RE: [PATCH iwl-net 12/12] ice: don't clear in_use until HW clears ready bitmap

> -----Original Message-----
> From: Keller, Jacob E <[email protected]> 
> Sent: Friday, August 21, 2026 5:13 PM
> To: Intel Wired LAN <[email protected]>
> Cc: [email protected]; Machnikowski, Maciej <[email protected]>; Nguyen, Anthony L <[email protected]>; Korba, Przemyslaw <[email protected]>; Nitka, Grzegorz <[email protected]>; Oros, Petr <[email protected]>; Nowlin, Alexander <[email protected]>; Bross, Kevin <[email protected]>; Cavatur, Ranjit <[email protected]>; Keller, Jacob E <[email protected]>
> Subject: [PATCH iwl-net 12/12] ice: don't clear in_use until HW clears ready bitmap
> 
> During a link down transition, the E825 PHY has a small window where it does not properly respond to reading the PHY timestamp registers. When this occurs, the PHY does not automatically clear the ready bitmap or the valid bit for the timestamp. This begins happening slightly before a link transition even before the firmware has notified the driver of the state change.
> 
> The driver happily completes the timestamp, releasing the in_use bit. This allows another request to reuse the bit potentially reporting an invalid stale timestamp. Additionally, with the ready bit still set high the > driver continues to re-trigger the IRQ and check for timestamps in a tight loop, wasting CPU cycles.
> 
> To fix this, re-read the PHY timestamp memory status after each read of a PHY index. Double check if the hardware cleared the index properly. If it hasn't, mark the timestamp index as stale and skip processing it.
> 
> Stale timestamps are already ignored by the ice_any_port_has_timestamps() function. However, the ice_ptp_tx_tstamps_pending() function also checks the ready bitmap. Instead, modify it to only check the software tracker.
> Additionally, stop re-triggering the interrupt from the IRQ if the timestamp tracker is calibrating or has the link marked as down. Continue to check the hardware ready bitmap from the watchdog to catch cases of unexpected timestamps.
> 
> With these changes, the timestamp processing no longer triggers a repeated spamming of the IRQ during link down events where timestamps get stuck as the PHY transitions to link down. Once link is restored, the PHY will be reset and the stuck timestamps are cleared.
> 
> Measuring CPU utilization of the miscellaneous IRQ thread function during timestamp storms near a link reset shows that this prevents the spikes caused by the "stuck" ready bit. Without this fix, the CPU handling the IRQ becomes slammed due to the IRQ re-triggering logic.
> 
> Measuring latency using the ice Tx timestamp traces does show that this fix comes at a latency cost. Latency is measured using the ice Tx timestamp traces for the request to completion time. I measured a couple of different workloads both before and after this fix:
> 
>  * ptp4l using a profile with ~16 SYNC messages per second
> 
>     before: 159.40 microseconds mean, stdev 45.28
>      after: 182.07 microseconds mean,  stdev 43.43
> 
>  * a C program generating 16 timestamp requests every 10 milliseconds on
>    two different ports:
> 
>     before: 604.35 microseconds mean, stdev 345.32
>      after: 990.13 microseconds mean, stdev 625.64
> 
> In the normal work flows this comes with about a 20 microsecond penalty on the average, and the standard deviation remains approximately the same. For heavy workloads with many more timestamps than expected for typical applications this comes at a significant cost. This is because we handle all timestamps in a single thread. If there are many concurrent timestamps being requested at once, any which use the later slots on ports later in the port list will take much longer to be processed once the interrupt is fired. Since each timestamp now requires an additional PHY register access, this cost is much higher in the case where the device is under unusually heavy load.
> 
> However, *correctness* is more important than speed here. Additionally, we still remain well below the default limit of 10 milliseconds that ptp4l will wait before complaining about missing timestamps.
> 
> Fixes: 7cab44f1c35f ("ice: Introduce ETH56G PHY model for E825C products")
> Signed-off-by: Jacob Keller <[email protected]>
> ---
>  drivers/net/ethernet/intel/ice/ice_ptp.c | 54 +++++++++++++++-----------------
>  1 file changed, 26 insertions(+), 28 deletions(-)

Tested-by: Alexander Nowlin <[email protected]>