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

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Message-ID: <a3802100-21b0-4a8d-9b4a-d557996e7c2d@intel.com>
Date: Thu, 25 Apr 2024 12:52:17 +0200
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
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" <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

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 :/


> 
> Thanks,
> Jake
>