[Gumstix-users] GPIO interrupts using sysfs

[Paul B. <pb...@si...>]

I've been testing the sysfs interface to use GPIOs on a Gumstix Overo 
and it seems to work properly except for interrupts. It would be handy 
to be able to user GPIO interrupts in user space.
As described in http://www.kernel.org/doc/Documentation/gpio.txt, under 
"Sysfs Interface for Userspace (OPTIONAL)":
Once a particular GPIO has been exported, a string of "rising", 
"falling", etc. can be written to /sys/class/gpio/gpioN/edge to 
configure it as an interrupt. To block on the interrupt, a poll(2) is 
done on /sys/class/gpio/gpioN/value (where N is the GPIO number). When I 
try this using GPIO14 it doesn't block on the interrupt, although I can 
read the correct value of the pin.
Here's the code:

#include <stdio.h>       /* For printf() */
#include <string.h>      /* For strerror() */
#include <unistd.h>      /* For read(), close() */
#include <fcntl.h>       /* For open() */
#include <errno.h>       /* For errno */
#include <sys/poll.h>    /* For poll() */

#define ERREXIT(str) {printf("err %s, %s\n", str, strerror(errno)); 
return -1;}

int main(int argc, char** argv)
{
   struct pollfd xfds[1];
   const char *fn;
   char buf[4];
   int rc;
   int fd;
   int i;

   /* export */
   fn = "/sys/class/gpio/export";
   fd = open(fn, O_WRONLY); if(fd < 0)  ERREXIT("open export")
   rc = write(fd, "14", 3); if(rc != 3) ERREXIT("write export")
   close(fd);

   /* direction */
   fn = "/sys/class/gpio/gpio14/direction";
   fd = open(fn, O_RDWR);  if(fd < 0)  ERREXIT("open direction")
   rc = write(fd, "in", 3);if(rc != 3) ERREXIT("write direction")
   close(fd);

   /* edge */
   fn = "/sys/class/gpio/gpio14/edge";
   fd = open(fn, O_RDWR);        if(fd < 0)  ERREXIT("open edge")
   rc = write(fd, "falling", 8); if(rc != 8) ERREXIT("write edge")
   rc = lseek(fd, 0, SEEK_SET);  if(rc < 0)  ERREXIT("lseek edge")
   rc = read(fd, buf, 10);       if(rc <= 0) ERREXIT("read edge")
   buf[10] = '\0';
   printf("read gpio14/edge:%s\n", buf);
   close(fd);

   /* wait for interrupt - try it a few times */
   fn = "/sys/class/gpio/gpio14/value";
   fd = open(fn, O_RDWR);  if(fd < 0)  ERREXIT("open value")
   xfds[0].fd       = fd;
   xfds[0].events   = POLLPRI | POLLERR;
   xfds[0].revents  = 0;
   for (i=0; i<3; i++)
   {
     printf("Waiting for interrupt..\n");
     rc = poll(xfds, 1, 10000); if(rc == -1) ERREXIT("poll value")
     printf("poll rc=%d, revents=0x%x\n", rc, xfds[0].revents);
   }
   /* get value */
   rc = lseek(fd, 0, SEEK_SET); if (rc < 0)  ERREXIT("lseek value")
   rc = read(fd, buf, 2);       if (rc != 2) ERREXIT("read value")
   close(fd);
   buf[1] = '\0'; /* Overwrite the newline character with terminator */
   printf("read rc=%d, val=%s\n", rc, buf);

   /* unexport */
   fn = "/sys/class/gpio/unexport";
   fd = open(fn, O_WRONLY); if(fd < 0)  ERREXIT("open unexport")
   rc = write(fd, "14", 3); if(rc != 3) ERREXIT("write unexport")
   close(fd);

   return 0;
}
Here's the result:
root@overo:~# ./irquser
read gpio14/edge:falling

Waiting for interrupt..
poll rc=1, revents=0xa
Waiting for interrupt..
poll rc=1, revents=0xa
Waiting for interrupt..
poll rc=1, revents=0xa
read rc=2, val=1
root@overo:~#


The poll() call returns immediately. If I hold down the GPIO14 button 
while running it, the "val=1" becomes "val=0."
The returned bits in 'revents' (0xa) are:
     POLLPRI: There is urgent data to read (e.g., out-of-band data on 
TCP socket; pseudoterminal master in packet mode has seen state change 
in slave).
     POLLERR: Error condition (output only).

I have tested on both these Linux versions:
Linux overo 2.6.34 #1 Wed Oct 20 10:22:48 PDT 2010 armv7l GNU/Linux
Linux overo 2.6.39 #1 Mon Aug 15 19:04:09 PDT 2011 armv7l GNU/Linux

It appears as though this hasn't been implemented, but any ideas are 
welcome.

-- 
Paul Brown
www.signalcraft.com