gpiolib: allow poll() on value

Many gpio chips allow to generate interrupts when the value of a pin
changes.  This patch gives usermode application the opportunity to make
use of this feature by calling poll(2) on the /sys/class/gpio/gpioN/value
sysfs file.  The edge to trigger can be set in the edge file in the same
directory.  Possible values are "none", "rising", "falling", and "both".

Using level triggers is not possible with current sysfs since nothing
changes the GPIO value (and the IRQ keeps triggering).  Edge triggering
will "just work".  Note that if there was an event between read() and
poll(), the poll() returns immediately.

Also note that this version only supports true GPIO interrupts.  Some
later patch might be able to synthesize this behavior by timer-driven
polling; some systems seem to need that.

[dbrownell@users.sourceforge.net: align ids to 16 bit ids; whitespace]
Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-gpio b/Documentation/ABI/testing/sysfs-gpio
index 8aab8092ad35..80f4c94c7bef 100644
--- a/Documentation/ABI/testing/sysfs-gpio
+++ b/Documentation/ABI/testing/sysfs-gpio
@@ -19,6 +19,7 @@ Description:
         /gpioN ... for each exported GPIO #N
             /value ... always readable, writes fail for input GPIOs
             /direction ... r/w as: in, out (default low); write: high, low
+            /edge ... r/w as: none, falling, rising, both
         /gpiochipN ... for each gpiochip; #N is its first GPIO
             /base ... (r/o) same as N
             /label ... (r/o) descriptive, not necessarily unique
diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index 566edaa56a53..fa4dc077ae0e 100644
--- a/Documentation/gpio.txt
+++ b/Documentation/gpio.txt
@@ -524,6 +524,13 @@ and have the following read/write attributes:
                 is configured as an output, this value may be written;
                 any nonzero value is treated as high.
 
+        "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.
+
+                This file exists only if the pin can be configured as an
+                interrupt generating input pin.
+
 GPIO controllers have paths like /sys/class/gpio/chipchip42/ (for the
 controller implementing GPIOs starting at #42) and have the following
 read-only attributes: