Opening a kernel driver inside a module/driver

Question

Hi i wrote a user app that passes data to /dev/ttyS0 .. I used php to send the data to the user app then had an if else inside the user app then passes the new data to /dev/ttyS0 ..

Eg

PHP -> user app - > driver(/dev/ttyS0);

php pass "go up" to user app then user app then it does an if else for string comparison that when it's true passes the new data(eg.0xff) to the driver(dev/ttyS0);

I also wrote my very first char driver which reads and write.. this is the site that im following http://lwn.net/Kernel/LDD3/

Now the question is.. is it possible to open a driver inside a driver?.. I know it's very very wrong to do it.. but i want to make my own driver which does an if else comparison inside so no need for the user app so passing data would just be like this:

PHP - > mydriver(which translates the data like my user app does)..

Here's sample of my user app code..

int main(int argc,char *argv[])   

{

unsigned char pCom[2][7]={{0xFF,0x01,0x00,0x08,0x00,0x3F,0x48},   
                            {0xFF,0x01,0x00,0x10,0x00,0x3F,0x50}};   


int fd;        
int p;       
    fd=open("/dev/ttyS0",O_RDWR | O_NOCTTY | O_NDELAY);

    if(fd<0) exit(1); 
    set_port(fd);

    if(strcmp(argv[1], "go up")==0){

        for(p=0; p<8; p++){
        write(fd,pCom[0],8);
        }

How can i modify my code so i can put in my char driver?.. eg. modify the ssize_t (*read) or ssize_t (*write) ? or can i just write this directly inside module_init(); Can you show me sample of how can i do it?..

Thank you in advance..

Answer 1

Don't do that. As you yourself noted, it's not the way it's supposed to be done, and I can see no real benefits either:

Benefits:

1. No need to install user-mode application.

Drawbacks:

1. Need to install kernel mode driver.
2. Need to update the driver when kernel is updated.
3. If there is an error in the driver code, whole system crashes.

On the other hand, you may want to create eg a fifo or a tcp socket so that the PHP application can easily communicate with your user application. Other option would be to code the if-else-code directly in PHP or as a library that could be called from PHP.