netdev - Re: [PATCH iwl-next,v4 1/1] igc: Add Tx hardware timestamp request for AF

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Message-ID: <87h6gtb0p0.fsf@kurt.kurt.home>
Date: Tue, 26 Mar 2024 14:08:11 +0100
From: Kurt Kanzenbach <kurt@...utronix.de>
To: Florian Bezdeka <florian.bezdeka@...mens.com>, Song Yoong Siang
 <yoong.siang.song@...el.com>, Jesse Brandeburg
 <jesse.brandeburg@...el.com>, Tony Nguyen <anthony.l.nguyen@...el.com>,
 "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>, Alexei
 Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend
 <john.fastabend@...il.com>, Stanislav Fomichev <sdf@...gle.com>, Vinicius
 Costa Gomes <vinicius.gomes@...el.com>, Maciej Fijalkowski
 <maciej.fijalkowski@...el.com>
Cc: intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 xdp-hints@...-project.net
Subject: Re: [PATCH iwl-next,v4 1/1] igc: Add Tx hardware timestamp request
 for AF_XDP zero-copy packet

Hi Florian,

On Tue Mar 26 2024, Florian Bezdeka wrote:
> On Mon, 2024-03-25 at 10:09 +0800, Song Yoong Siang wrote:
>> This patch adds support to per-packet Tx hardware timestamp request to
>> AF_XDP zero-copy packet via XDP Tx metadata framework. Please note that
>> user needs to enable Tx HW timestamp capability via igc_ioctl() with
>> SIOCSHWTSTAMP cmd before sending xsk Tx hardware timestamp request.
>> 
>> Same as implementation in RX timestamp XDP hints kfunc metadata, Timer 0
>> (adjustable clock) is used in xsk Tx hardware timestamp. i225/i226 have
>> four sets of timestamping registers. *skb and *xsk_tx_buffer pointers
>> are used to indicate whether the timestamping register is already occupied.
>
> Let me make sure that I fully understand that: In my own words:
>
> With that applied I'm able to get the point in time from the device
> when a specific frame made it to the wire. I have to enable that
> functionality using the mentioned ioctl() call first, and then check
> the meta area (located in the umem right before the frame payload)
> while consuming the completion queue/ring. Correct?
>
> If so, we now have a feedback channel for meta information for/from TX.
> Are there any plans - or would it be possible - to support Earliest
> TxTime First (NET_SCHED_ETF) QDisc based on that channel? In the past
> we had the problem that we we're missing a feedback channel to
> communicate back invalid lunch times.

Just asking: How would that work? AFAIK XDP bypasses the Qdisc
layer. Currently invalid Launch Times are accounted in the ETF Qdisc
itself. Does that mean every driver has to take care of it?

Thanks,
Kurt

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