netdev - Re: [PATCH v5 RESEND 2/9] ethtool: enable GSO and GRO by default

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Date:	Wed, 16 Feb 2011 03:42:37 +0100
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, bhutchings@...arflare.com
Subject: Re: [PATCH v5 RESEND 2/9] ethtool: enable GSO and GRO by default

On Tue, Feb 15, 2011 at 02:00:43PM -0800, David Miller wrote:
> From: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> Date: Tue, 15 Feb 2011 22:46:49 +0100
> > On Sun, Feb 13, 2011 at 10:50:23AM -0800, David Miller wrote:
> >> From: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> >> Date: Sun, 13 Feb 2011 12:11:45 +0100 (CET)
> >> > Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> >> This is not appropriate.
> >> 
> >> Now, every driver that lacks SG support will spit out that warning
> >> message in netdev_fix_features().
> >> 
> >> That's why the check is there conditionalizing NETIF_F_GSO on
> >> NETIF_F_SG in register_netdevice().
> > I think all those messages should be converted to DEBUG level. Those
> > conditions are constant and can be better described in
> > Documentation/networking/ or ethtool manpage. Preferably along the
> > device-specific conditions when implemented.
> > 
> > Or I could just drop the message for the GSO case as it's something new here
> > anyway (I added it to make it consistent with handling of other features).
> The messages exist to let driver authors know they've constructed an
> illegal set of feature bits.

Only multiple checksum flags are actual driver bugs - those are handled
in register_netdevice(). netdev_fix_features() represents limitations of core
networking code . Hardware might as well handle TSO without SG or SG without
checksumming. What's the point in duplicating those restrictions in drivers?

> Since you're now adding the GSO bit yourself, you should perform
> due diligence and prevent the illegal combination yourself.
>
> This has no other impact on the other messages and cases, which
> definitely should stay intact.
> 
> Your change is just wrong and knowingly introduces useless log
> messages, please just fix it up.

The messages from netdev_fix_features() will also be triggered by user
disabling required feature (ie. HW checksum for most of TX offloads)
for a device or its slave or sometimes on MTU changes (ie. for jme and
possibly others).  Even after I fix triggering of "GSO needs SG"
message in register_netdevice(), this and other messages will show up
more often because of the way the new scheme works.

Best Regards,
Michał Mirosław
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