gpio: <linux/gpio.h> and "no GPIO support here" stubs

Add a <linux/gpio.h> defining fail/warn stubs for GPIO calls on platforms that
don't support the GPIO programming interface.  That includes the arch-specific
implementation glue otherwise.

This facilitates a new model for GPIO usage: drivers that can use GPIOs if
they're available, but don't require them.  One example of such a driver is
NAND driver for various FreeScale chips.  On platforms update with GPIO
support, they can be used instead of a worst-case delay to verify that the
BUSY signal is off.

(Also includes a couple minor unrelated doc updates.)
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/gpio.txt

@@ -2,6 +2,9 @@ GPIO Interfaces
 
 This provides an overview of GPIO access conventions on Linux.
 
+These calls use the gpio_* naming prefix.  No other calls should use that
+prefix, or the related __gpio_* prefix.
+
 
 What is a GPIO?
 ===============
@@ -69,11 +72,13 @@ in this document, but drivers acting as clients to the GPIO interface must
 not care how it's implemented.)
 
 That said, if the convention is supported on their platform, drivers should
-use it when possible.  Platforms should declare GENERIC_GPIO support in
-Kconfig (boolean true), which multi-platform drivers can depend on when
-using the include file:
+use it when possible.  Platforms must declare GENERIC_GPIO support in their
+Kconfig (boolean true), and provide an <asm/gpio.h> file.  Drivers that can't
+work without standard GPIO calls should have Kconfig entries which depend
+on GENERIC_GPIO.  The GPIO calls are available, either as "real code" or as
+optimized-away stubs, when drivers use the include file:
 
-	#include <asm/gpio.h>
+	#include <linux/gpio.h>
 
 If you stick to this convention then it'll be easier for other developers to
 see what your code is doing, and help maintain it.
@@ -316,6 +321,9 @@ pulldowns integrated on some platforms.  Not all platforms support them,
 or support them in the same way; and any given board might use external
 pullups (or pulldowns) so that the on-chip ones should not be used.
 (When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
+Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
+platform-specific issue, as are models like (not) having a one-to-one
+correspondence between configurable pins and GPIOs.
 
 There are other system-specific mechanisms that are not specified here,
 like the aforementioned options for input de-glitching and wire-OR output.

include/linux/gpio.h

+#ifndef __LINUX_GPIO_H
+#define __LINUX_GPIO_H
+
+/* see Documentation/gpio.txt */
+
+#ifdef CONFIG_GENERIC_GPIO
+#include <asm/gpio.h>
+
+#else
+
+/*
+ * Some platforms don't support the GPIO programming interface.
+ *
+ * In case some driver uses it anyway (it should normally have
+ * depended on GENERIC_GPIO), these routines help the compiler
+ * optimize out much GPIO-related code ... or trigger a runtime
+ * warning when something is wrongly called.
+ */
+
+static inline int gpio_is_valid(int number)
+{
+	return 0;
+}
+
+static inline int gpio_request(unsigned gpio, const char *label)
+{
+	return -ENOSYS;
+}
+
+static inline void gpio_free(unsigned gpio)
+{
+	/* GPIO can never have been requested */
+	WARN_ON(1);
+}
+
+static inline int gpio_direction_input(unsigned gpio)
+{
+	return -ENOSYS;
+}
+
+static inline int gpio_direction_output(unsigned gpio, int value)
+{
+	return -ENOSYS;
+}
+
+static inline int gpio_get_value(unsigned gpio)
+{
+	/* GPIO can never have been requested or set as {in,out}put */
+	WARN_ON(1);
+	return 0;
+}
+
+static inline void gpio_set_value(unsigned gpio, int value)
+{
+	/* GPIO can never have been requested or set as output */
+	WARN_ON(1);
+}
+
+static inline int gpio_cansleep(unsigned gpio)
+{
+	/* GPIO can never have been requested or set as {in,out}put */
+	WARN_ON(1);
+	return 0;
+}
+
+static inline int gpio_get_value_cansleep(unsigned gpio)
+{
+	/* GPIO can never have been requested or set as {in,out}put */
+	WARN_ON(1);
+	return 0;
+}
+
+static inline void gpio_set_value_cansleep(unsigned gpio, int value)
+{
+	/* GPIO can never have been requested or set as output */
+	WARN_ON(1);
+}
+
+static inline int gpio_to_irq(unsigned gpio)
+{
+	/* GPIO can never have been requested or set as input */
+	WARN_ON(1);
+	return -EINVAL;
+}
+
+static inline int irq_to_gpio(unsigned irq)
+{
+	/* irq can never have been returned from gpio_to_irq() */
+	WARN_ON(1);
+	return -EINVAL;
+}
+
+#endif
+
+#endif /* __LINUX_GPIO_H */