netdev - Re: [PATCH V2 net] netdevice: Include NETIF_F_HW

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Message-ID: <20170421053325.GH13789@unicorn.suse.cz>
Date:   Fri, 21 Apr 2017 07:33:25 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Vlad Yasevich <vyasevic@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Vladislav Yasevich <vyasevich@...il.com>,
        Netdev <netdev@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH V2 net] netdevice: Include NETIF_F_HW_CSUM when
 intersecting features

On Thu, Apr 20, 2017 at 07:19:55PM -0400, Vlad Yasevich wrote:
> 
> Having said that, the other alternative is to inherit hw_features from
> lower devices.  BTW, bonding I think has a similar "issue" you are
> describing since it prefers HW_CSUM if any of the slaves have it set.

It does but bonding uses netdev_increment_features() to combine slave
features and this function handles checksumming like "or", not "and"
(not only checksumming, also flags in NETIF_F_ONE_FOR_ALL).

That said, it's legitimate to ask if we want some of the features to be
handled differently when computing features for a vlan device. My point
before was that if the helper is called netdev_intersect_features(), it
shouldn't return any features that are not supported by both argument
sets, even if all its current users would benefit from slightly
different behaviour. If it does, it's a trap that someone might one day
fall in.

                                                         Michal Kubecek