gpio: doc updates

There's been some recent confusion about error checking GPIO numbers.
briefly, it should be handled mostly during setup, when gpio_request() is
called, and NEVER by expectig gpio_is_valid to report more than
never-usable GPIO numbers.

[akpm@linux-foundation.org: terminate unterminated comment]
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Eric Miao" <eric.y.miao@gmail.com>
Cc: "Ryan Mallon" <ryan@bluewatersys.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/include/asm-generic/gpio.h b/include/asm-generic/gpio.h
index c7376bf80b06..8ca18e26d7e3 100644
--- a/include/asm-generic/gpio.h
+++ b/include/asm-generic/gpio.h
@@ -16,15 +16,27 @@
  * While the GPIO programming interface defines valid GPIO numbers
  * to be in the range 0..MAX_INT, this library restricts them to the
  * smaller range 0..ARCH_NR_GPIOS-1.
+ *
+ * ARCH_NR_GPIOS is somewhat arbitrary; it usually reflects the sum of
+ * builtin/SoC GPIOs plus a number of GPIOs on expanders; the latter is
+ * actually an estimate of a board-specific value.
  */
 
 #ifndef ARCH_NR_GPIOS
 #define ARCH_NR_GPIOS           256
 #endif
 
+/*
+ * "valid" GPIO numbers are nonnegative and may be passed to
+ * setup routines like gpio_request().  only some valid numbers
+ * can successfully be requested and used.
+ *
+ * Invalid GPIO numbers are useful for indicating no-such-GPIO in
+ * platform data and other tables.
+ */
+
 static inline int gpio_is_valid(int number)
 {
-        /* only some non-negative numbers are valid */
         return ((unsigned)number) < ARCH_NR_GPIOS;
 }