linux-kernel - Re: [PATCH] leds/of: leds-gpio.c: Use gpio_get_value

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Date:	Thu, 8 Sep 2011 22:44:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Trent Piepho <tpiepho@...il.com>
Cc:	David Daney <david.daney@...ium.com>, linux-kernel@...r.kernel.org,
	Richard Purdie <rpurdie@...ys.net>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH] leds/of: leds-gpio.c: Use gpio_get_value_cansleep()
 when initializing.

On Thu, 8 Sep 2011 22:30:58 -0700 Trent Piepho <tpiepho@...il.com> wrote:

> >
> > They're very different. __Why is it OK to replace one with the other??
> 
> What's supposed to happen is chip->get() will be a method that does
> "readl(GPIO_GPLR) & GPIO_GPIO(gpio);" or whatever the inlined bit in
> gpio_get_value() is.  So calling gpio_get_value_cansleep() should
> still get the correct value for the gpio.  It just won't be an inlined
> register read anymore.
> 
> For instance, all the arch versions that use builtin_constant_p() will
> not take the inline path, since the gpio number if obviously not a
> constant when gpio_get_value() is called in this leds function.  So
> they inline into a call to __gpio_get_value().  Which as you've
> pointed out is nearly exactly the same as gpio_get_value_cansleep().
> The only change is debugging related, that of the might_sleep_if() to
> a WARN_ON().
> 
> One could have:
> static inline int __gpio_get_value(gpio) { return
> _gpio_get_value(gpio, GFP_ATOMIC); }
> static inline int gpio_get_value_cansleep(gpio) { return
> _gpio_get_value(gpio, GFP_KERNEL); }
> 
> Then _gpio_get_value(gpio, context) would be the current code that's
> common to both __gpio_get_value() and gpio_get_value_cansleep(),
> except it uses context solely to spit a warning if the gpio can't be
> done from the requested context or if the context isn't allowable from
> whence the call was made.

Well, that may be the case with the current in-tree implementations (I
didn't check), but from a design point of view the core code shouldn't
"know" how the architecture is implementing gpio_get_value().

> And then one would ask if all this complexity in the interface is
> really the best way to throw warnings about sleeping in atomic
> context?  Because if there was a better way to do that, we could just
> s/(gpio_get_value)_cansleep/\1/g and not worry about these someone
> did/didn't call cansleep() when they should done the opposite patches.

Yes, it sounds like the interface around there needs a rethink.

I don't actually know why gpio_get_value_cansleep() exists.  I looked
at the silly comment and for some reason didn't feel like working this
out.

> >> > Asides:
> >> >
> >> > The duplication of code between __gpio_get_value() and
> >> > gpio_get_value_cansleep() is daft.
> >> >
> >> > The comment over gpio_get_value_cansleep() sucks mud rocks.
> >
> > Preserving this...
> 
> Well, inlining code that sits on a waitq doesn't make a lot of sense.

No need for inlining - just a function call.  One which the compiler
might turn into a tailcall for us.

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