From: Andrew Lunn <andrew@lunn.ch>
To: "Théo Lebrun" <theo.lebrun@bootlin.com>
Cc: "Andrew Lunn" <andrew+netdev@lunn.ch>,
"David S. Miller" <davem@davemloft.net>,
"Eric Dumazet" <edumazet@google.com>,
"Jakub Kicinski" <kuba@kernel.org>,
"Paolo Abeni" <pabeni@redhat.com>,
"Rob Herring" <robh@kernel.org>,
"Krzysztof Kozlowski" <krzk+dt@kernel.org>,
"Conor Dooley" <conor+dt@kernel.org>,
"Nicolas Ferre" <nicolas.ferre@microchip.com>,
"Claudiu Beznea" <claudiu.beznea@tuxon.dev>,
"Russell King" <linux@armlinux.org.uk>,
netdev@vger.kernel.org, devicetree@vger.kernel.org,
linux-kernel@vger.kernel.org,
"Benoît Monin" <benoit.monin@bootlin.com>,
"Grégory Clement" <gregory.clement@bootlin.com>,
"Maxime Chevallier" <maxime.chevallier@bootlin.com>,
"Tawfik Bayouk" <tawfik.bayouk@mobileye.com>,
"Thomas Petazzoni" <thomas.petazzoni@bootlin.com>,
"Vladimir Kondratiev" <vladimir.kondratiev@mobileye.com>
Subject: Re: [PATCH net-next v3 2/5] net: macb: match skb_reserve(skb, NET_IP_ALIGN) with HW alignment
Date: Thu, 23 Oct 2025 21:07:29 +0200 [thread overview]
Message-ID: <ef986d47-10ed-49af-9576-b0c1cda7cc5b@lunn.ch> (raw)
In-Reply-To: <20251023-macb-eyeq5-v3-2-af509422c204@bootlin.com>
On Thu, Oct 23, 2025 at 06:22:52PM +0200, Théo Lebrun wrote:
> If HW is RSC capable, it cannot add dummy bytes at the start of IP
> packets. Alignment (ie number of dummy bytes) is configured using the
> RBOF field inside the NCFGR register.
>
> On the software side, the skb_reserve(skb, NET_IP_ALIGN) call must only
> be done if those dummy bytes are added by the hardware; notice the
> skb_reserve() is done AFTER writing the address to the device.
>
> We cannot do the skb_reserve() call BEFORE writing the address because
> the address field ignores the low 2/3 bits. Conclusion: in some cases,
> we risk not being able to respect the NET_IP_ALIGN value (which is
> picked based on unaligned CPU access performance).
>
> Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Andrew
next prev parent reply other threads:[~2025-10-23 19:07 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-23 16:22 [PATCH net-next v3 0/5] net: macb: EyeQ5 support Théo Lebrun
2025-10-23 16:22 ` [PATCH net-next v3 1/5] dt-bindings: net: cdns,macb: add Mobileye EyeQ5 ethernet interface Théo Lebrun
2025-10-23 16:22 ` [PATCH net-next v3 2/5] net: macb: match skb_reserve(skb, NET_IP_ALIGN) with HW alignment Théo Lebrun
2025-10-23 19:07 ` Andrew Lunn [this message]
2025-10-30 14:31 ` Nicolas Ferre
2025-10-23 16:22 ` [PATCH net-next v3 3/5] net: macb: add no LSO capability (MACB_CAPS_NO_LSO) Théo Lebrun
2025-10-30 14:32 ` Nicolas Ferre
2025-10-23 16:22 ` [PATCH net-next v3 4/5] net: macb: rename bp->sgmii_phy field to bp->phy Théo Lebrun
2025-10-30 14:34 ` Nicolas Ferre
2025-10-23 16:22 ` [PATCH net-next v3 5/5] net: macb: Add "mobileye,eyeq5-gem" compatible Théo Lebrun
2025-10-23 19:08 ` Andrew Lunn
2025-10-30 14:37 ` Nicolas Ferre
2026-02-26 10:46 ` Conor Dooley
2026-02-26 19:24 ` Russell King (Oracle)
2026-02-26 20:20 ` Conor Dooley
2026-02-26 20:44 ` Russell King (Oracle)
2026-02-26 21:05 ` Conor Dooley
2026-02-26 21:13 ` Andrew Lunn
2025-10-28 14:20 ` [PATCH net-next v3 0/5] net: macb: EyeQ5 support patchwork-bot+netdevbpf
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