Re: [patch/rfc 2.6.19-rc5] arch-neutral GPIO calls

David Brownell
2006-11-13T19:26:05Z

On Monday 13 November 2006 9:56 am, Thiago Galesi wrote:
> I guess that a kind of "name addressing" would be the way to go, we
> need to get to it by "location" (I'm thinking PortA 20, PortB 5 rather
> than pin number or some other arbitrary convention; we need a way to
> not need to look up what 'Port X pin B' should be called.

That's highly platform-specific. AT91 and AVR32 use that style of
addressing ... most others just talk about GPIO numbers. In all cases,
the platform can assign _some_ number to each GPIO signal.

> Another thing that may be considered is the ability to get 'pointers'
> for GPIOs.

The GPIO identifiers are unsigned integers. If you really crave pointers,
you can cast those integers to pointers, and back. But why bother? :)

> And, of course, protecting GPIOs from concurrent accesses

Did you read the proposal? There are *already* two mechanisms for that:

- gpio_request()/gpio_free() ... protecting from concurrent access
between drivers, also known as bugs.

- gpio_set_value() ... implementations can lock internally, as needed.
ditto gpio_get_value(), but only bizarre hardware would need it.

Re that last, consider two different types of GPIO controller:

* One of them has just one register with the output values, and a
RISC CPU without bit set/clear instructions to use on it. So a
spinlock is needed to cover reading the register, modifying that
value, then rewriting the value.

* Better GPIO controller designs (normal ones) have separate registers
for "write mask to set" and "write mask to clear" ... no spinlock
needed, the hardware sorts everything out.

I did update the docs to make clear that gpio get/set calls are atomic;
hard to imagine single bit operations that aren't, but it's worth
removing potential confusion.

- Dave