Re: [PATCH v2] net: macb: Disable macb pad and fcs for fragmented packets

[Harini Katakam <harinik@xilinx.com>]

Hi Jakub,

On Thu, May 12, 2022 at 4:10 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 10 May 2022 21:58:09 +0530 Harini Katakam wrote:
> > data_len in skbuff represents bytes resident in fragment lists or
> > unmapped page buffers. For such packets, when data_len is non-zero,
> > skb_put cannot be used - this will throw a kernel bug. Hence do not
> > use macb_pad_and_fcs for such fragments.
> >
> > Fixes: 653e92a9175e ("net: macb: add support for padding and fcs computation")
> > Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
> > Signed-off-by: Michal Simek <michal.simek@xilinx.com>
> > Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
> > Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
>
> I'm confused. When do we *have to* compute the FCS?
>
> This commit seems to indicate that we can't put the FCS so it's okay to
> ask the HW to do it. But that's backwards. We should ask the HW to
> compute the FCS whenever possible, to save the CPU cycles.
>
> Is there an unstated HW limitation here?

Thanks for the review. The top level summary is that there CSUM
offload is enabled by
via NETIF_F_HW_CSUM (and universally in IP registers) and then
selectively disabled for
certain packets (using NOCRC bit in buffer descriptors) where the
application intentionally
performs CSUM and HW should not replace it, for ex. forwarding usecases.
I'm modifying this list of exceptions with this patch.

This was due to HW limitation (see
https://www.spinics.net/lists/netdev/msg505065.html).
Further to this, Claudiu added macb_pad_and_fcs support. Please see
comment starting
with "It was reported in" below:
https://lists.openwall.net/netdev/2018/10/30/76

Hope this helps.
I'll fix the nit and send another version.

Regards,
Harini