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

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Date: Tue, 26 Mar 2024 09:56:54 +0100
From: Florian Bezdeka <florian.bezdeka@...mens.com>
To: 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>, Kurt Kanzenbach
 <kurt@...utronix.de>, 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

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.

> 
> Furthermore, a boolean variable named xsk_pending_ts is used to hold the
> transmit completion until the tx hardware timestamp is ready. This is
> because, for i225/i226, the timestamp notification event comes some time
> after the transmit completion event. The driver will retrigger hardware irq
> to clean the packet after retrieve the tx hardware timestamp.
> 
> Besides, xsk_meta is added into struct igc_tx_timestamp_request as a hook
> to the metadata location of the transmit packet. When the Tx timestamp
> interrupt is fired, the interrupt handler will copy the value of Tx hwts
> into metadata location via xsk_tx_metadata_complete().
> 
> This patch is tested with tools/testing/selftests/bpf/xdp_hw_metadata
> on Intel ADL-S platform. Below are the test steps and results.