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

[Théo Lebrun]

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.

Regards,

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