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, 19 insertions, 0 deletions

diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.h b/drivers/net/ethernet/intel/i40e/i40e_txrx.h
index 5dbc958293f7..56009709528a 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_txrx.h
+++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.h
@@ -337,6 +337,7 @@ int i40e_tx_prepare_vlan_flags(struct sk_buff *skb,
 void i40e_force_wb(struct i40e_vsi *vsi, struct i40e_q_vector *q_vector);
 u32 i40e_get_tx_pending(struct i40e_ring *ring, bool in_sw);
 int __i40e_maybe_stop_tx(struct i40e_ring *tx_ring, int size);
+bool __i40e_chk_linearize(struct sk_buff *skb);
 
 /**
  * i40e_get_head - Retrieve head from head writeback
@@ -392,4 +393,22 @@ static inline int i40e_maybe_stop_tx(struct i40e_ring *tx_ring, int size)
 		return 0;
 	return __i40e_maybe_stop_tx(tx_ring, size);
 }
+
+/**
+ * i40e_chk_linearize - Check if there are more than 8 fragments per packet
+ * @skb:      send buffer
+ * @count:    number of buffers used
+ *
+ * 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 inline bool i40e_chk_linearize(struct sk_buff *skb, int count)
+{
+	/* we can only support up to 8 data buffers for a single send */
+	if (likely(count <= I40E_MAX_BUFFER_TXD))
+		return false;
+
+	return __i40e_chk_linearize(skb);
+}
 #endif /* _I40E_TXRX_H_ */