linux-kernel - Re: [PATCH] gpiolib: introduce for_each_gpio_in

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Message-ID: <45a44e480909182140s6df2cc73td6e2fd0362fc49f9@mail.gmail.com>
Date:	Sat, 19 Sep 2009 12:40:44 +0800
From:	Jaya Kumar <jayakumar.lkml@...il.com>
To:	H Hartley Sweeten <hartleys@...ionengravers.com>
Cc:	Ben Nizette <bn@...sdigital.com>,
	David Brownell <david-b@...bell.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpiolib: introduce for_each_gpio_in_chip macro

On Sat, Sep 19, 2009 at 8:42 AM, H Hartley Sweeten
<hartleys@...ionengravers.com> wrote:
> On Friday, September 18, 2009 5:03 PM, Jaya Kumar wrote:
>
> Hmm.. That patchset is a lot different than what I was thinking of.  Your
> patch allows a variable width to the number of gpio's in the "port".  But
> it also still gets/sets the "port" by individual bit accesses to the
> gpio_chip.  By doing this I don't see how you could get a performance
> increase.

It only does individual bit set/get if the specific arch does not
offer a multi bit set/get. See __gpio_set_batch_generic which calls
chip->set versus __gpio_set_batch which calls chip->set_batch so that
if the underlying architecture supports it then we do multi-bit
access. I provided one for pxa here:
http://marc.info/?l=linux-kernel&m=123287743104542&w=2 . So, the
following scenarios should all be optimized for:

a) if the user calls gpio_s/get_batch with a width that fits the
native register width and all those bits fit into the said register,
then it results in a call directly to the multibit register set/get
function/inline. For example, on pxa with above implementation,
gpio_set_batch(1, mask=0xFFFFFFF, value=0xc001f00d), at compile time,
that will result in a single GPSR/GPCR.
b) if the user calls gpio_s/get_batch with a width that more than one
register, the implementation walks the bits and splits it across the
registers as needed. This is done in __gpio_s/get_batch
c) if the user calls the gpio_s/get_batch with non-consecutive bits
using the 32-bit mask, then the implementation walks the bits and
splits it across the registers as needed.

So, it is fairly optimized. If I understood David's suggested design
correctly, it is one where the user does not need to worry about masks
or widths at all. Instead, you register with the library, a list of
bits you want to make a bus out of, and then the library is
responsible for taking that and optimizing that further as much as
possible. I agree that that would be a more elegant API. What I was
hoping for is that my implementation above could be a starting point
for that by first providing optimized multi-bit access that could be
used by the more elegant API.

Thanks,
jaya
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