Documentation/gpio.txt: explain poll/select usage

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@corscience.de>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index 9633da0..792faa3 100644 (file)
--- a/Documentation/gpio.txt
+++ b/Documentation/gpio.txt
@@ -617,6 +617,16 @@ and have the following read/write attributes:
                is configured as an output, this value may be written;
                any nonzero value is treated as high.
 
                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.
        "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.