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From: David Brownell <david-b@pacbell.net>
To: "Jaya Kumar" <jayakumar.lkml@gmail.com>
Subject: Re: [RFC 2.6.27 1/1] gpiolib: add support for batch set of pins
Date: Mon, 29 Dec 2008 12:32:08 -0800
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Cc: "Ben Nizette" <bn@niasdigital.com>,
       "Robin Getz" <rgetz@blackfin.uclinux.org>,
       "Eric Miao" <eric.y.miao@gmail.com>, "Sam Ravnborg" <sam@ravnborg.org>,
       "Eric Miao" <eric.miao@marvell.com>,
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       "Philipp Zabel" <philipp.zabel@gmail.com>,
       "Russell King" <rmk@arm.linux.org.uk>,
       "Ben Gardner" <bgardner@wabtec.com>, "Greg KH" <greg@kroah.com>,
       linux-arm-kernel@lists.arm.linux.org.uk,
       linux-fbdev-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org,
       linux-embedded@vger.kernel.org
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On Sunday 28 December 2008, Jaya Kumar wrote:
> I'd like to get the start/length approach out there and in-play to
> find out if other people want to use it and how they end up using it.

As an *implementation* constraint, I might agree ... so
long as it's easily changed later.

As an *interface* constraint, I don't ... interfaces are
rarely easy to change.

However, in terms of implementation, most gpio chips have
primitives that work in terms of bitmasks rather than any
kind of start/length primitive.  Example:

 - To set bits in "u32 mask":
	iowrite32(mask, bank_base + SET_REG)
 - To clear bits in "u32 mask"
	iowrite32(mask, bank_base + CLR_REG)
 - To read bits in "u32 mask",
	return mask & ioread32(bank_base + VALUE_REG)

In short, start/length looks most like a policy, of the
"keep them out of interfaces!" flavor, than something
appropriate for an interface.  As noted above, gpio_chip
interfaces would more naturally use masks.

- Dave
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