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

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Message-ID: <20220921141206.rvdsrp7lmm2fk5ub@skbuf>
Date:   Wed, 21 Sep 2022 14:12:07 +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>,
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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 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?