netdev - 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 11:34:33 +0100
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Vladimir Oltean <vladimir.oltean@....com>
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, Vladimir Oltean wrote:
> 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.

Yes, exactly. It's visible by the user.

> 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.

Yup. It doesn't really matter that much.

Reviewed-by: Kurt Kanzenbach <kurt@...utronix.de>

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