On 4/17/07, David Brownell <[email protected]> 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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