Linux Char Driver: blocking ioctl call - linux

Question

I am new to driver development, and I am trying to write a simple char driver that has ioctl that allows user process to get the time(timespec) that my char driver took on last read and write.
long charmem_ioctl(struct file *filp, unsigned int cmd, unsigned long arg) {
struct charmem_dev *dev = filp->private_data;
if (down_interruptible(&dev->sem)) {
printk(KERN_WARNING "I got booted!!\n");
return -ERESTARTSYS;
}
printk(KERN_WARNING "charmem: in ioctl; cmd = %d, arg = %d\n", (int)cmd, (int)arg);
switch(cmd) {
case IOCTL_GET_LAST_READ_TIME:
printk("charmem_ioctl: returning last read time delta, exiting...\n");
up(&dev->sem);
return dev->last_read_delta.tv_nsec;
break;
case IOCTL_GET_LAST_WRITE_TIME:
printk("charmem_ioctl: returning last write time delta, exiting...\n");
up(&dev->sem);
return dev->last_write_delta.tv_nsec;
break;
case IOCTL_RESET_READ: /*return read-pointer to the start of buffer*/
dev->rp = dev->buffer;
break;
case IOCTL_RESET_WRITE: /*return write-pointer to the start of buffer*/
dev->wp = dev->buffer;
break;
case IOCTL_LOAD_BUFFER_TO_CACHE:
load_buffer_to_cache(dev->buffer, dev->buffer_size);
break;
default:
printk("charmem_ioctl: invalid ioctl command, exiting...\n");
up(&dev->sem);
return -EFAULT;
}
up(&dev->sem);
return 0;
}
struct file_operations charmem_fops = {
.owner = THIS_MODULE,
.llseek = no_llseek,
.read = charmem_read,
.write = charmem_write,
.unlocked_ioctl = charmem_ioctl,
.open = charmem_open,
.release = charmem_release,
};
main.c -- user program that tests my char device:
int fd = 0, ret = 0;
fd = open("/dev/charmem0", O_RDWR);
if (fd < 0) {
printf("/dev/charmem0 unable to access (fd = %d)... EXITING\n", fd);
return -1;
}
ret = write(fd,msg1,10);
ret = read(fd,user_buffer,10);
read_delta = ioctl(fd, IOCTL_GET_LAST_READ_TIME);
printf("read_delta : %d\n ", read_delta);
write_delta = ioctl(fd, IOCTL_GET_LAST_WRITE_TIME);
printf("write_delta : %d\n ", write_delta);
main.c is the program that tests my char device; the program blocks after printing out read_delta value, and I am assuming that it blocks on ioctl. What am I doing wrong in my code?

Answer 1

I don't see any problems with the up/down of the semaphore in your code. The most likely place that your program is blocking is in the call to down_interruptible(). If you press control-c, that will force the down_interruptible to return, and you should see your printk of "I got booted" in dmesg or your console or syslog. Then the task is the figure out what other thing in your driver is holding that semaphore.
One other thought that occurs to me... printf is buffered. So it is possible that your GET_LAST_WRITE_TIME ioctl did return, and the output is in the stdout buffer, and your program is actually stuck on some code further down. Recommend adding a fflush(stdout) after the printf("write delta... to eliminate this possibility.
Michael