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

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Date: Wed, 24 Apr 2024 22:03:09 +0000
From: "Keller, Jacob E" <jacob.e.keller@...el.com>
To: "Polchlopek, Mateusz" <mateusz.polchlopek@...el.com>, Rahul Rameshbabu
	<rrameshbabu@...dia.com>
CC: "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, "horms@...nel.org"
	<horms@...nel.org>, "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
	"Drewek, Wojciech" <wojciech.drewek@...el.com>
Subject: RE: [Intel-wired-lan] [PATCH iwl-next v5 08/12] iavf: periodically
 cache PHC time



> -----Original Message-----
> From: Polchlopek, Mateusz <mateusz.polchlopek@...el.com>
> Sent: Monday, April 22, 2024 2:23 AM
> To: Rahul Rameshbabu <rrameshbabu@...dia.com>
> Cc: intel-wired-lan@...ts.osuosl.org; netdev@...r.kernel.org; horms@...nel.org;
> Nguyen, Anthony L <anthony.l.nguyen@...el.com>; Keller, Jacob E
> <jacob.e.keller@...el.com>; Drewek, Wojciech <wojciech.drewek@...el.com>
> Subject: Re: [Intel-wired-lan] [PATCH iwl-next v5 08/12] iavf: periodically cache
> PHC time
> 
> 
> 
> On 4/18/2024 9:51 PM, Rahul Rameshbabu wrote:
> > 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?
> >

I've always found it more intuitive to think in terms of msecs myself, but HZ / 2 is ok if other folks agree.

Thanks,
Jake