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

From: Kurt Kanzenbach <[email-masked]>
Date: Wed, 21 Sep 2022 13:46:22 +0200

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