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I have a device driver I am working on. It works this way:

1. User app sends ioctl cmd 0x01 to driver that puts it to sleep.
2. User app sends another ioctl cmd 0x02 to driver that wakes it up.

I am doing this because my hardware is not ready, so I don't have a device to talk to yet.

Issue: When I send ioctl cmd 0x01 to put the driver to sleep, my user app sleeps/hangs but when I send ioctl cmd 0x02, it does not get signaled to wake up. I print out the semaphore count to see whats going on and notice that for each case, my_sem.count == 0 before the call to down()/up().

In general what is it about drivers I don't understand? It appears that my ioctl handler has a different copy of my_sem for every ioctl cmd I send with my user app. When I send several consecutive cmds for up(), I expect to see the count increase for every ioctl cmd that calls up(), but it always prints out ZERO before up() and ONE after up().

/* PSEUDO CODE */
// global defn
DEFINE_SEMAPHORE(my_sem);
.
.
.
// file ops ioctl handler
int foo_CharIoctl(struct file * file, 
              unsigned int cmd, 
              unsigned long arg)
{
    sema_init(&my_sem, 0);
.
.
.
    switch(cmd)
    {
        case 0x01:
            printf my_sem.count;
            down_killable(&my_sem);
            printf my_sem.count;
        case 0x02:
            printf my_sem.count;
            up(&my_sem);
            printf my_sem.count;
        ...
    }
}
user1316601
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  • 1

1 Answers1

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It looks like you are re-initializing the semaphore every time foo_CharIoctl() is called. sema_init() should be placed in your driver initialization.

iamstinke
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