netdev - RE: [PATCH 4/7] cxgb4: Add packet queues and packet DMA code

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Message-ID: <8A71B368A89016469F72CD08050AD33401359B69@maui.asicdesigners.com>
Date:	Fri, 26 Feb 2010 16:10:07 -0800
From:	"Dimitrios Michailidis" <dm@...lsio.com>
To:	"Stephen Hemminger" <shemminger@...tta.com>
Cc:	<netdev@...r.kernel.org>
Subject: RE: [PATCH 4/7] cxgb4: Add packet queues and packet DMA code



> -----Original Message-----
> From: Stephen Hemminger [mailto:shemminger@...tta.com]
> Sent: Friday, February 26, 2010 3:58 PM
> To: Dimitrios Michailidis
> Cc: netdev@...r.kernel.org
> Subject: Re: [PATCH 4/7] cxgb4: Add packet queues and packet DMA code
> 
> On Fri, 26 Feb 2010 15:35:36 -0800
> Dimitris Michailidis <dm@...lsio.com> wrote:
> 
> > +
> > +/**
> > + *	need_skb_unmap - does the platform need unmapping of sk_buffs?
> > + *
> > + *	Returns true if the platfrom needs sk_buff unmapping.  The
compiler
> > + *	optimizes away unecessary code if this returns true.
> > + */
> > +static inline int need_skb_unmap(void)
> > +{
> > +	/*
> > +	 * This structure is used to tell if the platfrom needs buffer
> > +	 * unmapping by checking if DECLARE_PCI_UNMAP_ADDR defines
anything.
> > +	 */
> > +	struct dummy {
> > +		DECLARE_PCI_UNMAP_ADDR(addr);
> > +	};
> > +
> > +	return sizeof(struct dummy) != 0;
> > +}
> > +
> 
> I would prefer one code path and let the compiler decide if unmap
> should be nop; rather than this kind of trick code.

I believe this function does not generate any code, the compiler
statically figures out the result and optimizes any conditionals that
call it.  What option do you have in mind that would tell the compiler
if unmap is nop?
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