Re: [PATCH] net: stmmac: Disable checksum insertion for XDP frame TX

[Jakub Kicinski]

On Tue, 29 Sep 2026 22:29:14 +0200 Andrew Lunn wrote:
> On Tue, Sep 29, 2026 at 08:10:25PM +0800, Jisheng Zhang wrote:
> > stmmac enables TX checksum insertion for XDP frames whenever the queue
> > supports it. XDP frames carry no TX checksum offload request, so this
> > can overwrite a checksum already present in the packet.
> > 
> > Pass false to stmmac_prepare_tx_desc() when transmitting an XDP frame
> > so that the packet is sent with its checksum fields unchanged.  
> 
> This seems odd to me.
> 
> If the frame contains a checksum, it is either correct, and the
> hardware calculated one will come out the same, not an issue.  Or the
> checksum in the frame is actually wrong, because the frame has got
> mangled by eBPF before sending it out, and you want the hardware to
> calculate the correct value.
> 
> What an i missing?

Oops, missed this before applying. Fair, but also we shouldn't modify
the frame if user didn't ask for it. Perhaps 0 UDP csum is used, or
user is trying to build a testing tool and intentionally send bad csum.
I applied the patch to net-next. It can hurt as much as help, but the
direction seems right. At the very least any AF_XDP app expecting csum
insertion without asking for it would not be portable to other drivers.