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

On 22/07/2025 3:09, Jakub Kicinski wrote:

On Tue, 15 Jul 2025 08:15:30 +0300 Tariq Toukan wrote:

quoted

This patch series introduces support for exposing the raw free-running
cycle counter of PTP hardware clocks.

Could you say more about use cases? I realized when massaging the cover
letter to apply the series that all the use cases are vague and
hypothetical.

quoted

Some telemetry and low-level logging use cycle counter timestamps
rather than nanoseconds.

What is that "some telemetry"?

quoted

Currently, there is no generic interface to
correlate these raw values with system time.

To address this, the series introduces two new ioctl commands that
allow userspace to query the device's raw cycle counter together with
host time:

  - PTP_SYS_OFFSET_PRECISE_CYCLES

  - PTP_SYS_OFFSET_EXTENDED_CYCLES

These commands work like their existing counterparts but return the
device timestamp in cycle units instead of real-time nanoseconds.

This can also be useful in the XDP fast path: if a driver inserts the
raw cycle value into metadata instead of a real-time timestamp, it can
avoid the overhead of converting cycles to time in the kernel. Then
userspace can resolve the cycle-to-time mapping using this ioctl when
needed.

There is no API to achieve that today, right? The XDP access helpers
are supposed to return converted time. Are you planning to add new
callbacks?
 > If there are solid networking use cases for this I'd prefer we fully
iron them out before merging this uAPI. If there are RDMA use cases
please spell them out in more detail.

Hi Jakub

Thanks for the feedback.

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.

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.

Carolina