netdev - Re: [PATCH iwl-next v10 08/14] iavf: periodically cache PHC time

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Message-ID: <45029a59-70ce-4bec-8586-f3ac35eec045@intel.com>
Date: Wed, 28 Aug 2024 13:31:27 +0200
From: Wojciech Drewek <wojciech.drewek@...el.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
CC: <netdev@...r.kernel.org>, <intel-wired-lan@...ts.osuosl.org>,
	<horms@...nel.org>, <anthony.l.nguyen@...el.com>, <kuba@...nel.org>,
	<alexandr.lobakin@...el.com>
Subject: Re: [PATCH iwl-next v10 08/14] iavf: periodically cache PHC time



On 21.08.2024 16:43, Alexander Lobakin wrote:
> From: Wojciech Drewek <wojciech.drewek@...el.com>
> Date: Wed, 21 Aug 2024 14:15:33 +0200
> 
>> 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.
>>
>> 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.
>>
>> Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
>> Reviewed-by: Wojciech Drewek <wojciech.drewek@...el.com>
>> Reviewed-by: Simon Horman <horms@...nel.org>
>> Co-developed-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
>> Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
>> Signed-off-by: Wojciech Drewek <wojciech.drewek@...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
>> index d709d381958f..7124a717cd03 100644
>> --- a/drivers/net/ethernet/intel/iavf/iavf_ptp.c
>> +++ b/drivers/net/ethernet/intel/iavf/iavf_ptp.c
>> @@ -153,6 +153,55 @@ static int iavf_ptp_gettimex64(struct ptp_clock_info *info,
>>  	return iavf_read_phc_indirect(adapter, ts, sts);
>>  }
>>  
>> +/**
>> + * iavf_ptp_cache_phc_time - Cache PHC time for performing timestamp extension
>> + * @adapter: private adapter structure
>> + *
>> + * Periodically cache the PHC time in order to allow for timestamp extension.
>> + * This is required because the Tx and Rx timestamps only contain 32bits of
>> + * nanoseconds. Timestamp extension allows calculating the corrected 64bit
>> + * timestamp. This algorithm relies on the cached time being within ~1 second
>> + * of the timestamp.
>> + */
>> +static void iavf_ptp_cache_phc_time(struct iavf_adapter *adapter)
>> +{
>> +	if (time_is_before_jiffies(adapter->ptp.cached_phc_updated + HZ)) {
> 
> Also invert the condition to avoid +1 indent level?

Agree

> 
>> +		/* The response from virtchnl will store the time into
>> +		 * cached_phc_time.
>> +		 */
>> +		iavf_send_phc_read(adapter);
>> +	}
>> +}
>> +
>> +/**
>> + * iavf_ptp_do_aux_work - Perform periodic work required for PTP support
>> + * @info: 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 *info)
> 
> Why isn't it static since it's used only within this one file?

Agree

> 
>> +{
>> +	struct iavf_adapter *adapter = iavf_clock_to_adapter(info);
>> +
>> +	iavf_ptp_cache_phc_time(adapter);
>> +
>> +	/* Check work about twice a second */
>> +	return msecs_to_jiffies(500);
> 
> Thanks,
> Olek