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

[Pascal Kneuper <PKneuper@dspace.de>]

On Tue, 08 Sep 2026 22:58:05 +0200, Maxime Chevallier wrote:
> 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
>

Great! Thanks for testing and keeping us posted :)

Best
Pascal