linux-kernel - Re: [PATCH v2 1/1] net/macb: add TX multiqueue support for gem

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Date:	Fri, 12 Dec 2014 10:52:12 +0100
From:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Cyrille Pitchen <cyrille.pitchen@...el.com>,
	"nicolas.ferre@...el.com" <nicolas.ferre@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"soren.brinkmann@...inx.com" <soren.brinkmann@...inx.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] net/macb: add TX multiqueue support for gem

Dear David Laight,

On Fri, 12 Dec 2014 09:45:30 +0000, David Laight wrote:
> From: Thomas Petazzoni
> > On Thu, 11 Dec 2014 11:16:51 +0100, Cyrille Pitchen wrote:
> > 
> > > +#define GEM_ISR1				0x0400
> > > +#define GEM_ISR2				0x0404
> > > +#define GEM_ISR3				0x0408
> > > +#define GEM_ISR4				0x040c
> > > +#define GEM_ISR5				0x0410
> > > +#define GEM_ISR6				0x0414
> > > +#define GEM_ISR7				0x0418
> > 
> > What about doing instead:
> > 
> > #define GEM_ISR(q)				((q) == 0 ? MACB_ISR : 0x400 + (q) << 2)
> > 
> > And ditto for all other registers, which will save a lot of boring repeated code.
> 
> It will probably add a lot of object code and, depending on how often
> the registers are accesses, might have performance impact.
> 
> Having:
> #define GEM_ISR(n) (0x400 + (n) << 4)
> will save source code.

Except that this won't work for n == 0, because for n == 0, the
register offset is not 0x400.

And in fact, my code was also wrong, it should be:

	((q) == 0 ? MACB_ISR : 0x400 + ((q)-1) << 2))

Since q=1 is at 0x400.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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