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To:     Kurt Kanzenbach <kurt@linutronix.de>
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Subject: Re: [PATCH net-next 08/13] net: dsa: hellcreek: deny tc-taprio
 changes to per-tc max SDU
Thread-Topic: [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 14:12:07 +0000
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On Wed, Sep 21, 2022 at 01:46:22PM +0200, Kurt Kanzenbach wrote:
> >> So, configured to 128 and 132 bytes (including VLAN Ethernet header) i=
s
> >> the maximum frame size which passes through.
> >
> > Frame size means MAC DA + MAC SA + VLAN header + Ethertype + L2 payload=
,
> > and without FCS, right?
>=20
> Yes.
>=20
> >
> > 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.
>=20
> Ok, i see. So, for 128 max-sdu we should end up with something like this
> in the prio config register:
>=20
>  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 =3D 142, and this will pass packets (VLAN-tagged
or not) with an skb->len (on xmit) <=3D 142, right?=