path: root/Documentation
diff options
authorDavid Brownell <david-b@pacbell.net>2008-03-04 14:28:27 -0800
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2008-03-04 16:35:10 -0800
commit7560fa60fcdcdb0da662f6a9fad9064b554ef46c (patch)
tree2089d826b0b9230752f318b20884dca20b3847d5 /Documentation
parent83c7c693ed3e61535ad6a097ad991a88aafc54b8 (diff)
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>
Diffstat (limited to 'Documentation')
1 files changed, 12 insertions, 4 deletions
diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index 8da724e2a0f..54630095aa3 100644
--- a/Documentation/gpio.txt
+++ b/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.