netdev - Re: [Intel-wired-lan] [PATCH iwl-next v7 08/12] iavf: periodically cache PHC time

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Message-ID: <20240608125920.GA27689@kernel.org>
Date: Sat, 8 Jun 2024 13:59:20 +0100
From: Simon Horman <horms@...nel.org>
To: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
Cc: intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	Jacob Keller <jacob.e.keller@...el.com>,
	Wojciech Drewek <wojciech.drewek@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v7 08/12] iavf: periodically
 cache PHC time

On Tue, Jun 04, 2024 at 09:13:56AM -0400, Mateusz Polchlopek 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.
> 
> 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>
> Co-developed-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
> Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>

Reviewed-by: Simon Horman <horms@...nel.org>