linux-kernel - Re: [PATCH iwl-next v3] igb: Retrieve Tx timestamp directly from interrupt for i210

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Message-ID: <20260205121229.AwxNfEl_@linutronix.de>
Date: Thu, 5 Feb 2026 13:12:29 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Kurt Kanzenbach <kurt@...utronix.de>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Vinicius Costa Gomes <vinicius.gomes@...el.com>,
	Paul Menzel <pmenzel@...gen.mpg.de>,
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
	Miroslav Lichvar <mlichvar@...hat.com>,
	Jacob Keller <jacob.e.keller@...el.com>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH iwl-next v3] igb: Retrieve Tx timestamp directly from
 interrupt for i210

On 2026-02-05 08:54:34 [+0100], Kurt Kanzenbach wrote:
> Retrieve Tx timestamp directly from interrupt handler for i210.
> 
> The current implementation uses schedule_work() which is executed by the
> system work queue to retrieve Tx timestamps. This increases latency and can
> lead to timeouts in case of heavy system load. i210 is often used in
> industrial systems, where timestamp timeouts can be fatal.
> 
> Therefore, fetch the timestamp directly from the interrupt handler.
> 
> The work queue code stays for all other NICs supported by igb.
> 
> Tested on Intel i210 and i350.
> 
> Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

IMHO this is a compromise with Miroslav where he observed less PTP
timestamps on the i350. While testing I did not get near Miroslav's
difference but there was a small change. I don't understand *why*
because the current workqueue usage reads the timestamp on the same CPU
on which the interrupt occurred. Doing it directly just avoids the
context switch. This feels beneficial.

Sebastian