netdev - Re: [PATCH net-next 1/2] net: extend ndo_get

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Message-ID: <88efe7ae-7ffd-4088-a81c-b3761bd1ac20@engleder-embedded.com>
Date: Fri, 16 Jan 2026 21:36:07 +0100
From: Gerhard Engleder <gerhard@...leder-embedded.com>
To: Kevin Yang <yyd@...gle.com>
Cc: netdev@...r.kernel.org, Richard Cochran <richardcochran@...il.com>,
 Jakub Kicinski <kuba@...nel.org>, Joshua Washington <joshwash@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>,
 David Miller <davem@...emloft.net>, Andrew Lunn <andrew+netdev@...n.ch>,
 Harshitha Ramamurthy <hramamurthy@...gle.com>,
 Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 1/2] net: extend ndo_get_tstamp for other
 timestamp types

On 15.01.26 23:22, Kevin Yang wrote:
> Network device hardware timestamps (hwtstamps) and the system's
> clock (ktime) often originate from different clock domains.
> This makes it hard to directly calculate the duration between
> a hardware-timestamped event and a system-time event by simple
> subtraction.
> 
> This patch extends ndo_get_tstamp to allow a netdev to provide
> a hwtstamp into the system's CLOCK_REALTIME domain. This allows a
> driver to either perform a conversion by estimating or, if the
> clocks are kept synchronized, return the original timestamp directly.
> Other clock domains, e.g. CLOCK_MONOTONIC_RAW can also be added when
> a use surfaces.
> 
> This is useful for features that need to measure the delay between
> a packet's hardware arrival/departure and a later software event.
> For example, the TCP stack can use this to measure precise
> packet receive delays, which is a requirement for the upcoming
> TCP Swift [1] congestion control algorithm.
> 
> [1] Kumar, Gautam, et al. "Swift: Delay is simple and effective
> for congestion control in the datacenter." Proceedings of the
> Annual conference of the ACM Special Interest Group on Data
> Communication on the applications, technologies, architectures,
> and protocols for computer communication. 2020.
> 
> Signed-off-by: Kevin Yang <yyd@...gle.com>
> Reviewed-by: Willem de Bruijn <willemb@...gle.com>
> ---
>   drivers/net/ethernet/engleder/tsnep_main.c |  8 +++++---
>   drivers/net/ethernet/intel/igc/igc_main.c  |  8 +++++---
>   include/linux/netdevice.h                  | 21 ++++++++++++++-------
>   3 files changed, 24 insertions(+), 13 deletions(-)
>


...

>   
> +enum netdev_tstamp_type {
> +	NETDEV_TSTAMP_RAW = 0,
> +	NETDEV_TSTAMP_CYCLE,
> +	NETDEV_TSTAMP_REALTIME,
> +};

Would it make sense to document the types? By your change of the
ndo_get_tstamp() documentation below, the info that cycles is
about a free running cycle counter is lost.

IMO the name RAW for the timestamp of the adjustable physical clock is
not a good fit, because NETDEV_TSTAMP_CYCLE is more raw (not adjustable)
than NETDEV_TSTAMP_RAW (adjustable). Maybe NETDEV_TSTAMP_PHC instead of
NETDEV_TSTAMP_RAW?

If I understand your second patch correctly, then NETDEV_TSTAMP_REALTIME
can provide a timestamp with low accuracy, which can even jump backwards
(a later received package can get an earlier timestamp due to the simple
synchronisation in gve_hwts_realtime_update). Should the expected
quality of the timestamps be documented here?

> +
>   /*
>    * This structure defines the management hooks for network devices.
>    * The following hooks can be defined; unless noted otherwise, they are
> @@ -1406,11 +1412,10 @@ struct netdev_net_notifier {
>    *     Get the forwarding path to reach the real device from the HW destination address
>    * ktime_t (*ndo_get_tstamp)(struct net_device *dev,
>    *			     const struct skb_shared_hwtstamps *hwtstamps,
> - *			     bool cycles);
> - *	Get hardware timestamp based on normal/adjustable time or free running
> - *	cycle counter. This function is required if physical clock supports a
> - *	free running cycle counter.
> - *
> + *			     enum netdev_tstamp_type type);
> + *	Get hardware timestamp based on the type requested, or return 0 if the
> + *	requested type is not supported. This function is required if physical
> + *	clock supports a free running cycle counter.

...

Gerhard