Using GPIOs
-----------
-One of the first things to do with a GPIO, often in board setup code when
+The first thing a system should do with a GPIO is allocate it, using
+the gpio_request() call; see later.
+
+One of the next things to do with a GPIO, often in board setup code when
setting up a platform_device using the GPIO, is mark its direction:
/* set as input or output, returning 0 or negative errno */
For compatibility with legacy interfaces to GPIOs, setting the direction
of a GPIO implicitly requests that GPIO (see below) if it has not been
-requested already. That compatibility may be removed in the future;
-explicitly requesting GPIOs is strongly preferred.
+requested already. That compatibility is being removed from the optional
+gpiolib framework.
Setting the direction can fail if the GPIO number is invalid, or when
that particular GPIO can't be used in that mode. It's generally a bad
Platforms that support this type of GPIO distinguish them from other GPIOs
by returning nonzero from this call (which requires a valid GPIO number,
-either explicitly or implicitly requested):
+which should have been previously allocated with gpio_request):
int gpio_cansleep(unsigned gpio);
same as the spinlock-safe calls.
-Claiming and Releasing GPIOs (OPTIONAL)
----------------------------------------
+Claiming and Releasing GPIOs
+----------------------------
To help catch system configuration errors, two calls are defined.
-However, many platforms don't currently support this mechanism.
/* request GPIO, returning 0 or negative errno.
* non-null labels may be useful for diagnostics.
power management, such as by powering down unused chip sectors and, more
easily, gating off unused clocks.
-These two calls are optional because not not all current Linux platforms
-offer such functionality in their GPIO support; a valid implementation
-could return success for all gpio_request() calls. Unlike the other calls,
-the state they represent doesn't normally match anything from a hardware
-register; it's just a software bitmap which clearly is not necessary for
-correct operation of hardware or (bug free) drivers.
-
Note that requesting a GPIO does NOT cause it to be configured in any
way; it just marks that GPIO as in use. Separate code must handle any
pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).