Re: [RFC PATCH] gpio: support for GPIO forwarding

[Heikki Krogerus]

 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>, };

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?


Cheers,

-- 
heikki