linux-kernel - Re: [PATCH net-next 3/3] net/sched: taprio: dynamic max_sdu larger than the max

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Date:   Thu, 16 Feb 2023 12:29:14 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Kurt Kanzenbach <kurt@...utronix.de>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Gerhard Engleder <gerhard@...leder-embedded.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] net/sched: taprio: dynamic max_sdu larger
 than the max_mtu is unlimited

On Thu, Feb 16, 2023 at 10:28:48AM +0100, Kurt Kanzenbach wrote:
> On Thu Feb 16 2023, Vladimir Oltean wrote:
> > It makes no sense to keep randomly large max_sdu values, especially if
> > larger than the device's max_mtu. These are visible in "tc qdisc show".
> > Such a max_sdu is practically unlimited and will cause no packets for
> > that traffic class to be dropped on enqueue.
> >
> > Just set max_sdu_dynamic to U32_MAX, which in the logic below causes
> > taprio to save a max_frm_len of U32_MAX and a max_sdu presented to user
> > space of 0 (unlimited).
> >
> > Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> 
> Doesn't this deserve a Fixes tag as well?

No, I don't think so. It's just so that the user (and later, the offloading
driver) doesn't see arbitrarily large values, just a simplifying 0. I guess
it could potentially make a difference to the software taprio data path with
TSO, if the max MTU is comparable with the segment sizes.

Anyway, with or without the Fixes tag, the patch lands in the same place.