netdev - Re: [Intel-wired-lan] [PATCH iwl-next] igb: Retrieve Tx timestamp directly from interrupt

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Message-ID: <87y0rlm22a.fsf@jax.kurt.home>
Date: Fri, 15 Aug 2025 10:17:17 +0200
From: Kurt Kanzenbach <kurt@...utronix.de>
To: Paul Menzel <pmenzel@...gen.mpg.de>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>, Przemek Kitszel
 <przemyslaw.kitszel@...el.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
 <pabeni@...hat.com>, Richard Cochran <richardcochran@...il.com>, Vinicius
 Costa Gomes <vinicius.gomes@...el.com>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, intel-wired-lan@...ts.osuosl.org,
 netdev@...r.kernel.org
Subject: Re: [Intel-wired-lan] [PATCH iwl-next] igb: Retrieve Tx timestamp
 directly from interrupt

Hi Paul,

On Fri Aug 15 2025, Paul Menzel wrote:
> Dear Kurt,
>
>
> Thank you for your patch.
>
> Am 15.08.25 um 08:50 schrieb Kurt Kanzenbach:
>> Retrieve Tx timestamp directly from interrupt handler.
>> 
>> The current implementation uses schedule_work() which is executed by the
>> system work queue to retrieve Tx timestamps. This increases latency and can
>> lead to timeouts in case of heavy system load.
>> 
>> Therefore, fetch the timestamp directly from the interrupt handler.
>> 
>> The work queue code stays for the Intel 82576. Tested on Intel i210.
>
> Excuse my ignorance, I do not understand the first sentence in the last 
> line. Is it because the driver support different models? Why not change 
> it for Intel 82576 too?

Yes, the driver supports lots of different NIC(s). AFAICS Intel 82576 is
the only one which does not use time sync interrupts. Probably it does
not have this feature. Therefore, the 82576 needs to schedule a work
queue item.

>
> Do you have a reproducer for the issue, so others can test.

Yeah, I do have a reproducer:

 - Run ptp4l with 40ms tx timeout (--tx_timestamp_timeout)
 - Run periodic RT tasks (e.g. with SCHED_FIFO 1) with run time of
   50-100ms per CPU core

This leads to sporadic error messages from ptp4l such as "increasing
tx_timestamp_timeout or increasing kworker priority may correct this
issue, but a driver bug likely causes it"

However, increasing the kworker priority is not an option, simply
because this kworker is doing non-related PTP work items as well.

As the time sync interrupt already signals that the Tx timestamp is
available, there's no need to schedule a work item in this case. I might
have missed something though. But my testing looked good. The warn_on
never triggered.

>
>> Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>
>> ---
>>   drivers/net/ethernet/intel/igb/igb.h      |  1 +
>>   drivers/net/ethernet/intel/igb/igb_main.c |  2 +-
>>   drivers/net/ethernet/intel/igb/igb_ptp.c  | 22 ++++++++++++++++++++++
>>   3 files changed, 24 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/net/ethernet/intel/igb/igb.h b/drivers/net/ethernet/intel/igb/igb.h
>> index c3f4f7cd264e9b2ff70f03b580f95b15b528028c..102ca32e8979fa3203fc2ea36eac456f1943cfca 100644
>> --- a/drivers/net/ethernet/intel/igb/igb.h
>> +++ b/drivers/net/ethernet/intel/igb/igb.h
>> @@ -776,6 +776,7 @@ int igb_ptp_hwtstamp_get(struct net_device *netdev,
>>   int igb_ptp_hwtstamp_set(struct net_device *netdev,
>>   			 struct kernel_hwtstamp_config *config,
>>   			 struct netlink_ext_ack *extack);
>> +void igb_ptp_tx_tstamp_event(struct igb_adapter *adapter);
>>   void igb_set_flag_queue_pairs(struct igb_adapter *, const u32);
>>   unsigned int igb_get_max_rss_queues(struct igb_adapter *);
>>   #ifdef CONFIG_IGB_HWMON
>> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
>> index a9a7a94ae61e93aa737b0103e00580e73601d62b..8ab6e52cb839bbb698007a74462798faaaab0071 100644
>> --- a/drivers/net/ethernet/intel/igb/igb_main.c
>> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
>> @@ -7080,7 +7080,7 @@ static void igb_tsync_interrupt(struct igb_adapter *adapter)
>>   
>>   	if (tsicr & E1000_TSICR_TXTS) {
>>   		/* retrieve hardware timestamp */
>> -		schedule_work(&adapter->ptp_tx_work);
>> +		igb_ptp_tx_tstamp_event(adapter);
>>   	}
>>   
>>   	if (tsicr & TSINTR_TT0)
>> diff --git a/drivers/net/ethernet/intel/igb/igb_ptp.c b/drivers/net/ethernet/intel/igb/igb_ptp.c
>> index a7876882aeaf2b2a7fb9ec6ff5c83d8a1b06008a..20ecafecc60557353f8cc5ab505030246687c8e4 100644
>> --- a/drivers/net/ethernet/intel/igb/igb_ptp.c
>> +++ b/drivers/net/ethernet/intel/igb/igb_ptp.c
>> @@ -796,6 +796,28 @@ static int igb_ptp_verify_pin(struct ptp_clock_info *ptp, unsigned int pin,
>>   	return 0;
>>   }
>>   
>> +/**
>> + * igb_ptp_tx_tstamp_event
>> + * @adapter: pointer to igb adapter
>> + *
>> + * This function checks the TSYNCTXCTL valid bit and stores the Tx hardware
>> + * timestamp at the current skb.
>> + **/
>> +void igb_ptp_tx_tstamp_event(struct igb_adapter *adapter)
>> +{
>> +	struct e1000_hw *hw = &adapter->hw;
>> +	u32 tsynctxctl;
>> +
>> +	if (!adapter->ptp_tx_skb)
>> +		return;
>> +
>> +	tsynctxctl = rd32(E1000_TSYNCTXCTL);
>> +	if (WARN_ON_ONCE(!(tsynctxctl & E1000_TSYNCTXCTL_VALID)))
>> +		return;
>> +
>> +	igb_ptp_tx_hwtstamp(adapter);
>> +}
>> +
>>   /**
>>    * igb_ptp_tx_work
>>    * @work: pointer to work struct
>
> The diff looks fine.
>
> Reviewed-by: Paul Menzel <pmenzel@...gen.mpg.de>

Thanks!

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