netdev - Re: [PATCH net-next v2 2/5] net: macb: match skb_reserve(skb, NET_IP

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Message-ID: <7950f287-f025-40d9-b182-c1002d955a5b@lunn.ch>
Date: Wed, 22 Oct 2025 21:27:58 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Théo Lebrun <theo.lebrun@...tlin.com>
Cc: Andrew Lunn <andrew+netdev@...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>,
	Nicolas Ferre <nicolas.ferre@...rochip.com>,
	Claudiu Beznea <claudiu.beznea@...on.dev>,
	Russell King <linux@...linux.org.uk>, netdev@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Benoît Monin <benoit.monin@...tlin.com>,
	Grégory Clement <gregory.clement@...tlin.com>,
	Maxime Chevallier <maxime.chevallier@...tlin.com>,
	Tawfik Bayouk <tawfik.bayouk@...ileye.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
	Vladimir Kondratiev <vladimir.kondratiev@...ileye.com>
Subject: Re: [PATCH net-next v2 2/5] net: macb: match skb_reserve(skb,
 NET_IP_ALIGN) with HW alignment

On Wed, Oct 22, 2025 at 09:38:11AM +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).
> 
> Fixes: 4df95131ea80 ("net/macb: change RX path for GEM")

Is this a real fix? You should not mix new development with
fixes. Either post this patch to net, or drop the Fixes: tag for
net-next.

    Andrew

---
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