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

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Date:	Tue, 10 Feb 2015 18:32:46 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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 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?
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