Linux Device Drivers, Second Edition

Handling Requests: A Simple Introduction

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 discusses the basic design of the request procedure.

The Request Queue

When the kernel schedules a data transfer, it queues the request in a list, ordered in such a way that it maximizes system performance. The queue of requests is then passed to the driver’s request function, which has the following prototype:

void request_fn(request_queue_t *queue);

The request function should perform the following tasks for each request in the queue:

1. Check the validity of the request. This test is performed by the macro INIT_REQUEST, defined in blk.h; the test consists of looking for problems that could indicate a bug in the system’s request queue handling.

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

3. Clean up the request just processed. This operation is performed by end_request, a static function whose code resides in blk.h. end_request handles the management of the request queue and wakes up processes waiting on the I/O operation. It also manages the CURRENT variable, ensuring that it points to the next unsatisfied request. The driver passes the function a single argument, which is 1 in case of success and ...