net: Add a software TSO helper API

Although the implementation probably needs a lot of work, this initial API
allows to implement software TSO in mvneta and mv643xx_eth drivers in a not
so intrusive way.
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tso.h

+#ifndef _TSO_H
+#define _TSO_H
+
+#include <net/ip.h>
+
+struct tso_t {
+	int next_frag_idx;
+	void *data;
+	size_t size;
+	u16 ip_id;
+	u32 tcp_seq;
+};
+
+int tso_count_descs(struct sk_buff *skb);
+void tso_build_hdr(struct sk_buff *skb, char *hdr, struct tso_t *tso,
+		   int size, bool is_last);
+void tso_build_data(struct sk_buff *skb, struct tso_t *tso, int size);
+void tso_start(struct sk_buff *skb, struct tso_t *tso);
+
+#endif	/* _TSO_H */

net/core/Makefile

@@ -9,7 +9,7 @@ obj-$(CONFIG_SYSCTL) += sysctl_net_core.o
 
 obj-y		     += dev.o ethtool.o dev_addr_lists.o dst.o netevent.o \
 			neighbour.o rtnetlink.o utils.o link_watch.o filter.o \
-			sock_diag.o dev_ioctl.o
+			sock_diag.o dev_ioctl.o tso.o
 
 obj-$(CONFIG_XFRM) += flow.o
 obj-y += net-sysfs.o

net/core/tso.c

+#include <net/ip.h>
+#include <net/tso.h>
+
+/* Calculate expected number of TX descriptors */
+int tso_count_descs(struct sk_buff *skb)
+{
+	/* The Marvell Way */
+	return skb_shinfo(skb)->gso_segs * 2 + skb_shinfo(skb)->nr_frags;
+}
+
+void tso_build_hdr(struct sk_buff *skb, char *hdr, struct tso_t *tso,
+		   int size, bool is_last)
+{
+	struct iphdr *iph;
+	struct tcphdr *tcph;
+	int hdr_len = skb_transport_offset(skb) + tcp_hdrlen(skb);
+	int mac_hdr_len = skb_network_offset(skb);
+
+	memcpy(hdr, skb->data, hdr_len);
+	iph = (struct iphdr *)(hdr + mac_hdr_len);
+	iph->id = htons(tso->ip_id);
+	iph->tot_len = htons(size + hdr_len - mac_hdr_len);
+	tcph = (struct tcphdr *)(hdr + skb_transport_offset(skb));
+	tcph->seq = htonl(tso->tcp_seq);
+	tso->ip_id++;
+
+	if (!is_last) {
+		/* Clear all special flags for not last packet */
+		tcph->psh = 0;
+		tcph->fin = 0;
+		tcph->rst = 0;
+	}
+}
+
+void tso_build_data(struct sk_buff *skb, struct tso_t *tso, int size)
+{
+	tso->tcp_seq += size;
+	tso->size -= size;
+	tso->data += size;
+
+	if ((tso->size == 0) &&
+	    (tso->next_frag_idx < skb_shinfo(skb)->nr_frags)) {
+		skb_frag_t *frag = &skb_shinfo(skb)->frags[tso->next_frag_idx];
+
+		/* Move to next segment */
+		tso->size = frag->size;
+		tso->data = page_address(frag->page.p) + frag->page_offset;
+		tso->next_frag_idx++;
+	}
+}
+
+void tso_start(struct sk_buff *skb, struct tso_t *tso)
+{
+	int hdr_len = skb_transport_offset(skb) + tcp_hdrlen(skb);
+
+	tso->ip_id = ntohs(ip_hdr(skb)->id);
+	tso->tcp_seq = ntohl(tcp_hdr(skb)->seq);
+	tso->next_frag_idx = 0;
+
+	/* Build first data */
+	tso->size = skb_headlen(skb) - hdr_len;
+	tso->data = skb->data + hdr_len;
+	if ((tso->size == 0) &&
+	    (tso->next_frag_idx < skb_shinfo(skb)->nr_frags)) {
+		skb_frag_t *frag = &skb_shinfo(skb)->frags[tso->next_frag_idx];
+
+		/* Move to next segment */
+		tso->size = frag->size;
+		tso->data = page_address(frag->page.p) + frag->page_offset;
+		tso->next_frag_idx++;
+	}
+}