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

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Message-ID: <20250729154012.5d540144@kernel.org>
Date: Tue, 29 Jul 2025 15:40:12 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Carolina Jubran <cjubran@...dia.com>
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 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.