i40e/i40evf: Rewrite logic for 8 descriptor per packet check

This patch is meant to rewrite the logic for how we determine if we can
transmit the frame or if it needs to be linearized.

The previous code for this function was using a mix of division and modulus
division as a part of computing if we need to take the slow path.  Instead
I have replaced this by simply working with a sliding window which will
tell us if the frame would be capable of causing a single packet to span
several descriptors.

The logic for the scan is fairly simple.  If any given group of 6 fragments
is less than gso_size - 1 then it is possible for us to have one byte
coming out of the first fragment, 6 fragments, and one or more bytes coming
out of the last fragment.  This gives us a total of 8 fragments
which exceeds what we can allow so we send such frames to be linearized.

Arguably the use of modulus might be more exact as the approach I propose
may generate some false positives.  However the likelihood of us taking much
of a hit for those false positives is fairly low, and I would rather not
add more overhead in the case where we are receiving a frame composed of 4K
pages.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

1 files changed, 59 insertions, 46 deletions

diff --git a/drivers/net/ethernet/intel/i40evf/i40e_txrx.c b/drivers/net/ethernet/intel/i40evf/i40e_txrx.c
index 9f38127bdc11..2369db58cdb1 100644
--- a/drivers/net/ethernet/intel/i40evf/i40e_txrx.c
+++ b/drivers/net/ethernet/intel/i40evf/i40e_txrx.c
@@ -1796,59 +1796,71 @@ static void i40e_create_tx_ctx(struct i40e_ring *tx_ring,
 }
 
 /**
- * i40e_chk_linearize - Check if there are more than 8 fragments per packet
+ * __i40evf_chk_linearize - Check if there are more than 8 fragments per packet
  * @skb:      send buffer
- * @tx_flags: collected send information
  *
  * Note: Our HW can't scatter-gather more than 8 fragments to build
  * a packet on the wire and so we need to figure out the cases where we
  * need to linearize the skb.
  **/
-static bool i40e_chk_linearize(struct sk_buff *skb, u32 tx_flags)
+bool __i40evf_chk_linearize(struct sk_buff *skb)
 {
-	struct skb_frag_struct *frag;
-	bool linearize = false;
-	unsigned int size = 0;
-	u16 num_frags;
-	u16 gso_segs;
+	const struct skb_frag_struct *frag, *stale;
+	int gso_size, nr_frags, sum;
 
-	num_frags = skb_shinfo(skb)->nr_frags;
-	gso_segs = skb_shinfo(skb)->gso_segs;
+	/* check to see if TSO is enabled, if so we may get a repreive */
+	gso_size = skb_shinfo(skb)->gso_size;
+	if (unlikely(!gso_size))
+		return true;
 
-	if (tx_flags & (I40E_TX_FLAGS_TSO | I40E_TX_FLAGS_FSO)) {
-		u16 j = 0;
+	/* no need to check if number of frags is less than 8 */
+	nr_frags = skb_shinfo(skb)->nr_frags;
+	if (nr_frags < I40E_MAX_BUFFER_TXD)
+		return false;
 
-		if (num_frags < (I40E_MAX_BUFFER_TXD))
-			goto linearize_chk_done;
-		/* try the simple math, if we have too many frags per segment */
-		if (DIV_ROUND_UP((num_frags + gso_segs), gso_segs) >
-		    I40E_MAX_BUFFER_TXD) {
-			linearize = true;
-			goto linearize_chk_done;
-		}
-		frag = &skb_shinfo(skb)->frags[0];
-		/* we might still have more fragments per segment */
-		do {
-			size += skb_frag_size(frag);
-			frag++; j++;
-			if ((size >= skb_shinfo(skb)->gso_size) &&
-			    (j < I40E_MAX_BUFFER_TXD)) {
-				size = (size % skb_shinfo(skb)->gso_size);
-				j = (size) ? 1 : 0;
-			}
-			if (j == I40E_MAX_BUFFER_TXD) {
-				linearize = true;
-				break;
-			}
-			num_frags--;
-		} while (num_frags);
-	} else {
-		if (num_frags >= I40E_MAX_BUFFER_TXD)
-			linearize = true;
+	/* We need to walk through the list and validate that each group
+	 * of 6 fragments totals at least gso_size.  However we don't need
+	 * to perform such validation on the first or last 6 since the first
+	 * 6 cannot inherit any data from a descriptor before them, and the
+	 * last 6 cannot inherit any data from a descriptor after them.
+	 */
+	nr_frags -= I40E_MAX_BUFFER_TXD - 1;
+	frag = &skb_shinfo(skb)->frags[0];
+
+	/* Initialize size to the negative value of gso_size minus 1.  We
+	 * use this as the worst case scenerio in which the frag ahead
+	 * of us only provides one byte which is why we are limited to 6
+	 * descriptors for a single transmit as the header and previous
+	 * fragment are already consuming 2 descriptors.
+	 */
+	sum = 1 - gso_size;
+
+	/* Add size of frags 1 through 5 to create our initial sum */
+	sum += skb_frag_size(++frag);
+	sum += skb_frag_size(++frag);
+	sum += skb_frag_size(++frag);
+	sum += skb_frag_size(++frag);
+	sum += skb_frag_size(++frag);
+
+	/* Walk through fragments adding latest fragment, testing it, and
+	 * then removing stale fragments from the sum.
+	 */
+	stale = &skb_shinfo(skb)->frags[0];
+	for (;;) {
+		sum += skb_frag_size(++frag);
+
+		/* if sum is negative we failed to make sufficient progress */
+		if (sum < 0)
+			return true;
+
+		/* use pre-decrement to avoid processing last fragment */
+		if (!--nr_frags)
+			break;
+
+		sum -= skb_frag_size(++stale);
 	}
 
-linearize_chk_done:
-	return linearize;
+	return false;
 }
 
 /**
@@ -2095,6 +2107,12 @@ static netdev_tx_t i40e_xmit_frame_ring(struct sk_buff *skb,
 	prefetch(skb->data);
 
 	count = i40e_xmit_descriptor_count(skb);
+	if (i40e_chk_linearize(skb, count)) {
+		if (__skb_linearize(skb))
+			goto out_drop;
+		count = TXD_USE_COUNT(skb->len);
+		tx_ring->tx_stats.tx_linearize++;
+	}
 
 	/* need: 1 descriptor per page * PAGE_SIZE/I40E_MAX_DATA_PER_TXD,
 	 *       + 1 desc for skb_head_len/I40E_MAX_DATA_PER_TXD,
@@ -2130,11 +2148,6 @@ static netdev_tx_t i40e_xmit_frame_ring(struct sk_buff *skb,
 	else if (tso)
 		tx_flags |= I40E_TX_FLAGS_TSO;
 
-	if (i40e_chk_linearize(skb, tx_flags)) {
-		if (skb_linearize(skb))
-			goto out_drop;
-		tx_ring->tx_stats.tx_linearize++;
-	}
 	skb_tx_timestamp(skb);
 
 	/* always enable CRC insertion offload */