Rx buffer and loads DMA map. Also add a function
fxp_discard_rfabuf that handles reusing Rx buffer/DMA map. With
this change fxp_add_rfabuf just handles appending a new RFA to
existing chain.
o Initialize mbuf length in fxp_new_rfabuf.
o Don't reset rnr and have fxp(4) handle received frames even if
it couldn't allocate new Rx buffer. This will make fxp(4) reload
updated RFA under rnr case. The rnr would still be reset to 0 if
polling is active and fxp(4) processed number of allowed Rx
events.
o Update if_iqdrops if fxp(4) couldn't allocate Rx buffer.
Previously fxp(4) used to try to reuse Rx buffer when new buffer
allocation is failed. But fxp(4) didn't take into account loaded
DMA map such that the same DMA map was loaded again without
unloading the map. There is no reason to unload the loaded map and
reload the same map again, just reusing the map is enough. I
believe the spare DMA map in softc was introduced to implement this
behaviour. Also fxp(4) used to stop Rx processing if once Rx buffer
allocation or DMA map load fails which in turn resulted in losing
incoming frames under heavy network load. With this change fxp(4)
should survive from resource shortage condition.
8505042178