netdev - Re: [PATCH 3/3] net: ethernet: ti: am65-cpsw: Add device tree property to set max MTU

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Date: Wed, 3 Jan 2024 10:11:57 +0000
From: Sanjuán García, Jorge
	<Jorge.SanjuanGarcia@...gon.com>
To: "andrew@...n.ch" <andrew@...n.ch>
CC: "s-vadapalli@...com" <s-vadapalli@...com>, "davem@...emloft.net"
	<davem@...emloft.net>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "grygorii.strashko@...com"
	<grygorii.strashko@...com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "kuba@...nel.org" <kuba@...nel.org>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "pabeni@...hat.com"
	<pabeni@...hat.com>
Subject: Re: [PATCH 3/3] net: ethernet: ti: am65-cpsw: Add device tree
 property to set max MTU

On Wed, 2024-01-03 at 02:42 +0100, Andrew Lunn wrote:
> [No suele recibir correo electrónico de andrew@...n.ch. Descubra por
> qué esto es importante en
> https://aka.ms/LearnAboutSenderIdentification ]
> 
> On Tue, Jan 02, 2024 at 08:19:15AM +0000, Sanjuán García, Jorge
> wrote:
> > The switch supports ethernet frame sizes between 64 and 2024 bytes
> > (including VLAN) as stated in the technical reference manual.
> > 
> > This patch adds a new devicetree property so the switch ports can
> > be configured with an MTU higher than the standar 1500 bytes,
> > making
> > the max frame length configured on the registers and the max_mtu
> > advertised on the network device consistent.
> 
> Why do you need a device tree property for this? How many other
> drivers have a device tree property like this? Why not set
> ndev->max_mtu to 2024 minus overheads?

Hi Andrew,

There are a few drivers that set the max_mtu based on "max_frame_size"
parsed from device tree. Here is a list:

driver/net/ethernet/
  stmicro/stmmac/stmmac_platform.c
  altera/altera_tse_main.c
  socionext/netsec.c
  ibm/emac/core.c

I also considered hardcoding this to the maximum capabilities of the HW
but I ended making this a configurable frame size. I beleive this way
it is more stable as I don't know whether there may be any performance
issues if we default the max frame size on the swith registers to be
something different than the standard 1522 bytes. I need it for my use
case where there is a DSA switch connected to one port and I need some
extra room for the DSA headers.

Regards,
Jorge

> 
>         Andrew