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

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Message-Id: <DDPRMKIPAO90.3RFDT36LZP7CE@bootlin.com>
Date: Thu, 23 Oct 2025 16:19:07 +0200
From: Théo Lebrun <theo.lebrun@...tlin.com>
To: "Andrew Lunn" <andrew@...n.ch>, 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 9:27 PM CEST, Andrew Lunn wrote:
> 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.

No, it isn't fixing any platform that currently uses the driver.
Dropped the "Fixes:" trailer for next revision.

Thanks Andrew,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com