From 28735a7253a6c24364765e80a5428b4a151fccc2 Mon Sep 17 00:00:00 2001 From: David Brownell Date: Fri, 16 Mar 2007 13:38:14 -0800 Subject: [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 Acked-by: Andrew Victor Acked-by: Milan Svoboda Acked-by: Haavard Skinnemoen Cc: Russell King Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/gpio.txt | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'Documentation/gpio.txt') diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt index 576ce463cf4..989f1130f4f 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 -- cgit v1.2.3