linux-kernel - Re: [PATCH net-next 08/13] net: dsa: hellcreek: deny tc-taprio changes to per-tc max SDU

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Date:   Wed, 21 Sep 2022 13:46:22 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     "netdev@...r.kernel.org" <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>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>,
        Rui Sousa <rui.sousa@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        "UNGLinuxDriver@...rochip.com" <UNGLinuxDriver@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Michael Walle <michael@...le.cc>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Maxim Kochetkov <fido_max@...ox.ru>,
        Colin Foster <colin.foster@...advantage.com>,
        Richie Pearn <richard.pearn@....com>,
        Vladimir Oltean <olteanv@...il.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Gerhard Engleder <gerhard@...leder-embedded.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 08/13] net: dsa: hellcreek: deny tc-taprio
 changes to per-tc max SDU

On Wed Sep 21 2022, Vladimir Oltean wrote:
> On Wed, Sep 21, 2022 at 01:23:24PM +0200, Kurt Kanzenbach wrote:
>> On Thu Sep 15 2022, Vladimir Oltean wrote:
>> > On Thu, Sep 15, 2022 at 08:15:54AM +0200, Kurt Kanzenbach wrote:
>> >> > So the maxSDU hardware register tracks exactly the L2 payload size, like
>> >> > the software variable does, or does it include the Ethernet header size
>> >> > and/or FCS?
>> >> 
>> >> This is something I'm not sure about. I'll ask the HW engineer when he's
>> >> back from vacation.
>> >
>> > You can also probably figure this out by limiting the max-sdu to a value
>> > like 200 and seeing what frame sizes pass through.
>> 
>> So, configured to 128 and 132 bytes (including VLAN Ethernet header) is
>> the maximum frame size which passes through.
>
> Frame size means MAC DA + MAC SA + VLAN header + Ethertype + L2 payload,
> and without FCS, right?

Yes.

>
> Because max_sdu 128 only counts the L2 payload, so the maximum frame
> size that passes should be 142 octets, or 146 octets with VLAN.

Ok, i see. So, for 128 max-sdu we should end up with something like this
in the prio config register:

 schedule->max_sdu[tc] + VLAN_ETH_HLEN - 4

Thanks,
Kurt

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