Created attachment 225638 [details]
In gen_encap() #n1053 it always calls bus_dmamap_load_mbuf_sg() into 'map' (which is the current tx_queue). If the tx_queue is full, it will load with a 'map' that already has a currently active mapping.
bus_dmamap_load: "map A DMA map without a currently active mapping."
Also "(if nsegs == 0)" #n1077 shouldn't bus_dmamap_unload() be called as bus_dmamap_load_mbuf_sq() was successful?
See https://cgit.freebsd.org/src/tree/sys/arm64/broadcom/genet/if_genet.c?id=f66a1f40740c63741b6ebe48cb0cbce9e52ef34e for line number references.
Attached is diff with a potential fix.
Checking for a full queue and returning ENOMEM will allow gen_start_locked() to set the IFF_DRV_OACTIVE faster without having to needlessly check if the mbuf will fit (it won't).