[PATCH net-next 4/9] net: dsa: Keep private info in the skb->cb

[Vladimir Oltean]

Map a DSA structure over the 48-byte control block that will hold
persistent skb info on transmit and receive.

Also add a DSA_SKB_CB_PRIV() macro which retrieves a pointer to the
space up to 48 bytes that the DSA structure does not use. This space can
be used for drivers to add their own private info.

One use is for the PTP timestamping code path. When cloning a skb,
annotate the original with a pointer to the clone, which the driver can
then find easily and place the timestamp to. This avoids the need of a
separate queue to hold clones and a way to match an original to a cloned
skb.

Signed-off-by: Vladimir Oltean <olteanv@xxxxxxxxx>
---
 include/net/dsa.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/include/net/dsa.h b/include/net/dsa.h
index c90ceeec7d1f..8f5fcec30b13 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -83,6 +83,34 @@ struct dsa_device_ops {
 #define MODULE_ALIAS_DSA_TAG_DRIVER(__proto)				\
 	MODULE_ALIAS(DSA_TAG_DRIVER_ALIAS __stringify(__proto##_VALUE))
 
+struct dsa_skb_cb {
+	struct sk_buff *clone;
+};
+
+struct __dsa_skb_cb {
+	struct dsa_skb_cb cb;
+	u8 priv[48 - sizeof(struct dsa_skb_cb)];
+};
+
+#define __DSA_SKB_CB(skb) ((struct __dsa_skb_cb *)((skb)->cb))
+
+#define DSA_SKB_CB(skb) ((struct dsa_skb_cb *)((skb)->cb))
+
+#define DSA_SKB_CB_COPY(nskb, skb) \
+	{ *__DSA_SKB_CB(nskb) = *__DSA_SKB_CB(skb); }
+
+#define DSA_SKB_CB_ZERO(skb) \
+	{ *__DSA_SKB_CB(skb) = (struct __dsa_skb_cb) {0}; }
+
+#define DSA_SKB_CB_PRIV(skb) \
+	((void *)(skb)->cb + offsetof(struct __dsa_skb_cb, priv))
+
+#define DSA_SKB_CB_CLONE(clone, skb) \
+	{ \
+		DSA_SKB_CB_COPY(clone, skb); \
+		DSA_SKB_CB(skb)->clone = clone; \
+	}
+
 struct dsa_switch_tree {
 	struct list_head	list;
 
-- 
2.17.1