linux-kernel - Re: [RFC 2.6.28 1/2] gpiolib: add set/get batch v4

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Date:	Mon, 19 Jan 2009 22:19:40 +0800
From:	Jaya Kumar <jayakumar.lkml@...il.com>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Cc:	David Brownell <david-b@...bell.net>,
	Eric Miao <eric.miao@...vell.com>,
	Paulius Zaleckas <paulius.zaleckas@...tonika.lt>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Sam Ravnborg <sam@...nborg.org>,
	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 2.6.28 1/2] gpiolib: add set/get batch v4

On Mon, Jan 19, 2009 at 6:03 PM, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
> Hi Jaya,
>
> On Sat, Jan 17, 2009 at 05:57:17PM +0800, Jaya Kumar wrote:
>> Hi friends,
>>
>> This is v4 of batch support for gpiolib. Thanks to David Brownell, Eric Miao,
>> David Hylands, Robin, Ben, Jamie and others for prior feedback. The post for
>> v3 summarized the previous discussion. Since then the changes I've made are:
>> - split the patches into generic and arch specific
> IMHO this should be three patches: "gpiolib", "pxa" and "am300epd".
> Well, ...

Hi Uwe,

Ok, will do.

>
>> +[OPTIONAL] Spinlock-Safe GPIO Batch access
> Is it really spinlock safe in general?  Or only if gpio_cansleep(gpio)
> if false for each gpio to get or set?

You are correct to raise this issue. It is only spinlock safe if
chip->cansleep is false. Initially, I wasn't sure what to do. The
original gpio set/get_value() just does;
        WARN_ON(extra_checks && chip->can_sleep);
and it is documented as:

"
Spinlock-Safe GPIO access
-------------------------
<snip>
return zero.  Also, using these calls for GPIOs that can't safely be accessed
without sleeping (see below) is an error.
"

I will change this in the batch code to return an error if can_sleep
is detected on any involved gpio_chip.

>
>> +static int __generic_gpio_set_batch(struct gpio_chip *chip, unsigned offset,
>> +                                     u32 values, u32 bitmask, int width)
> IMHO a better name is __gpio_set_batch_generic (or
> __gpiolib_set_batch_generic?), YMMV.

Agreed. Will change.

>
>> +int __gpio_set_batch(unsigned gpio, u32 values, u32 bitmask, int bitwidth)
> Sometimes your width parameter is called bitwidth, sometimes width.  I'd
> like to have that consistant.

Ok, you're right. I'll fix this.

>
> While talking about this parameter.  I don't really like it, because you
> can calculate it from bitmask.  In an earlier mail you write:

Agreed.

>
>        bitwidth (needed to iterate and map to chip ngpios) could be
>        calculated from bitmask, but that involves iteratively counting
>        bits from the mask, so we would have to do 800*600 bit counts.
>        Unless, we do ugly things like cache the previous bitwidth/mask
>        and compare against the current caller arguments. Given that the
>        bitwidth would typically be a fixed value, I believe it is more
>        intuitive for the caller to provide it, ...
>
> I think it's easier than that.  bitwidth is just fls(bitmask) which
> should be efficient enough not to bother the programmer.  If bitmask is
> constant it's even the compiler that does the work here.
>

That is a good point. I agree that width is ugly in the main API. It
is just fls(mask) and I now realize that this is an inline so you're
right it would get taken care of by the compiler. fls is checked with
__constant_fls first. Beauty! Thanks Uwe! I'll make these changes.

Thanks,
jaya
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