linux-kernel - Re: [RFC 2.6.27 1/1] gpiolib: add support for batch set of pins

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Date:	Mon, 29 Dec 2008 12:32:08 -0800
From:	David Brownell <david-b@...bell.net>
To:	"Jaya Kumar" <jayakumar.lkml@...il.com>
Cc:	"Ben Nizette" <bn@...sdigital.com>,
	"Robin Getz" <rgetz@...ckfin.uclinux.org>,
	"Eric Miao" <eric.y.miao@...il.com>,
	"Sam Ravnborg" <sam@...nborg.org>,
	"Eric Miao" <eric.miao@...vell.com>,
	"Haavard Skinnemoen" <hskinnemoen@...el.com>,
	"Philipp Zabel" <philipp.zabel@...il.com>,
	"Russell King" <rmk@....linux.org.uk>,
	"Ben Gardner" <bgardner@...tec.com>, "Greg KH" <greg@...ah.com>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-fbdev-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, linux-embedded@...r.kernel.org
Subject: Re: [RFC 2.6.27 1/1] gpiolib: add support for batch set of pins

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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