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emacs 28.2 From: Rahul Rameshbabu To: "Keller, Jacob E" Cc: "Kitszel, Przemyslaw" , "Polchlopek, Mateusz" , "intel-wired-lan@lists.osuosl.org" , "netdev@vger.kernel.org" , "horms@kernel.org" , "Nguyen, Anthony L" , "Drewek, Wojciech" Subject: Re: [Intel-wired-lan] [PATCH iwl-next v5 08/12] iavf: periodically cache PHC time Date: Thu, 25 Apr 2024 10:10:56 -0700 In-reply-to: Message-ID: <87wmol1i4j.fsf@nvidia.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: SJ0PR13CA0080.namprd13.prod.outlook.com (2603:10b6:a03:2c4::25) To BYAPR12MB2743.namprd12.prod.outlook.com (2603:10b6:a03:61::28) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BYAPR12MB2743:EE_|MN0PR12MB6004:EE_ X-MS-Office365-Filtering-Correlation-Id: 333c5130-f4d7-498f-17af-08dc654ae48d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Polchlopek, Mateusz >> ; Rahul Rameshbabu >> >> Cc: intel-wired-lan@lists.osuosl.org; netdev@vger.kernel.org; horms@kern= el.org; >> Nguyen, Anthony L ; Drewek, Wojciech >> >> Subject: Re: [Intel-wired-lan] [PATCH iwl-next v5 08/12] iavf: periodica= lly cache >> PHC time >>=20 >> On 4/25/24 00:03, Keller, Jacob E wrote: >> > >> > >> >> -----Original Message----- >> >> From: Polchlopek, Mateusz >> >> Sent: Monday, April 22, 2024 2:23 AM >> >> To: Rahul Rameshbabu >> >> Cc: intel-wired-lan@lists.osuosl.org; netdev@vger.kernel.org; >> horms@kernel.org; >> >> Nguyen, Anthony L ; Keller, Jacob E >> >> ; Drewek, Wojciech >> >> Subject: Re: [Intel-wired-lan] [PATCH iwl-next v5 08/12] iavf: period= ically cache >> >> PHC time >> >> >> >> >> >> >> >> On 4/18/2024 9:51 PM, Rahul Rameshbabu wrote: >> >>> On Thu, 18 Apr, 2024 01:24:56 -0400 Mateusz Polchlopek >> >> wrote: >> >>>> From: Jacob Keller >> >>>> >> >>>> The Rx timestamps reported by hardware may only have 32 bits of sto= rage >> >>>> for nanosecond time. These timestamps cannot be directly reported t= o 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 algor= ithm >> >>>> relies on a cached value that is periodically updated. >> >>>> >> >>>> Keep this cached time up to date by using the PTP .do_aux_work kthr= ead >> >>>> function. >> >>> >> >>> Seems reasonable. >> >>> >> >>>> >> >>>> The iavf_ptp_do_aux_work will reschedule itself about twice a secon= d, >> >>>> and will check whether or not the cached PTP time needs to be updat= ed. >> >>>> If so, it issues a VIRTCHNL_OP_1588_PTP_GET_TIME to request the tim= e >> >>>> 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 >> >>>> Signed-off-by: Jacob Keller >> >>>> Co-developed-by: Mateusz Polchlopek >> >>>> Signed-off-by: Mateusz Polchlopek >> >>>> --- >> >>>> 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 >> >>> >> >>>> +/** >> >>>> + * iavf_ptp_do_aux_work - Perform periodic work required for PTP s= upport >> >>>> + * @ptp: PTP clock info structure >> >>>> + * >> >>>> + * Handler to take care of periodic work required for PTP operatio= n. This >> >>>> + * includes the following tasks: >> >>>> + * >> >>>> + * 1) updating cached_phc_time >> >>>> + * >> >>>> + * cached_phc_time is used by the Tx and Rx timestamp flows i= n order to >> >>>> + * perform timestamp extension, by carefully comparing the ti= mestamp >> >>>> + * 32bit nanosecond timestamps and determining the corrected = 64bit >> >>>> + * timestamp value to report to userspace. This algorithm onl= y works if >> >>>> + * the cached_phc_time is within ~1 second of the Tx or Rx ti= mestamp >> >>>> + * event. This task periodically reads the PHC time and store= s it, to >> >>>> + * ensure that timestamp extension operates correctly. >> >>>> + * >> >>>> + * Returns: time in jiffies until the periodic task should be re-s= cheduled. >> >>>> + */ >> >>>> +long iavf_ptp_do_aux_work(struct ptp_clock_info *ptp) >> >>>> +{ >> >>>> + struct iavf_adapter *adapter =3D 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. >>=20 >> 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 ;) >>=20 >> 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 :/ >>=20 > > That=E2=80=99s 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 personal= ly find > intuitive. Those used to seeing and reading HZ may find it easier. > Makes sense to stick with the same if ice is using jiffies_to_msec in general. I, recently, was re-reading the Linux Device Drivers book, which has a section that elaborates on HZ a bit. -- Thanks, Rahul Rameshbabu