netdev - 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 14:21:16 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Kurt Kanzenbach <kurt@...utronix.de>
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 at 05:12:06PM +0300, Vladimir Oltean wrote:
> On Wed, Sep 21, 2022 at 01:46:22PM +0200, Kurt Kanzenbach wrote:
> > >> 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
> 
> What does 4 represent? ETH_FCS_LEN?
> 
> So when schedule->max_sdu[tc] is 128, you write to HR_PTPRTCCFG_MAXSDU
> the value of 128 + 18 - 4 = 142, and this will pass packets (VLAN-tagged
> or not) with an skb->len (on xmit) <= 142, right?

Mistake, I meant skb->len <= 146 (max_sdu[tc] + VLAN_ETH_HLEN). So the
hardware calculates the max frame len to include FCS, and skb->len is
without FCS, hence the 4 discrepancy.