netdev - Re: [PATCH net 4/4] net: ethernet: cortina: Handle large frames

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Date: Sun, 5 Nov 2023 21:56:23 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Hans Ulli Kroll <ulli.kroll@...glemail.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Michał Mirosław <mirq-linux@...e.qmqm.pl>, 
	Vladimir Oltean <olteanv@...il.com>, linux-arm-kernel@...ts.infradead.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 4/4] net: ethernet: cortina: Handle large frames

On Sat, Nov 4, 2023 at 3:57 PM Andrew Lunn <andrew@...n.ch> wrote:

> > +              * Just bypass on bigger frames.
> > +              */
> > +             word1 |= TSS_BYPASS_BIT;
> > +     } else if (skb->ip_summed != CHECKSUM_NONE) {
>
> I've never looked at how the network stack does checksums. But looking
> at this patch, it made me wounder, how do you tell the stack it needs
> to do a software checksum because the hardware cannot?

I read up on it: the documentation is in
Documentation/networking/checksum-offloads.rst
and in the header for skbuff, include/linux/skbuff.h

Actually we should check for == CHECKSUM_PARTIAL which means
we need to do the checksum (!= CHECKSUM_NONE is not inclusive)
then I call a software fallback directly from the driver if I need to.

> Or for this
> driver, is it always calculating a checksum, which is then ignored?
> Maybe you can improve performance a little but disabling software
> checksum when it is not needed?

The ping was somehow working without proper checksum
before, but I think I'm doing the right thing now, also tested with
HTTP traffic, check out v2.

Thanks for pointing it out, the patch looks way better now.

Yours,
Linus Walleij