--- kernel-2.6.27.orig/Documentation/gpio.txt
+++ kernel-2.6.27/Documentation/gpio.txt
@@ -240,6 +240,10 @@
 needed to manage a signal that's in active use.  That is, requesting a
 GPIO can serve as a kind of lock.
 
+Some platforms may also use knowledge about what GPIOs are active for
+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,
@@ -264,7 +268,7 @@
 	/* map GPIO numbers to IRQ numbers */
 	int gpio_to_irq(unsigned gpio);
 
-	/* map IRQ numbers to GPIO numbers */
+	/* map IRQ numbers to GPIO numbers (avoid using this) */
 	int irq_to_gpio(unsigned irq);
 
 Those return either the corresponding number in the other namespace, or
@@ -284,7 +288,8 @@
 
 Non-error values returned from irq_to_gpio() would most commonly be used
 with gpio_get_value(), for example to initialize or update driver state
-when the IRQ is edge-triggered.
+when the IRQ is edge-triggered.  Note that some platforms don't support
+this reverse mapping, so you should avoid using it.
 
 
 Emulating Open Drain Signals