linux-kernel - Re: [PATCH net v2 2/2] net: stmmac: Consider Tx VLAN offload tag length for maxSDU

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Message-ID: <20250924160535.12c14ae9@kernel.org>
Date: Wed, 24 Sep 2025 16:05:35 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: "G Thomas, Rohan" <rohan.g.thomas@...era.com>
Cc: Rohan G Thomas via B4 Relay
 <devnull+rohan.g.thomas.altera.com@...nel.org>, Andrew Lunn
 <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Maxime
 Coquelin <mcoquelin.stm32@...il.com>, Alexandre Torgue
 <alexandre.torgue@...s.st.com>, Jose Abreu <Jose.Abreu@...opsys.com>, Rohan
 G Thomas <rohan.g.thomas@...el.com>, netdev@...r.kernel.org,
 linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, Matthew
 Gerlach <matthew.gerlach@...era.com>, "Ng, Boon Khai"
 <boon.khai.ng@...era.com>
Subject: Re: [PATCH net v2 2/2] net: stmmac: Consider Tx VLAN offload tag
 length for maxSDU

On Wed, 24 Sep 2025 10:24:44 +0530 G Thomas, Rohan wrote:
> >> Is the device adding the same VLAN tag twice if the proto is 8021AD?
> >> It looks like it from the code, but how every strange..
> >>
> >> In any case, it doesn't look like the driver is doing anything with
> >> the NETIF_F_HW_VLAN_* flags right? stmmac_vlan_insert() works purely
> >> off of vlan proto. So I think we should do the same thing here?  
> > 
> > I suppose the double tagging depends on the exact SKU but first check
> > looks unnecessary. Maybe stmmac_vlan_insert() should return the number
> > of vlans it decided to insert?
> >   
> 
> I overlooked the behavior of stmmac_vlan_insert(). It seems the hardware
> supports inserting only one VLAN tag at a time, with the default setting
> being SVLAN for 802.1AD and CVLAN for 802.1Q. I'll update the patch to
> simply add VLAN_HLEN when stmmac_vlan_insert() returns true. Please let
> me know if you have any further concerns.

SG, no further concerns.

Please make sure to CC "Ng, Boon Khai" <boon.khai.ng@...era.com>
who wrote the VLAN support (IIRC).