netdev - Re: [PATCH] net: drop NETIF_F_GSO from hw_features without NETIF_F

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Message-ID: <AANLkTinCdaEs=G=AC45ePA-J99Vik9fYE+ZyF1miCiVy@mail.gmail.com>
Date:	Sat, 19 Mar 2011 17:18:17 +0100
From:	Michał Mirosław <mirqus@...il.com>
To:	Roger Luethi <rl@...lgate.ch>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] net: drop NETIF_F_GSO from hw_features without NETIF_F_SG

2011/3/19 Roger Luethi <rl@...lgate.ch>:
> On Fri, 18 Mar 2011 21:43:23 +0100, Micha? Miros?aw wrote:
>> 2011/3/18 Roger Luethi <rl@...lgate.ch>:
>> > register_netdevice() removes NETIF_F_GSO from dev->features and
>> > dev->wanted_features for hardware without NETIF_F_SG to "avoid warning from
>> > netdev_fix_features()".
>> >
>> > However, as the flag remains set in dev->hw_features, users trying to
>> > enable GSO via ethtool will still end up with NETIF_F_GSO stuck in
>> > wanted_features, resulting in a warning every time they try to change any
>> > feature (until they use ethtool to "disable" GSO):
>>
>> That's expected, as it allows the user to debug why his request didn't
>> work as expected. I think that those messages should get demoted to
>> DEBUG if that's too noisy. The network core imposed conditions could
>> be explained in Documentation/ or ethtool manpage instead.
>
> I agree that it is good to have a note explaining what happened in the
> kernel log, and the patch I posted does not do that; that is easy to fix.
>
> However, I still think the current, unpatched behavior is confusing users.
> ethtool_get_features() will happily tell the user that GSO is available for
> a NIC even if it is not (because the NIC does not support SG). Then, if the
> user tries to enable GSO, the operation fails silently: ethtool won't get
> an error code, so it looks as if the operation succeeded. The user needs to
> look into the kernel log or actually verify the offload settings to realize
> that something went wrong.

Actually, the user will get the information that GSO wasn't enabled,
though not while using ETHTOOL_SGSO.

OTOH, I think I can see what's your point - I got confused by your
proposed patch. The correct way is to remove NETIF_F_GSO from
hw_features when NETIF_F_SG is not present in hw_features (and not
wanted_features as you're patch is doing). The idea here is that a
driver might advertise SG in changeable features (that's hw_features)
but not enable it by default for whatever reason.

Do you want to prepare a patch for this? Please remember about
compatibility with per-offload ethtool ops if you do - this needs to
be there until all drivers are converted to hw_features.

Best Regards,
Michał Mirosław
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