->device, desc->des2))
-			goto dma_map_err;
-		priv->tx_skbuff_dma[entry].buf = desc->des2;
-		priv->tx_skbuff_dma[entry].len = nopaged_len;
-		/* do not set the own at this stage */
-		priv->hw->desc->prepare_tx_desc(desc, 1, nopaged_len,
-						csum_insertion, priv->mode, 0,
-						nfrags == 0);
-	} else {
-		desc = first;
+	if (unlikely(is_jumbo)) {
 		entry = priv->hw->mode->jumbo_frm(priv, skb, csum_insertion);
 		if (unlikely(entry < 0))
 			goto dma_map_err;
@@ -2003,10 +1994,9 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		int len = skb_frag_size(frag);
 		bool last_segment = (i == (nfrags - 1));
 
-		priv->tx_skbuff[entry] = NULL;
 		entry = STMMAC_GET_ENTRY(entry, DMA_TX_SIZE);
 
-		if (priv->extend_desc)
+		if (likely(priv->extend_desc))
 			desc = (struct dma_desc *)(priv->dma_etx + entry);
 		else
 			desc = priv->dma_tx + entry;
@@ -2016,41 +2006,25 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		if (dma_mapping_error(priv->device, desc->des2))
 			goto dma_map_err; /* should reuse desc w/o issues */
 
+		priv->tx_skbuff[entry] = NULL;
 		priv->tx_skbuff_dma[entry].buf = desc->des2;
 		priv->tx_skbuff_dma[entry].map_as_page = true;
 		priv->tx_skbuff_dma[entry].len = len;
+		priv->tx_skbuff_dma[entry].last_segment = last_segment;
+
+		/* Prepare the descriptor and set the own bit too */
 		priv->hw->desc->prepare_tx_desc(desc, 0, len, csum_insertion,
 						priv->mode, 1, last_segment);
-		priv->tx_skbuff_dma[entry].last_segment = last_segment;
 	}
 
-	priv->tx_skbuff[entry] = skb;
-
-	/* According to the coalesce parameter the IC bit for the latest
-	 * segment could be reset and the timer re-started to invoke the
-	 * stmmac_tx function. This approach takes care about the fragments.
-	 */
-	priv->tx_count_frames += nfrags + 1;
-	if (priv->tx_coal_frames > priv->tx_count_frames) {
-		priv->hw->desc->clear_tx_ic(desc);
-		priv->xstats.tx_reset_ic_bit++;
-		mod_timer(&priv->txtimer,
-			  STMMAC_COAL_TIMER(priv->tx_coal_timer));
-	} else
-		priv->tx_count_frames = 0;
-
-	/* To avoid raise condition */
-	priv->hw->desc->set_tx_owner(first);
-	wmb();
-
 	entry = STMMAC_GET_ENTRY(entry, DMA_TX_SIZE);
 
 	priv->cur_tx = entry;
 
 	if (netif_msg_pktdata(priv)) {
-		pr_debug("%s: curr %d dirty=%d entry=%d, first=%p, nfrags=%d",
-			__func__, (priv->cur_tx % DMA_TX_SIZE),
-			(priv->dirty_tx % DMA_TX_SIZE), entry, first, nfrags);
+		pr_debug("%s: curr=%d dirty=%d f=%d, e=%d, first=%p, nfrags=%d",
+			 __func__, priv->cur_tx, priv->dirty_tx, first_entry,
+			 entry, first, nfrags);
 
 		if (priv->extend_desc)
 			stmmac_display_ring((void *)priv->dma_etx,
@@ -2062,6 +2036,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		pr_debug(">>> frame to be transmitted: ");
 		print_pkt(skb->data, skb->len);
 	}
+
 	if (unlikely(stmmac_tx_avail(priv) <= (MAX_SKB_FRAGS + 1))) {
 		if (netif_msg_hw(priv))
 			pr_debug("%s: stop transmitted packets\n", __func__);
@@ -2070,16 +2045,59 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	dev->stats.tx_bytes += skb->len;
 
-	if (unlikely((skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) &&
-		     priv->hwts_tx_en)) {
-		/* declare that device is doing timestamping */
-		skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
-		priv->hw->desc->enable_tx_timestamp(first);
+	/* According to the coalesce parameter the IC bit for the latest
+	 * segment is reset and the timer re-started to clean the tx status.
+	 * This approach takes care about the fragments: desc is the first
+	 * element in case of no SG.
+	 */
+	priv->tx_count_frames += nfrags + 1;
+	if (likely(priv->tx_coal_frames > priv->tx_count_frames)) {
+		mod_timer(&priv->txtimer,
+			  STMMAC_COAL_TIMER(priv->tx_coal_timer));
+	} else {
+		priv->tx_count_frames = 0;
+		priv->hw->desc->set_tx_ic(desc);
+		priv->xstats.tx_set_ic_bit++;
 	}
 
 	if (!priv->hwts_tx_en)
 		skb_tx_timestamp(skb);
 
+	/* Ready to fill the first descriptor and set the OWN bit w/o any
+	 * problems because all the descriptors are actually ready to be
+	 * passed to the DMA engine.
+	 */
+	if (likely(!is_jumbo)) {
+		bool last_segment = (nfrags == 0);
+
+		first->des2 = dma_map_single(priv->device, skb->data,
+					     nopaged_len, DMA_TO_DEVICE);
+		if (dma_mapping_error(priv->device, first->des2))
+			goto dma_map_err;
+
+		priv->tx_skbuff_dma[first_entry].buf = first->des2;
+		priv->tx_skbuff_dma[first_entry].len = nopaged_len;
+		priv->tx_skbuff_dma[first_entry].last_segment = last_segment;
+
+		if (unlikely((skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) &&
+			     priv->hwts_tx_en)) {
+			/* declare that device is doing timestamping */
+			skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
+			priv->hw->desc->enable_tx_timestamp(first);
+		}
+
+		/* Prepare the first descriptor setting the OWN bit too */
+		priv->hw->desc->prepare_tx_desc(first, 1, nopaged_len,
+						csum_insertion, priv->mode, 1,
+						last_segment);
+
+		/* The own bit must be the latest setting done when prepare the
+		 * descriptor and then barrier is needed to make sure that
+		 * all is coherent before granting the DMA engine.
+		 */
+		smp_wmb();
+	}
+
 	netdev_sent_queue(dev, skb->len);
 	priv->hw->dma->enable_dma_transmission(priv->ioaddr);