netdev - Re: [PATCH] net: Change netdev_fix

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Message-ID: <20110517082700.GA18423@rere.qmqm.pl>
Date:	Tue, 17 May 2011 10:27:00 +0200
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	mst@...hat.com, bhutchings@...arflare.com
Subject: Re: [PATCH] net: Change netdev_fix_features messages loglevel

On Tue, May 17, 2011 at 09:48:16AM +1000, Herbert Xu wrote:
> On Mon, May 16, 2011 at 03:14:34PM -0400, David Miller wrote:
> > From: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > Date: Mon, 16 May 2011 15:17:57 +0200 (CEST)
> > 
> > > Those reduced to DEBUG can possibly be triggered by unprivileged processes
> > > and are nothing exceptional. Illegal checksum combinations can only be
> > > caused by driver bug, so promote those messages to WARN.
> > > 
> > > Since GSO without SG will now only cause DEBUG message from
> > > netdev_fix_features(), remove the workaround from register_netdevice().
> > > 
> > > Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > Applied, thanks.
> I think this patch is not a good idea.  The only problem here
> is the tun driver which should just fix up the feature bits
> silently (we need to do that anyway as right now it doesn't
> verify the feature bits at all, those warnings are only trigger
> on registering the tun device).
> 
> With this patch future buggy drivers will instead trigger hard-
> to-debug warnings further down the stack in the TSO code path,
> like they have before (or silent packet drops if those warnings
> aren't there anymore).

If core TSO code really depends on checksumming features being turned on
then this dependency should be checked in netdev_fix_features. There
will be no possibility for a driver to cause bugs then.

Network core is making up for devices which can't checksum just before
calling ndo_hard_start_xmit. It's a possibility that packet to be TSOed
will be unnecessarily checksummed in dev_hard_start_xmit() if HW checksum
is off while TSO is on (I deduce this based on the code, can't test as
TSO in my test-box is busted).

Best Regards,
Michał Mirosław
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