linux-kernel - Re: How about a gpio_get(device *, char *) function?

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Date:	Mon, 26 Nov 2012 11:14:31 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Linus Walleij <linus.walleij@...aro.org>,
	Stephen Warren <swarren@...dotorg.org>
Cc:	Alex Courbot <acourbot@...dia.com>,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: How about a gpio_get(device *, char *) function?

On Wed, 7 Nov 2012 22:28:01 +0100, Linus Walleij <linus.walleij@...aro.org> wrote:
> On Mon, Nov 5, 2012 at 6:35 PM, Stephen Warren <swarren@...dotorg.org> wrote:
> > [Me]
> >> gpio_get() should get an abstract handle just like clk_get() or
> >> regulator_get(), not a fixed numeral.
> >
> > I don't really see why the return type of gpio_get() influences whether
> > it can be implemented or not.
> 
> It doesn't influence that, but I want to follow the opaqueness design
> pattern from irq descriptors and struct clk.

Right. I like the pattern too. Unforutunately that means dealing with
somewhere on the order of 2500 callers of the old API. :-(

However, I don't think that the GPIO numberspace issue is completely
intertwined with opaqifying the gpio handles. The numberspace can be
fixed with the current API if someone creates a sparse gpio
registrations.

I don't have any problem with a gpio_get function, but I do agree that
making it return an opaque handle is how it should be written with a new
set of accessors. The handle should probably be simply the pointer to
the &gpio_desc[number] which is a private table in gpiolib.c. The
definition of it isn't available outside of gpiolib.c

In fact, the old functions should be redefined in terms of getting the
gpio_desc from the irq number and calling the new functions.

> 
> > With board files, some "gpio map" table would simply contain the same
> > int GPIO ID value the table as is used anywhere else already. With DT,
> > the same xlate function would translate from DT GPIO-chip-relative
> > IDs/specifiers into the global number space in the same way that we do
> > today via other APIs.
> 
> Yes, this part I buy into, just want to see how we can move forward
> from there. The coplete nightmare is to introduce something into DT
> that nails down a global GPIO numberspace... but I think that is not
> the case atleast.
> 
> > If the GPIO subsystem were reworked as you propose, this API could be
> > reworked in exactly the same way, or if implemented after the rework, it
> > would return whatever handle type was in use at the time.
> 
> Yes, I just think we should return an opaque struct from day 1, so
> just a little, little bit more to shield us.
> 
> Yours,
> Linus Walleij

-- 
Grant Likely, B.Sc, P.Eng.
Secret Lab Technologies, Ltd.
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