On Thursday 19 April 2007 1:05 am, Francis Moreau wrote:
> On 4/17/07, David Brownell <[email protected]> wrote:
With regards to a userspace interface to GPIOs (rather than to
devices such as leds or switches they control):
> > 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:
That addresses only (b). But (a) would still need a solution.
It's common that any given pin be usable for multiple different
purposes ... it's rarely hardwired only to a GPIO controller.
More usually it will be multiplexed to several other controllers
under software control. (Some chips allow up to eight choices,
Others only have two or three.)
>
> static struct resource foo_gpio_resource[] = {
> [0] = {
> .start = 10,
> .end = 11,
> .flags = IORESOURCE_GPIO,
As you probably know, there is no such thing as IORESOURCE_GPIO;
that could be changed if necessary.
> },
> [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.
If there's going to be a "user-gpio" driver it could just as easily
take an array of GPIOs in its platform data; and that would usuallly
take a lot less space, even if it's not compressed to a bitmask!
> > - 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...
Actually GPIOs don't need to start at zero, and the numbering doesn't
need to be continuous. On one system I use, GPIOs start at 32; on
another, they go 0..63 and then 192..207; and that's just for the
once on the same chip as the CPU!
As far as configuration goes, what would be useful would be to have
someone do a survey of the existing userspace tools that work with
GPIOs, and see what they're trying to do. I'd start with that one
for Gumstix, since it's got a fairly broad problem domain.
> > - 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
Yes, using gpio_request() is the natural solution. I hope to see more
platforms actually implementing it before too long.
- Dave
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