linux-kernel - Re: [PATCH net-next v2 12/18] net: macb: match skb_reserve(skb, NET_IP

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Message-ID: <d4bead1c-697a-46d8-ba9c-64292fccb19f@linux.dev>
Date: Mon, 11 Aug 2025 14:53:19 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Théo Lebrun <theo.lebrun@...tlin.com>,
 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>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>, Samuel Holland <samuel.holland@...ive.com>,
 Richard Cochran <richardcochran@...il.com>,
 Russell King <linux@...linux.org.uk>,
 Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
 Vladimir Kondratiev <vladimir.kondratiev@...ileye.com>,
 Gregory CLEMENT <gregory.clement@...tlin.com>,
 Cyrille Pitchen <cyrille.pitchen@...el.com>,
 Harini Katakam <harini.katakam@...inx.com>,
 Rafal Ozieblo <rafalo@...ence.com>,
 Haavard Skinnemoen <hskinnemoen@...el.com>
Cc: netdev@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
 linux-mips@...r.kernel.org, Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 Tawfik Bayouk <tawfik.bayouk@...ileye.com>
Subject: Re: [PATCH net-next v2 12/18] net: macb: match skb_reserve(skb,
 NET_IP_ALIGN) with HW alignment

On 8/7/25 11:24, Théo Lebrun wrote:
> On Tue Jul 1, 2025 at 6:40 PM CEST, Sean Anderson wrote:
>> On 6/27/25 05:08, Théo Lebrun wrote:
>>> If HW is RSC capable, it cannot add dummy bytes at the start of IP
>>
>> Receive-side coalescing? Can you add a brief description of this
>> feature to your commit message?
> 
> Yes that is Receive Side Coalescing. Clearly it needs to be mentioned
> out loud, and briefly described.
> 
>>> 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")
>>
>> Do any existing MACBs support RSC? Is this a fix? 
> 
> I have no idea. If any MACB supports RSC, it must be those running with
> NET_IP_ALIGN=0, so arm64/powerpc/x86.
> 
> Is it a fix? We can guess that all boards fall in either category:
>  - Don't support RSC (=> RBOF works fine).
>  - Support RSC (=> RBOF not working) AND NET_IP_ALIGN=0.
> 
> Both of those are not impacted, so we technically don't fix anything for
> current users.

OK, then please drop the fixes tag then.

--Sean