Re: [PATCH net-next v2 12/18] net: macb: match skb_reserve(skb, NET_IP_ALIGN) with HW alignment

[Sean Anderson <sean.anderson@linux.dev>]

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