linux-kernel - Re: question on generic gpio interface

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Message-ID: <38b2ab8a0704190105x68414f34w51f109f1f289ca7d@mail.gmail.com>
Date:	Thu, 19 Apr 2007 10:05:19 +0200
From:	"Francis Moreau" <francis.moro@...il.com>
To:	"David Brownell" <david-b@...bell.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: question on generic gpio interface

On 4/17/07, David Brownell <david-b@...bell.net> wrote:
> In this case I'm not entirely sure how it'd work.  I've seen a few
> drivers which let userspace peek and poke at GPIO signals -- like
> one for Gumstix boards -- but generalizing the model isn't simple.
> Sub-problems include:
>
>  - Configuring the relevant pins.  Especially for SOC cases, GPIO
>    roles are multiplexed with several others.  So there are two
>    issues:  (a) the platform-specific setup of that multiplexing,
>    plus (b) the board-specific knowledge of what pins are truly
>    available for use as GPIOs, and not otherwise in use.
>

what about create a module "user-gpio" for example that could request
some gpios that the board could have declared using resource
subsystem, like this:

static struct resource foo_gpio_resource[] = {
        [0] = {
                .start = 10,
                .end  = 11,
                .flags = IORESOURCE_GPIO,
        },
       [1] = {
                .start = 26,
                .end  = 31,
                .flags = IORESOURCE_GPIO,
        },
};

struct platform_device foo_device_usergpio = {
        .name           = "user-gpio",
        .id             = -1,
        .num_resources  = ARRAY_SIZE(foo_gpio_resource),
        .resource       = foo_gpio_resource,
};

This way "user-gpio" module knows which pins are avalaible to userspace.

>  - Enumerating those GPIOs to userspace.  One SOC might have just
>    a few dozen, another might have a few hundred; and then there
>    are all the board-specific ones, on FPGA or I2C chips etc.
>

This point is actully the one where I'm really not sure...

Enumerating user GPIOs would always start from 0 to GPIO_USER_NR - 1
and an application that need to be portable should use a config file
to specify which GPIO num to use...

>  - Exposing those pins to userspace.  It'd be unsafe to let pins
>    claimed by drivers be managed by userspace; the default should
>    be that only unclaimed GPIOs can be accessed.
>

Well an extreme solution would be to test in gpio_request(), if the
passed gpio nr is a user one then gpio_request() would return an
error. We could use is_user_gpio() function implemented by user-gpio
module

Thanks
-- 
Francis
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