netdev - Re: [PATCH net-next 0/3] Support exposing raw cycle counters in PTP and mlx5

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Message-ID: <c52004ea-16dd-4131-b58a-4a7f7c6be758@nvidia.com>
Date: Thu, 31 Jul 2025 22:03:02 +0300
From: Carolina Jubran <cjubran@...dia.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Tariq Toukan <tariqt@...dia.com>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Saeed Mahameed <saeedm@...dia.com>,
 Leon Romanovsky <leon@...nel.org>, Mark Bloch <mbloch@...dia.com>,
 Richard Cochran <richardcochran@...il.com>, netdev@...r.kernel.org,
 linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 0/3] Support exposing raw cycle counters in PTP
 and mlx5

On 30/07/2025 1:40, Jakub Kicinski wrote:
> On Tue, 29 Jul 2025 09:57:13 +0300 Carolina Jubran wrote:
>> One concrete use case is monitoring the frequency stability of the
>> device clock in FreeRunning mode. User space can periodically sample the
>> (cycle, time) pairs returned by the new ioctl to estimate the clock’s
>> frequency and detect anomalies, for example, drift caused by temperature
>> changes. This is especially useful in holdover scenarios.
> 
> Because the servo running on the host doesn't know the stability?
> Seems like your real use case is the one below.
> 
>> Another practical case is with DPDK. When the hardware is in FreeRunning
>> mode, the CQE contains raw cycle counter values. DPDK returns these
>> values directly to user space without converting them. The new ioctl
>> provides a generic and consistent way to translate those raw values to
>> host time.
>>
>> As for XDP, you’re right that it doesn’t expose raw cycles today. The
>> point here is more future-looking: if drivers ever choose to emit raw
>> cycles into metadata for performance, this API gives user space a clean
>> way to interpret those timestamps.
> 
> Got it, I can see how DPDK / kernel bypass may need this.
> 
> Please include this justification in the commit message for v2
> and let's see if anyone merges it.

Thanks, I’ll include the DPDK/kernel bypass justification clearly in the 
v2 commit message and cover letter.

Additionally, I wanted to mention another relevant use case that wasn’t 
brought up earlier: fwctl can expose event records tagged with raw cycle 
counter timestamps. When the device is in free-running mode, correlating 
those with host time becomes difficult unless user space has access to 
both cycle and system time snapshots.