iflib: Do not ring the transmit doorbell when nothing is pendingHEADmain

For a lightly used ring iflib_txd_db_check() may defer zero descriptors,
so its "pending >= limit" test is true even when nothing has been queued
since the last doorbell.  iflib_txq_drain() calls it before, inside and
after its loop, so a sender that drains its own packet wrote the tail
register three times per packet, twice with the value the hardware
already had.

The log of 81be655266fa ("iflib: ensure that tx interrupts enabled and
cleanups") calls skipping the doorbell when db_pending is zero "an
obvious missing optimization"; the comparison against a limit of zero
defeated it.  vmx(4) and mgb(4) have dropped such repeated requests in
the driver since 2019.  Return early when nothing is pending.

Reviewed by:		gallatin
MFC after:		2 weeks
Sponsored by:		Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D60290

0 files changed, 0 insertions, 0 deletions