netdev - Re: [PATCH iwl-next v10 08/14] iavf: periodically cache PHC time

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Message-ID: <4ae03918-32c7-4065-b1b3-71b0361d6187@intel.com>
Date: Wed, 21 Aug 2024 16:43:16 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Wojciech Drewek <wojciech.drewek@...el.com>
CC: <netdev@...r.kernel.org>, <intel-wired-lan@...ts.osuosl.org>,
	<horms@...nel.org>, <anthony.l.nguyen@...el.com>, <kuba@...nel.org>,
	<alexandr.lobakin@...el.com>
Subject: Re: [PATCH iwl-next v10 08/14] iavf: periodically cache PHC time

From: Wojciech Drewek <wojciech.drewek@...el.com>
Date: Wed, 21 Aug 2024 14:15:33 +0200

> From: Jacob Keller <jacob.e.keller@...el.com>
> 
> The Rx timestamps reported by hardware may only have 32 bits of storage
> for nanosecond time. These timestamps cannot be directly reported to the
> Linux stack, as it expects 64bits of time.
> 
> To handle this, the timestamps must be extended using an algorithm that
> calculates the corrected 64bit timestamp by comparison between the PHC
> time and the timestamp. This algorithm requires the PHC time to be
> captured within ~2 seconds of when the timestamp was captured.
> 
> Instead of trying to read the PHC time in the Rx hotpath, the algorithm
> relies on a cached value that is periodically updated.
> 
> Keep this cached time up to date by using the PTP .do_aux_work kthread
> function.
> 
> The iavf_ptp_do_aux_work will reschedule itself about twice a second,
> and will check whether or not the cached PTP time needs to be updated.
> If so, it issues a VIRTCHNL_OP_1588_PTP_GET_TIME to request the time
> from the PF. The jitter and latency involved with this command aren't
> important, because the cached time just needs to be kept up to date
> within about ~2 seconds.
> 
> Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
> Reviewed-by: Wojciech Drewek <wojciech.drewek@...el.com>
> Reviewed-by: Simon Horman <horms@...nel.org>
> Co-developed-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
> Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
> Signed-off-by: Wojciech Drewek <wojciech.drewek@...el.com>
> ---
>  drivers/net/ethernet/intel/iavf/iavf_ptp.c | 52 ++++++++++++++++++++++
>  drivers/net/ethernet/intel/iavf/iavf_ptp.h |  1 +
>  2 files changed, 53 insertions(+)
> 
> diff --git a/drivers/net/ethernet/intel/iavf/iavf_ptp.c b/drivers/net/ethernet/intel/iavf/iavf_ptp.c
> index d709d381958f..7124a717cd03 100644
> --- a/drivers/net/ethernet/intel/iavf/iavf_ptp.c
> +++ b/drivers/net/ethernet/intel/iavf/iavf_ptp.c
> @@ -153,6 +153,55 @@ static int iavf_ptp_gettimex64(struct ptp_clock_info *info,
>  	return iavf_read_phc_indirect(adapter, ts, sts);
>  }
>  
> +/**
> + * iavf_ptp_cache_phc_time - Cache PHC time for performing timestamp extension
> + * @adapter: private adapter structure
> + *
> + * Periodically cache the PHC time in order to allow for timestamp extension.
> + * This is required because the Tx and Rx timestamps only contain 32bits of
> + * nanoseconds. Timestamp extension allows calculating the corrected 64bit
> + * timestamp. This algorithm relies on the cached time being within ~1 second
> + * of the timestamp.
> + */
> +static void iavf_ptp_cache_phc_time(struct iavf_adapter *adapter)
> +{
> +	if (time_is_before_jiffies(adapter->ptp.cached_phc_updated + HZ)) {

Also invert the condition to avoid +1 indent level?

> +		/* The response from virtchnl will store the time into
> +		 * cached_phc_time.
> +		 */
> +		iavf_send_phc_read(adapter);
> +	}
> +}
> +
> +/**
> + * iavf_ptp_do_aux_work - Perform periodic work required for PTP support
> + * @info: PTP clock info structure
> + *
> + * Handler to take care of periodic work required for PTP operation. This
> + * includes the following tasks:
> + *
> + *   1) updating cached_phc_time
> + *
> + *      cached_phc_time is used by the Tx and Rx timestamp flows in order to
> + *      perform timestamp extension, by carefully comparing the timestamp
> + *      32bit nanosecond timestamps and determining the corrected 64bit
> + *      timestamp value to report to userspace. This algorithm only works if
> + *      the cached_phc_time is within ~1 second of the Tx or Rx timestamp
> + *      event. This task periodically reads the PHC time and stores it, to
> + *      ensure that timestamp extension operates correctly.
> + *
> + * Returns: time in jiffies until the periodic task should be re-scheduled.
> + */
> +long iavf_ptp_do_aux_work(struct ptp_clock_info *info)

Why isn't it static since it's used only within this one file?

> +{
> +	struct iavf_adapter *adapter = iavf_clock_to_adapter(info);
> +
> +	iavf_ptp_cache_phc_time(adapter);
> +
> +	/* Check work about twice a second */
> +	return msecs_to_jiffies(500);

Thanks,
Olek