Re: [PATCH net v2] net: stmmac: restore NET_IP_ALIGN in the RX DMA offset

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi again, 

On 9/8/26 10:16, Maxime Chevallier wrote:
> Hi,
> 
> On 8/24/26 14:50, Pascal Kneuper wrote:
>> Since the RX path was converted to zero-copy, the page pool page is handed
>> to the stack directly as the skb head, and the offset the DMA engine writes
>> at is what determines the alignment of the packet headers.
>>
>> Before the conversion the payload was copied into an skb obtained from
>> napi_alloc_skb(), which reserves NET_SKB_PAD + NET_IP_ALIGN. The
>> conversion moved the headroom into stmmac_rx_offset() but did not carry
>> over NET_IP_ALIGN, so on architectures where NET_IP_ALIGN is 2 the IP
>> header now lands misaligned:
>>
>>   64 (NET_SKB_PAD) + 14 (ethernet) + 20 (IP) = 98
>>
>> Same for the XDP branch:
>>
>>   256 (XDP_PACKET_HEADROOM) + 14 (ethernet) + 20 (IP) = 290
>>
>> On ARM32 this is fatal, because ldm and ldrd trap on unaligned addresses
>> even when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set.

> This patch breaks the stmmac selftests for Jumbo Frames on dwmac1000 :(
So looks like the problem isn't this patch, but rather both the way we select
the bufsize, and the Jumbo test itself :)

This merely surfaces existing problems, I have some fixes that I'll send shortly.

Maxime