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From: David Brownell <david-b@pacbell.net>
To: Bill Gatliff <bgat@billgatliff.com>
Subject: Re: [patch/rfc 2.6.19-rc5] arch-neutral GPIO calls
Date: Mon, 13 Nov 2006 12:53:00 -0800
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Cc: Paul Mundt <lethal@linux-sh.org>,
       Linux Kernel list <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>, Andrew Victor <andrew@sanpeople.com>,
       Haavard Skinnemoen <hskinnemoen@atmel.com>, jamey.hicks@hp.com,
       Kevin Hilman <khilman@mvista.com>, Nicolas Pitre <nico@cam.org>,
       Russell King <rmk@arm.linux.org.uk>, Tony Lindgren <tony@atomide.com>
References: <200611111541.34699.david-b@pacbell.net> <200611131215.39888.david-b@pacbell.net> <4558D4E6.4020601@billgatliff.com>
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On Monday 13 November 2006 12:26 pm, Bill Gatliff wrote:

> >Nah; look at arch/arm/plat-omap/gpio.c and ignore the mess, but observe
> >that what you see there is essentially a bunch of "gpio controller"
> >classes using the ugly "switch(type)" dispatch scheme instead of the
> >prettier "type->op()" dispatch scheme.  All that stuff needs to be
> >cleaner, but for now it'd suffice to add a new FPGA typecode.
> 
> Agreed.  But if we add to the machine descriptor, then not only do you 
> not need to touch arch-omap/gpio.c, but you can take that switch 
> statement out, too.  Just one less chunk of code to tweak when a new 
> platform is supported.

Do non-ARM platforms have board/machine descriptors on Linux, though?
I thought most didn't ...

One could come up with an implementation that uses GPIO numbers
as indices into a descriptor array, and using board-specific
initialization of that array ... just like with IRQs and irq_chip.

That could lead to heavier weight implementations than I'd prefer
to see (since GPIOs are a very light weight notion!), but it'd
certainly provide a more reusable way to add GPIO controllers.

All behind the API I proposed, note -- no changes needed.

- Dave


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