netdev - Re: extending feature word.

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Message-ID: <20110411191947.GC31338@rere.qmqm.pl>
Date:	Mon, 11 Apr 2011 21:19:47 +0200
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Mahesh Bandewar <maheshb@...gle.com>,
	linux-netdev <netdev@...r.kernel.org>,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>
Subject: Re: extending feature word.

On Mon, Apr 11, 2011 at 11:54:59AM -0700, Stephen Hemminger wrote:
> On Mon, 11 Apr 2011 11:45:05 -0700
> Mahesh Bandewar <maheshb@...gle.com> wrote:
> > >> 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, ...
> > Should the added code be of any concern? If that is happening in the
> > control-path and does not affect the data-path as such; those added
> > instructions is a cost of added flexibility to we got through bitmap.
> > If performance is not at risk then that shouldn't be a problem.
> Just to be dense... What is wrong with just using u64?

Hmm. Looks like this is so simple that nobody thought of it seriously. ;)

This of course needs a bit of glue code in G/SFEATURES handling, but most
of the change would be s/u32/u64/ in apropriate places.

Best Regards,
Michał Mirosław
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