netdev - Re: [Intel-wired-lan] [PATCH iwl-next v5 08/12] iavf: periodically cache PHC time

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Message-ID: <87jzkue99b.fsf@nvidia.com>
Date: Thu, 18 Apr 2024 12:51:19 -0700
From: Rahul Rameshbabu <rrameshbabu@...dia.com>
To: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
Cc: intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
 horms@...nel.org, anthony.l.nguyen@...el.com, Jacob Keller
 <jacob.e.keller@...el.com>, Wojciech Drewek <wojciech.drewek@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v5 08/12] iavf: periodically
 cache PHC time

On Thu, 18 Apr, 2024 01:24:56 -0400 Mateusz Polchlopek <mateusz.polchlopek@...el.com> wrote:
> 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.

Seems reasonable.

>
> 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.
>
> Reviewed-by: Wojciech Drewek <wojciech.drewek@...el.com>
> Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
> Co-developed-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
> Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@...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
<snip>
> +/**
> + * iavf_ptp_do_aux_work - Perform periodic work required for PTP support
> + * @ptp: 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 *ptp)
> +{
> +	struct iavf_adapter *adapter = clock_to_adapter(ptp);
> +
> +	iavf_ptp_cache_phc_time(adapter);
> +
> +	/* Check work about twice a second */
> +	return msecs_to_jiffies(500);

HZ / 2 might be more intuitive?

> +}
> +
<snip>