Re: [patch 2.6.20-rc1 1/6] GPIO core

David Brownell
2007-01-01T21:27:46Z

On Monday 01 January 2007 12:55 pm, Pavel Machek wrote:

> > Think of it as "cookies represented by integers" if you like.
>
> typedef int gpio_t would hurt, and would serve as a useful
> documentation hint.

Yes, I agree that such needless obfuscation hurts. ;)

Plus, such a typedef would disagree with Documentation/CodingStyle
which says "... the rule should basically be to NEVER EVER use a
typedef" (with some exceptions not matched here).

> > Should it instead say that's an (obviously unchecked) error?
>
> Saying it is an error would be okay by me. (Or "Behaviour of these calls for
> GPIOs that can't be safely accessed without sleeping is undefined.").

See the appended doc patch ... better?

- Dave

================= CUT HERE
Index: at91/Documentation/gpio.txt
===================================================================
--- at91.orig/Documentation/gpio.txt 2006-12-29 00:00:28.000000000 -0800
+++ at91/Documentation/gpio.txt 2006-12-29 15:47:18.000000000 -0800
@@ -78,7 +78,8 @@ Identifying GPIOs
-----------------
GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
reserves "negative" numbers for other purposes like marking signals as
-"not available on this board", or indicating faults.
+"not available on this board", or indicating faults. Code that doesn't
+touch the underlying hardware treats these integers as opaque cookies.

Platforms define how they use those integers, and usually #define symbols
for the GPIO lines so that board-specific setup code directly corresponds
@@ -139,8 +140,8 @@ issues including wire-OR and output late
The get/set calls have no error returns because "invalid GPIO" should have
been reported earlier in gpio_set_direction(). However, note that not all
platforms can read the value of output pins; those that can't should always
-return zero. Also, these calls will be ignored for GPIOs that can't safely
-be accessed wihtout sleeping (see below).
+return zero. Also, using these calls for GPIOs that can't safely be accessed
+without sleeping (see below) is an error.

Platform-specific implementations are encouraged to optimise the two
calls to access the GPIO value in cases where the GPIO number (and for
@@ -239,7 +240,8 @@ options are part of the IRQ interface, e
system wakeup capabilities.

Non-error values returned from irq_to_gpio() would most commonly be used
-with gpio_get_value().
+with gpio_get_value(), for example to initialize or update driver state
+when the IRQ is edge-triggered.

@@ -262,7 +264,8 @@ like the aforementioned options for inpu
Hardware may support reading or writing GPIOs in gangs, but that's usually
configuration dependednt: for GPIOs sharing the same bank. (GPIOs are
commonly grouped in banks of 16 or 32, with a given SOC having several such
-banks.) Code relying on such mechanisms will necessarily be nonportable.
+banks.) Some systems can trigger IRQs from output GPIOs. Code relying on
+such mechanisms will necessarily be nonportable.

Dynamic definition of GPIOs is not currently supported; for example, as
a side effect of configuring an add-on board with some GPIO expanders.