[PATCH] gpio_direction_output() needs an initial value

It's been pointed out that output GPIOs should have an initial value, to
avoid signal glitching ...  among other things, it can be some time before
a driver is ready.  This patch corrects that oversight, fixing

 - documentation
 - platforms supporting the GPIO interface
 - users of that call (just one for now, others are pending)

There's only one user of this call for now since most platforms are still
using non-generic GPIO setup code, which in most cases already couples the
initial value with its "set output mode" request.

Note that most platforms are clear about the hardware letting the output
value be set before the pin direction is changed, but the s3c241x docs are
vague on that topic ...  so those chips might not avoid the glitches.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Andrew Victor <andrew@sanpeople.com>
Acked-by: Milan Svoboda <msvoboda@ra.rockwell.com>
Acked-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index 576ce463cf44..989f1130f4f3 100644
--- a/Documentation/gpio.txt
+++ b/Documentation/gpio.txt
@@ -105,12 +105,15 @@ setting up a platform_device using the GPIO, is mark its direction:
 
         /* set as input or output, returning 0 or negative errno */
         int gpio_direction_input(unsigned gpio);
-        int gpio_direction_output(unsigned gpio);
+        int gpio_direction_output(unsigned gpio, int value);
 
 The return value is zero for success, else a negative errno.  It should
 be checked, since the get/set calls don't have error returns and since
 misconfiguration is possible.  (These calls could sleep.)
 
+For output GPIOs, the value provided becomes the initial output value.
+This helps avoid signal glitching during system startup.
+
 Setting the direction can fail if the GPIO number is invalid, or when
 that particular GPIO can't be used in that mode.  It's generally a bad
 idea to rely on boot firmware to have set the direction correctly, since