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

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Message-ID: <20260210121207.9kLHroS0@linutronix.de>
Date: Tue, 10 Feb 2026 13:12:07 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
	Willem de Bruijn <willemb@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Loktionov, Aleksandr" <aleksandr.loktionov@...el.com>,
	Kurt Kanzenbach <kurt@...utronix.de>,
	"Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
	"Kitszel, Przemyslaw" <przemyslaw.kitszel@...el.com>,
	Paul Menzel <pmenzel@...gen.mpg.de>,
	"Gomes, Vinicius" <vinicius.gomes@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Richard Cochran <richardcochran@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
	"Keller, Jacob E" <jacob.e.keller@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v3] igb: Retrieve Tx timestamp
 directly from interrupt for i210

On 2026-02-09 07:46:01 [-0500], Willem de Bruijn wrote:
> > > Yeah, but what is the legacy user here? If you enable HW-timestamps but
> > > never set OPT_TSONLY and the sysctl is also 0 then you reply on the
> > > CAP_NET_RAW later on. Right?
> > 
> > Legacy users here means users of HW TX timestamps expecting full skb to 
> > be returned back with the TX timestamp. Legacy here means that skb will 
> > be returned with headers modified by stack, which is kind of exposure of
> > data, which requires CAP_NET_RAW...

Ah okay. I assumed the err-queue was the standard way of receiving
timestamps.

> > > I just try to justify the CAP_NET_RAW check and if it is required to
> > > move it earlier (where HW timestamps are enabled). And if the sysctl
> > > check is enough then maybe it is not needed.
> > 
> > Capabilities should not change during lifetime of the process, should be
> > fine to move. On the other, sysctl can be changed system-wide which may 
> > affect users.
> 
> Ignore the hardware configuration. That is entirely optional. Some
> devices will timestamp every packet.
> 
> The capability check here is per-socket, independent from the system
> hardware configuration.
> 
> I don't see how it could be moved.
> 
> Before OPT_TSONLY was introduced packets were always queued with their
> payload. The sysctl check was added to optionally disallow this. The
> check could arguably be moved earlier in the socket lifecycle and the
> decision cached in the socket. But then flipping the sysctl would not
> affect existing sockets, so that is a change in ABI behavior.

You could cache only the part under sk_callback_lock.
Any other suggestions?
The access from IRQ is quick and avoids any detours.
The alternative would be to move the whole routine into an aux_worker.
For every driver doing it from the IRQ handler.

Sebastian