ixl: Honor iflib transmit completion batchingHEADmain

ixl uses head writeback by default.  Hardware publishes the transmit
ring head through DMA only after completing a descriptor marked RS.
Marking every packet requested much more frequent head updates than
iflib needs to reclaim descriptors.

iflib marks selected packets with IPI_TX_INTR as completion
checkpoints.  It forces a checkpoint as deferred work or ring pressure
grows.  Retain EOP on every packet, but set RS only at those
checkpoints.  This batches head writebacks while preserving bounded
descriptor reclamation.

The optional descriptor writeback mode benefits as well.  ixl already
recorded only IPI_TX_INTR descriptors in its report-status queue, so
status written for every other packet was not inspected.

DPDK uses the same sparse RS design.  Let iflib choose the adaptive
interval for FreeBSD.  This is a PCIe/memory bandwidth savings.

MFC after:	2 weeks

0 files changed, 0 insertions, 0 deletions