On Tue, May 2, 2017 at 6:10 AM, Miroslav Lichvar [off-list ref] wrote:

Since commit b68581778cd0 ("net: Make skb->skb_iif always track
skb->dev") skbs don't have the original index of the interface which
received the packet. This information is now needed for a new control
message related to hardware timestamping.

Instead of adding a new field to skb, we can find the device by the NAPI
ID if it is available, i.e. CONFIG_NET_RX_BUSY_POLL is enabled and the
driver is using NAPI. Add dev_get_by_napi_id() and also skb_napi_id() to
hide the CONFIG_NET_RX_BUSY_POLL ifdef.

CC: Richard Cochran <richardcochran@gmail.com>
CC: Willem de Bruijn <willemb@google.com>
Suggested-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Miroslav Lichvar <redacted>

 /**
+ *     dev_get_by_napi_id - find a device by napi_id
+ *     @napi_id: ID of the NAPI struct
+ *
+ *     Search for an interface by NAPI ID. Returns %NULL if the device
+ *     is not found or a pointer to the device. The device has not had
+ *     its reference counter increased so the caller must be careful
+ *     about locking. The caller must hold RCU lock.

Instead of a comment, can check with

  WARN_ON_ONCE(!rcu_read_lock_held());

+ */
+
+struct net_device *dev_get_by_napi_id(unsigned int napi_id)
+{
+       struct napi_struct *napi;
+
+       if (napi_id < MIN_NAPI_ID)
+               return NULL;
+
+       napi = napi_by_id(napi_id);
+       if (napi)
+               return napi->dev;
+
+       return NULL;

make return NULL the branch expected not taken.

  return napi ? napi->dev : NULL;