linux-kernel - Re: [PATCH net-next 09/11] net: dsa: rzn1-a5psw: Add support for management port frame length adjustment

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Message-ID: <20251121210504.ljeejnltaawahqtv@skbuf>
Date: Fri, 21 Nov 2025 23:05:04 +0200
From: Vladimir Oltean <olteanv@...il.com>
To: Prabhakar <prabhakar.csengg@...il.com>
Cc: Clément Léger <clement.leger@...tlin.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>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Simon Horman <horms@...nel.org>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Russell King <linux@...linux.org.uk>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Magnus Damm <magnus.damm@...il.com>,
	linux-renesas-soc@...r.kernel.org, netdev@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Biju Das <biju.das.jz@...renesas.com>,
	Fabrizio Castro <fabrizio.castro.jz@...esas.com>,
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH net-next 09/11] net: dsa: rzn1-a5psw: Add support for
 management port frame length adjustment

On Fri, Nov 21, 2025 at 11:35:35AM +0000, Prabhakar wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> 
> Extend the RZN1 A5PSW driver to support SoC-specific adjustments to the
> management (CPU) port frame length. Some SoCs, such as the RZ/T2H and
> RZ/N2H, require additional headroom on the management port to account
> for a special management tag added to frames. Without this adjustment,
> frames may be incorrectly detected as oversized and subsequently
> discarded.
> 
> Introduce a new field, `management_port_frame_len_adj`, in
> `struct a5psw_of_data` to represent this adjustment, and apply it in
> `a5psw_port_change_mtu()` when configuring the frame length for the
> CPU port.
> 
> This change prepares the driver for use on RZ/T2H and RZ/N2H SoCs.

In the next change you set this to 40. What's the reason behind such a
high value (need to set the management port A5PSW_FRM_LENGTH value to
1574 bytes to pass L2 payload of 1500 bytes)? It sounds like this needs
to be called out more clearly for what it is - a hardware bug.