Linux Device Drivers

Handling Requests

The most important function in a block driver is the request function, which performs the low-level operations related to reading and writing data. This section introduces the design of such a procedure.

When the kernel schedules a data transfer, it queues the ``request'' in a list, ordered so that it maximizes system performance. The linked list of requests is then passed to the driver’s request function, which should perform the following tasks for each request in the linked list:

• Check the validity of the current request. This task is performed by the macro INIT_REQUEST, defined in blk.h.

• Perform the actual data transfer. The CURRENT variable (macro, actually) can be used to retrieve the details of the outstanding request. CURRENT is a pointer to struct request, whose fields are described in the next section.

• Clean up the current request. This operation is performed by end_request, a static function whose code resides in blk.h. The driver passes the function a single argument, which is 1 in case of success and 0 in case of failure. When end_request is called with an argument of zero, an ``I/O error'' message is delivered to the system logs (via printk).

• Loop back to the beginning, to consume the next request. A goto, a surrounding for(;;), or a surrounding while(1) can be used, at the programmer’s will.

In practice, the code for the request function is structured like this:

void sbull_request(void) { while(1) { INIT_REQUEST; printk("request %p: cmd %i sec %li ...