linux-kernel - Re: [PATCH bpf-next v3 2/3] net: stmmac: add Launch Time support to XDP ZC

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Date:   Mon, 04 Dec 2023 09:54:40 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jesper Dangaard Brouer <hawk@...nel.org>,
        Song Yoong Siang <yoong.siang.song@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Bjorn Topel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Mykola Lysenko <mykolal@...com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>, Shuah Khan <shuah@...nel.org>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, bpf@...r.kernel.org,
        xdp-hints@...-project.net,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 2/3] net: stmmac: add Launch Time support to
 XDP ZC

Jesper Dangaard Brouer wrote:
> 
> 
> On 12/3/23 17:51, Song Yoong Siang wrote:
> > This patch enables Launch Time (Time-Based Scheduling) support to XDP zero
> > copy via XDP Tx metadata framework.
> > 
> > Signed-off-by: Song Yoong Siang<yoong.siang.song@...el.com>
> > ---
> >   drivers/net/ethernet/stmicro/stmmac/stmmac.h      |  2 ++
> 
> As requested before, I think we need to see another driver implementing 
> this.
> 
> I propose driver igc and chip i225.
> 
> The interesting thing for me is to see how the LaunchTime max 1 second
> into the future[1] is handled code wise. One suggestion is to add a 
> section to Documentation/networking/xsk-tx-metadata.rst per driver that 
> mentions/documents these different hardware limitations.  It is natural 
> that different types of hardware have limitations.  This is a close-to 
> hardware-level abstraction/API, and IMHO as long as we document the 
> limitations we can expose this API without too many limitations for more 
> capable hardware.

I would assume that the kfunc will fail when a value is passed that
cannot be programmed.

What is being implemented here already exists for qdiscs. The FQ
qdisc takes a horizon attribute and

    "
    when a packet is beyond the horizon
        at enqueue() time:
        - either drop the packet (default policy)
        - or cap its delivery time to the horizon.
    "
    commit 39d010504e6b ("net_sched: sch_fq: add horizon attribute")

Having the admin manually configure this on the qdisc based on
off-line knowledge of the device is more fragile than if the device
would somehow signal its limit to the stack.

But I don't think we should add enforcement of that as a requirement
for this xdp extension of pacing.