linux-kernel - Re: [RFC PATCH] gpio: support for GPIO forwarding

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Message-ID: <20150212124623.GC18860@kuha.fi.intel.com>
Date:	Thu, 12 Feb 2015 14:46:23 +0200
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Alexandre Courbot <gnurou@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: [RFC PATCH] gpio: support for GPIO forwarding

On Tue, Feb 10, 2015 at 04:10:04PM +0100, Rafael J. Wysocki wrote:
> On Tuesday, February 10, 2015 06:32:46 PM Alexandre Courbot wrote:
> > On Fri, Jan 23, 2015 at 8:21 PM, Heikki Krogerus
> > <heikki.krogerus@...ux.intel.com> wrote:
> > > Hi guys,
> > >
> > > On Thu, Jan 22, 2015 at 05:14:22PM +0100, Rafael J. Wysocki wrote:
> > >> On Thursday, January 22, 2015 11:57:55 AM Alexandre Courbot wrote:
> > >> > If we decide to go ahead with the solution proposed by this patch for
> > >> > practical reasons (which are good reasons indeed), I still have one
> > >> > problem with its current form.
> > >> >
> > >> > As the discussion highlighted, this is an ACPI problem, so I'd very
> > >> > much like it to be confined to the ACPI GPIO code, to be enabled only
> > >> > when ACPI is, and to use function names that start with acpi_gpio.
> > >>
> > >> I can agree with that.
> > >>
> > >> > The current implementation leverages platform lookup, making said lookup
> > >> > less efficient in the process and bringing confusion about its
> > >> > purpose. Although the two processes are indeed similar, they are
> > >> > separate things: one is a legitimate way to map GPIOs, the other is a
> > >> > fixup for broken firmware.
> > >> >
> > >> > I suppose we all agree this is a hackish fix, so let's confine it as
> > >> > much as we can.
> > >>
> > >> OK
> > >>
> > >> Heikki, any comments?
> > >
> > > I'm fine with that.
> > >
> > > That actually makes me think that we could then drop the lookup tables
> > > completely and use device properties instead with the help of "generic
> > > property" (attached):
> > >
> > > We would just need to agree on the format how to describe a gpio
> > > property, document it and of course convert the current users as
> > > usual. The format could be something like this as an example (I'm
> > > writing this out of my head so don't shoot me if you can see it would
> > > not work. Just an example):
> > >
> > > static const u32 example_gpio[] = { <gpio>, <flags>,爙;
> > >
> > > static struct dev_gen_prop example_prop[] =
> > >         {
> > >                 .type = DEV_PROP_U32,
> > >                 .name = "gpio,<con_id>",
> > >                 .nval = 2,
> > >                 .num = &example_gpio,
> > >         },
> > >         { },
> > > };
> > >
> > > static struct platform_device example_pdev = {
> > >         ...
> > >         .dev = {
> > >                 .gen_prop = &example_prop,
> > >         },
> > > }
> > >
> > >
> > > In gpiolib.c we would then, instead of going through the lookups,
> > > simply ask for that property:
> > >
> > >         ...
> > >         sprintf(propname, "gpio,%s", con_id);
> > >         device_property_read_u32_array(dev, propname, &val, 2);
> > >         ...
> > >         desc = gpio_to_desc(val[0]);
> > >         flags = val[1];
> > >         ...
> > >
> > >
> > > So this is just and idea. I think it would be relatively easy to
> > > implement. What do you guys think?
> > 
> > At first sight, that looks like a very good idea and a great use of
> > the device properties API. Are you willing to explore it further?

Yes. If I get green light for the generic property idea, I can start
thinking about this.


Thanks,

-- 
heikki
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