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

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Message-ID: <BANLkTimig_Fgi9DFOs4u_R1hAYLxfaa0cJXwO_rQ4P6bkm+-QA@mail.gmail.com>
Date:	Thu, 23 Jun 2011 13:38:51 -0700
From:	Mahesh Bandewar <maheshb@...gle.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	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 Thu, Jun 23, 2011 at 11:55 AM, Ben Hutchings
<bhutchings@...arflare.com> wrote:
> On Thu, 2011-06-23 at 11:21 -0700, Mahesh Bandewar wrote:
>> On Thu, Jun 23, 2011 at 11:03 AM, Ben Hutchings
>> <bhutchings@...arflare.com> wrote:
>> > On Thu, 2011-06-23 at 10:50 -0700, Mahesh Bandewar wrote:
>> >> 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.
>> >
>> > Yes, ethtool_{get,set}_features() will have to be changed if and when
>> > the representation of netdev_features_t is changed significantly.  I
>> > don't think there's any way of avoiding that and I don't think it really
>> > matters.
>> >
>> Well, if you have a conversion routine that converts (whatever the)
>> netdev_type_t type is to the ethtool representation (array of u32 for
>> example). So the changes would have to be done in that conversion
>> routine only and not get/set_features() ethtool methods as such.
>
> These are precisely those conversion routines, because there is no other
> place that needs to deal with the ethtool representation...
>
Oh, I was considering get/set_features ethtool methods as APIs  to
manipulate certain bits from user-space into kernel space. I was
disassociating features bit representation as a separate aspect and
(of course) the conversion. So depending on the type selection you may
or may not need these conversion routines.

--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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