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Question

I wrote a linux kernel module char device (SuSE 13.2 Kernel 3.16.6), and an application for using this device.

In principle, the driver, the application and the communication works. The application can open, read, write and close the char device.

But there is a sporadic issue, that the application somtime 'lost' the reference to the module's write function. After 3 to 4 (sometime much more) write commands from the application to the module, the module's write funtion isn't executed.

I reduced and the write function inside module:

static ssize_t fWrite(struct file *file, const char  *buffer, size_t length, loff_t *offset)
{
  printk(KERN_ERR "INSIDE\n");
  return 0;
}

And I also reduced the applcation:

int main (int argc, char **argv)
{
  unsigned short buffer=0xffff;
  fd = open ("/dev/MYPCI" , O_SYNC | O_RDWR );
  OUTPW (0, buffer ,fd );
  OUTPW (2, buffer,fd );
  OUTPW (4, buffer,fd ); 
  OUTPW (196, buffer,fd );
  OUTPW (70000, buffer,fd );   
  OUTPW (400, buffer,fd );

  printf("DEV-Handle: %d\n", fd);  
  close (fd );
}

int OUTPW (unsigned short ByteAddr, unsigned short value, int fd)
{
  int x;  
  int size = (ByteAddr/2) <<16; 
  size +=2;            

  printf ("\nByteAddr:0x%08hX value:0x%08hX size:0x%08hX WRITE-Handle: 0x%04hX\n" , ByteAddr,value,size, &write);

  x= write(fd, &value, size) ;

  return x;
}

The output looks loke this. sometime I can read 3 times, sometime I can read 4 times, sometimes much more.

The write function handle is still valid. If one write command fails, all following will also fail. If I close&open the device again (inside the application) the write is temporary fixed.

The same behaviour exists on read command.

Any ideas?

Answer 1

In the driver function fWrite() return length instead of 0.

static ssize_t fWrite(struct file *file, const char  *buffer, size_t length, loff_t *offset)
{
printk(KERN_ERR "INSIDE\n");
return length;
}