Need help parsing dropwatch results

Using iperf3 and dropwatch, I discovered that my EMAC driver is dropping 
packets, a lot of them.  This driver is based on an internal version 
(written by someone else) that does not have this problem, so obviously 
there's a bug in my driver.  Unfortunately, I need help understanding 
where in my driver the bug could be.

dropwatch display this:

3297 drops at net_tx_action+0 (0xffff000008aa1108)
3671 drops at net_tx_action+0 (0xffff000008aa1108)
4055 drops at net_tx_action+0 (0xffff000008aa1108)
3976 drops at net_tx_action+0 (0xffff000008aa1108)
3847 drops at net_tx_action+0 (0xffff000008aa1108)
3933 drops at net_tx_action+0 (0xffff000008aa1108)
3933 drops at net_tx_action+0 (0xffff000008aa1108)
1376 drops at net_tx_action+0 (0xffff000008aa1108)

Can someone tell me what "drops at net_tx_action+0" actually means? How 
does a packet drop at net_tx_action() actually occur?  Where in the 
driver should I look for the problem?

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