netdev - [PATCH RFC net-next] net: stmmac: document (buggy?) maximum MTU setup

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Message-Id: <E1vLJcL-0000000ExBL-0GFe@rmk-PC.armlinux.org.uk>
Date: Tue, 18 Nov 2025 11:11:25 +0000
From: "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>
To: Andrew Lunn <andrew@...n.ch>,
	Heiner Kallweit <hkallweit1@...il.com>
Cc: Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-stm32@...md-mailman.stormreply.com,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	netdev@...r.kernel.org,
	Paolo Abeni <pabeni@...hat.com>
Subject: [PATCH RFC net-next] net: stmmac: document (buggy?) maximum MTU setup

Add documentation for the ndev->max_mtu determination. I'm not sure
this is currect - specifically, having the "everything else" case
depend on the kernel's page size, which is a build-time choice for
some architectures and could be e.g. 4KiB, 16KiB or 64KiB.

E.g. when standard descriptors are used (enh_desc is false) and the
core is younger than v4.00, the buffer length is limited to 11 bits by
the format of the buffer size fields in the descriptors (TBS1/TBS2 in
TDES1 and RBS1/RBS2 in RDES1). So, when page size is greater than 4KiB,
a packet of max_mtu will not fit.

Also, dwmac4 cores (indicated with core_type == DWMAC_CORE_GMAC4) use
the descriptor format in dwmac4_descs.c, even when priv->synopsys_id
is less than DWMAC_CORE_4_00 (see the first DWMAC_CORE_GMAC4 entry in
the stmmac_hw table in hwif.c.)

So, I think this max_mtu is buggy.

Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 15b0c08ebd87..644dccb29f75 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -7813,6 +7813,12 @@ int stmmac_dvr_probe(struct device *device,
 	/* MTU range: 46 - hw-specific max */
 	ndev->min_mtu = ETH_ZLEN - ETH_HLEN;
 
+	/* Set the maximum MTU.
+	 *  For XGMAC cores, 16KiB.
+	 *  For cores using enhanced descriptors or GMAC cores >= v4.00, 9kB.
+	 *  For everything else, PAGE_SIZE - NET_SKB_PAD - NET_IP_ALIGN -
+	 *   aligned skb_shared_info.
+	 */
 	if (priv->plat->core_type == DWMAC_CORE_XGMAC)
 		ndev->max_mtu = XGMAC_JUMBO_LEN;
 	else if (priv->plat->enh_desc || priv->synopsys_id >= DWMAC_CORE_4_00)
-- 
2.47.3