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

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Message-ID: <20251121113553.2955854-10-prabhakar.mahadev-lad.rj@bp.renesas.com>
Date: Fri, 21 Nov 2025 11:35:35 +0000
From: Prabhakar <prabhakar.csengg@...il.com>
To: Clément Léger <clement.leger@...tlin.com>,
	Andrew Lunn <andrew@...n.ch>,
	Vladimir Oltean <olteanv@...il.com>,
	"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>
Cc: linux-renesas-soc@...r.kernel.org,
	netdev@...r.kernel.org,
	devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Prabhakar <prabhakar.csengg@...il.com>,
	Biju Das <biju.das.jz@...renesas.com>,
	Fabrizio Castro <fabrizio.castro.jz@...esas.com>,
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: [PATCH net-next 09/11] net: dsa: rzn1-a5psw: Add support for management port frame length adjustment

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.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
---
 drivers/net/dsa/rzn1_a5psw.c | 4 ++++
 drivers/net/dsa/rzn1_a5psw.h | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/net/dsa/rzn1_a5psw.c b/drivers/net/dsa/rzn1_a5psw.c
index dc42a409eaef..82f4236a726e 100644
--- a/drivers/net/dsa/rzn1_a5psw.c
+++ b/drivers/net/dsa/rzn1_a5psw.c
@@ -211,6 +211,10 @@ static int a5psw_port_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
 	struct a5psw *a5psw = ds->priv;
 
 	new_mtu += ETH_HLEN + A5PSW_EXTRA_MTU_LEN + ETH_FCS_LEN;
+
+	if (dsa_is_cpu_port(ds, port))
+		new_mtu += a5psw->of_data->management_port_frame_len_adj;
+
 	a5psw_reg_writel(a5psw, A5PSW_FRM_LENGTH(port), new_mtu);
 
 	return 0;
diff --git a/drivers/net/dsa/rzn1_a5psw.h b/drivers/net/dsa/rzn1_a5psw.h
index 0fef32451e4f..41c910d534cf 100644
--- a/drivers/net/dsa/rzn1_a5psw.h
+++ b/drivers/net/dsa/rzn1_a5psw.h
@@ -235,11 +235,15 @@ union lk_data {
  * @nports: Number of ports in the switch
  * @cpu_port: CPU port number
  * @tag_proto: DSA tag protocol used by the switch
+ * @management_port_frame_len_adj: Adjustment to apply to management
+ *   port frame length to account for accepting a frame with special
+ *   management tag.
  */
 struct a5psw_of_data {
 	unsigned int nports;
 	unsigned int cpu_port;
 	enum dsa_tag_protocol tag_proto;
+	unsigned int management_port_frame_len_adj;
 };
 
 /**
-- 
2.52.0