netdev - Re: [PATCH] dsa: tag_rtl4_a: Bump min packet size

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Date: Mon, 30 Oct 2023 15:32:10 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Vladimir Oltean <olteanv@...il.com>, DENG Qingfang <dqfext@...il.com>
Cc: Florian Fainelli <f.fainelli@...il.com>, Andrew Lunn <andrew@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dsa: tag_rtl4_a: Bump min packet size

Hi Deng,

do you have some comments on the below, pertaining to
commit 9eb8bc593a5eed167dac2029abef343854c5ba75
"net: dsa: tag_rtl4_a: fix egress tags"?

I plan to test without the ZLEN padding and see what happens.
IIRC it wasn't working without that, but I may just misremember
the whole thing so let's rehash this.

On Mon, Oct 30, 2023 at 1:51 PM Vladimir Oltean <olteanv@...il.com> wrote:

> What you are missing is that the existing padding done by rtl4a_tag_xmit()
> shouldn't be normally needed except for exceptional cases.
>
> Socket buffers smaller than ETH_ZLEN can be passed to any network
> device, and it is expected that either the driver or the hardware pads
> up to ETH_ZLEN automatically. Thus, the conduit driver should already
> know that it needs to pad packets to ETH_ZLEN.
>
> The exceptional cases are:
> - This is a tail tag (not the case here), which by definition needs to
>   be located at the end of the skb. If you first put the tag then let
>   the conduit interface pad, then the tail tag is no longer at the tail.
>   So in that case, DSA pads first in generic code - dsa_user_xmit().
> - The switch must handle the case where, after stripping the DSA tag
>   from a ETH_ZLEN sized packet coming from the CPU port, it re-pads the
>   packet on user port egress. Some switches don't handle that properly,
>   and thus, we have isolated __skb_put_padto() calls within certain
>   tagging protocols which address just that case.
>
> So, what Florian was asking is whether the conduit interface is not
> doing its expected job properly. You clarified that the problem is big
> rather than small packets, but we still need an explanation for the
> existing __skb_put_padto() call, given that it seems that it was placed
> there due to a misunderstanding rather than due to an explicit need for
> an exceptional case.

Yours,
Linus Walleij