netdev - Re: [RFT PATCH 7/9] ethtool: prepare for larger netdev_features

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Message-ID: <BANLkTi=RB1MSR=c6GhXr4bsrPtFJVZnaZ8ZLCN=o339S_BE7iw@mail.gmail.com>
Date:	Thu, 23 Jun 2011 10:50:59 -0700
From:	Mahesh Bandewar <maheshb@...gle.com>
To:	Ben Hutchings <bhutchings@...arflare.com>,
	Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFT PATCH 7/9] ethtool: prepare for larger netdev_features_t type

On Mon, Jun 20, 2011 at 2:16 PM, Ben Hutchings
<bhutchings@...arflare.com> wrote:
>
> On Mon, 2011-06-20 at 21:14 +0200, Michał Mirosław wrote:
> [...]
> > @@ -125,19 +131,26 @@ static int ethtool_set_features(struct net_device *dev, void __user *useraddr)
> >       if (copy_from_user(features, useraddr, sizeof(features)))
> >               return -EFAULT;
> >
> > -     if (features[0].valid & ~NETIF_F_ETHTOOL_BITS)
> > +     /* I wonder if the compiler will be smart enough to loop-unroll
> > +      * and optimize this... (no worries if not) --mq */
> > +     for (i = ETHTOOL_DEV_FEATURE_WORDS; i-- > 0; ) {
> > +             valid = (valid << 32)|features[i].valid;
> > +             wanted = (wanted << 32)|features[i].requested;
> > +     }
> [...]
>
> I don't know (or care) about optimisation of this, but I would expect
> gcc to complain about shifting a 32-bit value by 32 bits.  I suggest you
> write this as:
>
>        for (i = 0; i < ETHTOOL_DEV_FEATURE_WORDS; ++i) {
>                valid |= (netdev_features_t)features[i].valid << 32 *i;
>                wanted |= (netdev_features_t)features[i].requested << 32 *i;

It's a valid point but this type of typecast or similar usage would
imply that netdev_feature_t is an int of XXX bits. That's not opaque
and would hinder the way you can abstract the feature type.

--mahesh..
>
>        }
>
> Ben.
>
> --
> Ben Hutchings, Senior Software Engineer, Solarflare
> Not speaking for my employer; that's the marketing department's job.
> They asked us to note that Solarflare product names are trademarked.
>
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