linux-kernel - 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 14:39:04 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Sanjuán García, Jorge <Jorge.SanjuanGarcia@...gon.com>
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

> 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

So not many.

> 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.

Generally, you just set max_mtu should not have any performance
impacts, since the MTU will still default to 1500. The user has to
take an action to change the MTU and only then could it have any
impact, if implemented correctly.

DT should describe hardware, not configuration of the hardware, and
this is clearly configuration of hardware.

	Andrew