netdev - Re: [PATCH net v3 1/1] igc: read before write to SRRCTL register

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Date:   Sat, 15 Apr 2023 11:19:42 +0200
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>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Vedang Patel <vedang.patel@...el.com>,
        Jithu Joseph <jithu.joseph@...el.com>,
        Andre Guedes <andre.guedes@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Jacob Keller <jacob.e.keller@...el.com>,
        David Laight <David.Laight@...LAB.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, stable@...r.kernel.org
Subject: Re: [PATCH net v3 1/1] igc: read before write to SRRCTL register

On 14.04.23 17:49, Song Yoong Siang wrote:
> igc_configure_rx_ring() function will be called as part of XDP program
> setup. If Rx hardware timestamp is enabled prio to XDP program setup,
> this timestamp enablement will be overwritten when buffer size is
> written into SRRCTL register.
> 

Hi all,

I'm actually searching for the root cause of a similar problem (RX
timestamp lost) that I can reproduce here, but the setup is slightly
different. The setup:

- igc driver
- i225/226 network card
- When starting to transmit frames using XDP with zero copy enabled
  another application (ptp4l) complains about missing RX timestamps
- Loading the XDP program has already been removed (not needed for
  TX only)
- ptp4l is using the traditional socket API
- The RX timestamps seem to come back once we stop sending frames
  using XDP

The "zero copy support" enabled part is important. If ZC support is not
requested everything works fine.

Any ideas?

Best regards,
Florian