linux-kernel - [PATCH 3/7] Documentation/gpio.txt: explain poll/select usage

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Date:	Thu, 18 Nov 2010 12:27:34 -0800
From:	Randy Dunlap <rdunlap@...otime.net>
To:	linux-kernel@...r.kernel.org
Cc:	Bernhard Walle <walle@...science.de>
Subject: [PATCH 3/7] Documentation/gpio.txt: explain poll/select usage

From: Bernhard Walle <walle@...science.de>

Add a bit more information how to use poll(2) on GPIO value files
correctly. For me it was not clear that I need to poll(2) for
POLLPRI|POLLERR or select(2) for exceptfds.

Signed-off-by: Bernhard Walle <walle@...science.de>
Signed-off-by: Randy Dunlap <randy.dunlap@...cle.com>
---
 Documentation/gpio.txt |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- linux-2.6.37-rc2-git4.orig/Documentation/gpio.txt
+++ linux-2.6.37-rc2-git4/Documentation/gpio.txt
@@ -617,6 +617,16 @@ and have the following read/write attrib
 		is configured as an output, this value may be written;
 		any nonzero value is treated as high.
 
+		If the pin can be configured as interrupt-generating interrupt
+		and if it has been configured to generate interrupts (see the
+		description of "edge"), you can poll(2) on that file and
+		poll(2) will return whenever the interrupt was triggered. If
+		you use poll(2), set the events POLLPRI and POLLERR. If you
+		use select(2), set the file descriptor in exceptfds. After
+		poll(2) returns, either lseek(2) to the beginning of the sysfs
+		file and read the new value or close the file and re-open it
+		to read the value.
+
 	"edge" ... reads as either "none", "rising", "falling", or
 		"both". Write these strings to select the signal edge(s)
 		that will make poll(2) on the "value" file return.


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