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>

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index 9633da01ff46..792faa3c06cf 100644
--- 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.
 
+                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.