Message-ID: <CO1PR11MB5089937991BD0CAD0FA21859D6172@CO1PR11MB5089.namprd11.prod.outlook.com>
Date: Thu, 25 Apr 2024 16:28:22 +0000
From: "Keller, Jacob E" <jacob.e.keller@...el.com>
To: "Kitszel, Przemyslaw" <przemyslaw.kitszel@...el.com>, "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: Kitszel, Przemyslaw <przemyslaw.kitszel@...el.com>
> Sent: Thursday, April 25, 2024 3:52 AM
> To: Keller, Jacob E <jacob.e.keller@...el.com>; Polchlopek, Mateusz
> <mateusz.polchlopek@...el.com>; 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>; Drewek, Wojciech
> <wojciech.drewek@...el.com>
> Subject: Re: [Intel-wired-lan] [PATCH iwl-next v5 08/12] iavf: periodically cache
> PHC time
> 
> On 4/25/24 00:03, Keller, Jacob E wrote:
> >
> >
> >> -----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.
> 
> HZ/2 or HZ/3 as a timer period could be understood without thinking, but
> the same stands for 400ms. Problems starts when one thinks about it ;)
> 
> For me HZ, which could be both literally and colloquially understood as
> "per second" should not mean 1000ms (just evaluate to).
> 2Hz is a frequency with half second period, but 2*HZ evaluates to 2000ms
> which is 4 times more :/
> 

That’s part of why I switched ice over from using HZ generally to using jiffies_to_msec in a lot of cases. It really depends on what you personally find intuitive. Those used to seeing and reading HZ may find it easier.

Thanks,
Jake

> 
> >
> > Thanks,
> > Jake
> >

Powered by blists - more mailing lists