[PATCH iwl-net v4 12/13] ice: skip reading Tx ready bitmap on ports with no timestamps

On E82x devices, the interrupt for Tx timestamps are handled by the clock
owner. When an interrupt with the Tx timestamp cause is fired, the clock
owner PF iterates the list of ports and checks for timestamps across all
ports.

The existing logic reads the PHY timestamp ready bitmap before iterating
the list of in-use timestamp indexes, even for ports which have no
timestamps waiting in the software timestamp tracker. This has a
significant and measurable latency impact on reporting Tx timestamps.

Check the bitmap and exit early in the event that there are no timestamps
waiting on a port. Observant reviewers may notice that the check is done
without acquiring the lock. This is fine, as whether the thread sees or
fails to see a new outstanding timestamp does not affect correctness, only
determining whether or not it should do extra work.

Skipping the check has the highest impact on E822 an E825 devices which
iterate all ports in a single thread, but it is applied universally to all
device types since the extra read is unnecessary regardless.

The average latency of a Tx timestamp is impacted by several factors
including system load, the number of timestamp requests, and some random
factors that are difficult to control. However for comparison on my system
with these changes, while operating ptp4l on a single port with a sync rate
of 16/second:

  Before: mean 317.43 microseconds, standard deviation 34.51
  After: mean 189.71 microseconds, standard deviation 43.24

Of course this level of improvement may not be indicative of a production
setup as one might expect timestamps to be operating out of multiple ports
on the device. However, even in that case an improvement is still likely
depending on the actual timestamp rates.

Fixes: d938a8cca88a ("ice: Auxbus devices & driver for E822 TS")
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
---
 drivers/net/ethernet/intel/ice/ice_ptp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/ice/ice_ptp.c b/drivers/net/ethernet/intel/ice/ice_ptp.c
index 309fa2877cc3..b0ea01cfcc4f 100644
--- a/drivers/net/ethernet/intel/ice/ice_ptp.c
+++ b/drivers/net/ethernet/intel/ice/ice_ptp.c

@@ -574,7 +574,7 @@ static void ice_ptp_process_tx_tstamp(struct ice_ptp_tx *tx)
 	pf = ptp_port_to_pf(ptp_port);
 	hw = &pf->hw;
 
-	if (!tx->init)
+	if (!tx->init || bitmap_empty(tx->in_use, tx->len))
 		return;
 
 	/* Read the Tx ready status first */

-- 
2.56.0.212.g129ede20d986