cxgb4: Add comment for calculate tx flits and sge length code (0aac3f56) · Commits · 戴 / test

drivers/net/ethernet/chelsio/cxgb4/sge.c

+35 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}

Admin message