cxgb4: Add comment for calculate tx flits and sge length code

Add comment for tx filt and sge length calucaltion code, also remove
a hardcoded value

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/chelsio/cxgb4/sge.c b/drivers/net/ethernet/chelsio/cxgb4/sge.c
index 6af4bab6059c0eaca83d4c30f0a6992629210a26..1a31e40f0c2de7454879d5a2682b6bf616b79cb5 100644 (file)
--- a/drivers/net/ethernet/chelsio/cxgb4/sge.c
+++ b/drivers/net/ethernet/chelsio/cxgb4/sge.c
@@ -730,6 +730,22 @@ static void *alloc_ring(struct device *dev, size_t nelem, size_t elem_size,
  */
 static inline unsigned int sgl_len(unsigned int n)
 {
+       /* A Direct Scatter Gather List uses 32-bit lengths and 64-bit PCI DMA
+        * addresses.  The DSGL Work Request starts off with a 32-bit DSGL
+        * ULPTX header, then Length0, then Address0, then, for 1 <= i <= N,
+        * repeated sequences of { Length[i], Length[i+1], Address[i],
+        * Address[i+1] } (this ensures that all addresses are on 64-bit
+        * boundaries).  If N is even, then Length[N+1] should be set to 0 and
+        * Address[N+1] is omitted.
+        *
+        * The following calculation incorporates all of the above.  It's
+        * somewhat hard to follow but, briefly: the "+2" accounts for the
+        * first two flits which include the DSGL header, Length0 and
+        * Address0; the "(3*(n-1))/2" covers the main body of list entries (3
+        * flits for every pair of the remaining N) +1 if (n-1) is odd; and
+        * finally the "+((n-1)&1)" adds the one remaining flit needed if
+        * (n-1) is odd ...
+        */
        n--;
        return (3 * n) / 2 + (n & 1) + 2;
 }
@@ -777,12 +793,30 @@ static inline unsigned int calc_tx_flits(const struct sk_buff *skb)
        unsigned int flits;
        int hdrlen = is_eth_imm(skb);
 
+       /* If the skb is small enough, we can pump it out as a work request
+        * with only immediate data.  In that case we just have to have the
+        * TX Packet header plus the skb data in the Work Request.
+        */
+
        if (hdrlen)
                return DIV_ROUND_UP(skb->len + hdrlen, sizeof(__be64));
 
+       /* Otherwise, we're going to have to construct a Scatter gather list
+        * of the skb body and fragments.  We also include the flits necessary
+        * for the TX Packet Work Request and CPL.  We always have a firmware
+        * Write Header (incorporated as part of the cpl_tx_pkt_lso and
+        * cpl_tx_pkt structures), followed by either a TX Packet Write CPL
+        * message or, if we're doing a Large Send Offload, an LSO CPL message
+        * with an embedded TX Packet Write CPL message.
+        */
        flits = sgl_len(skb_shinfo(skb)->nr_frags + 1) + 4;
        if (skb_shinfo(skb)->gso_size)
-               flits += 2;
+               flits += (sizeof(struct fw_eth_tx_pkt_wr) +
+                         sizeof(struct cpl_tx_pkt_lso_core) +
+                         sizeof(struct cpl_tx_pkt_core)) / sizeof(__be64);
+       else
+               flits += (sizeof(struct fw_eth_tx_pkt_wr) +
+                         sizeof(struct cpl_tx_pkt_core)) / sizeof(__be64);
        return flits;
 }