[Bug 256482] [genet] gen_encap() dma loads into active map when 'tx_queue' is full

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256482

            Bug ID: 256482
           Summary: [genet] gen_encap() dma loads into active map when
                    'tx_queue' is full
           Product: Base System
           Version: CURRENT
          Hardware: arm64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: arm
          Assignee: freebsd-arm@FreeBSD.org
          Reporter: ghuckriede@blackberry.com

Created attachment 225638
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=225638&action=edit
Potential Fix

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.

https://www.freebsd.org/cgi/man.cgi?query=busdma&sektion=9
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).

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