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>
diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index 8da724e..5463009 100644
--- a/Documentation/gpio.txt
+++ b/Documentation/gpio.txt
@@ -2,6 +2,9 @@
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 @@
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 @@
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.