netdev - Re: extending feature word.

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Date:	Sun, 10 Apr 2011 12:19:54 +0200
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	Mahesh Bandewar <maheshb@...gle.com>
Cc:	linux-netdev <netdev@...r.kernel.org>,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>
Subject: Re: extending feature word.

On Fri, Apr 08, 2011 at 11:17:41AM -0700, Mahesh Bandewar wrote:
> On Fri, Apr 8, 2011 at 3:05 AM, Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:
> > On Fri, Apr 01, 2011 at 07:07:05PM -0700, Mahesh Bandewar wrote:
> >> Thanks for your comments on my loop-back patch. I was looking at the
> >> code today from the perspective of extending various "features" for
> >> word to an array of words and as Michael has pointed out, it's a huge
> >> change. So I'm thinking on the following lines
> >> (include/linux/netdevice.h)
> >>
> >> +#define DEV_FEATURE_WORDS      2
> >> +#define LEGACY_FEATURE_WORD    0
> >>        /* currently active device features */
> >> -       u32                     features;
> >> +       u32                     features[DEV_FEATURE_WORDS];
> >>        /* user-changeable features */
> >> -       u32                     hw_features;
> >> +       u32                     hw_features[DEV_FEATURE_WORDS];
> >>        /* user-requested features */
> >> -       u32                     wanted_features;
> >> +       u32                     wanted_features[DEV_FEATURE_WORDS];
> >>        /* VLAN feature mask */
> >> -       u32                     vlan_features;
> >> +       u32                     vlan_features[DEV_FEATURE_WORDS];
> >
> > Hmm. There might be no point in making features field an array.
> > This gives us nothing really. Maybe just add features_2 or similar?
> > If we ever get to the point there need to be more than two words for
> > features we can think of some abstraction layer then.
> >
> That is right! making it an array doesn't really buy us anything
> unless there is a uniform way of managing all the bits spread across
> multiple words inside that array. This was the reason why I have
> changed that array into a bitmap in the patch that I have posted
> earlier. This way the upper limit (currently only 32 bits) will be
> removed and we'll have a long term solution. There will be little bit
> of work involved but 'doing-things-right' has cost associated.

I really don't like the bitmap idea. It multiplies the amount of code
needed to manipulate multiple bits at once --- and that's a common
thing for drivers to do. Almost every driver that needs ndo_fix_features
will clear sets --- checkumming set, TSO set, all TX offloads set, ...

As a first step just add another set of words:

union {
	struct {
		u32 features;
		u32 features_2;
	} /* anonymous struct */;
	u32 features_array[2];
} /* anonymous union */;

This allows to change drivers after core supporting code gets implemented.

Best Regards,
Michał Mirosław
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