Writing Device Drivers

Enqueuing the Request

Unlike synchronous data transfers, a driver does not wait for an asynchronous request to complete. Instead, the driver adds the request to a queue. The head of the queue can be the current transfer. The head of the queue can also be a separate field in the state structure for holding the active request, as in Example 16–5.

If the queue is initially empty, then the hardware is not busy and strategy(9E) starts the transfer before returning. Otherwise, if a transfer completes with a non-empty queue, the interrupt routine begins a new transfer. Example 16–5 places the decision of whether to start a new transfer into a separate routine for convenience.

The driver can use the av_forw and the av_back members of the buf(9S) structure to manage a list of transfer requests. A single pointer can be used to manage a singly linked list, or both pointers can be used together to build a doubly linked list. The device hardware specification specifies which type of list management, such as insertion policies, is used to optimize the performance of the device. The transfer list is a per-device list, so the head and tail of the list are stored in the state structure.

The following example provides multiple threads with access to the driver shared data, such as the transfer list. You must identify the shared data and must protect the data with a mutex. See Chapter 3, Multithreading for more details about mutex locks.

Example 16–5 Enqueuing Data Transfer Requests for Block Drivers

static int
xxstrategy(struct buf *bp)
    struct xxstate *xsp;
    minor_t instance;
    instance = getminor(bp->b_edev);
    xsp = ddi_get_soft_state(statep, instance);
    /* ... */
    /* validate transfer request */
    /* ... */
     * Add the request to the end of the queue. Depending on the device, a sorting
     * algorithm, such as disksort(9F) can be used if it improves the
     * performance of the device.
    bp->av_forw = NULL;
    if (xsp->list_head) {
       /* Non-empty transfer list */
       xsp->list_tail->av_forw = bp;
       xsp->list_tail = bp;
    } else {
       /* Empty Transfer list */
       xsp->list_head = bp;
       xsp->list_tail = bp;
    /* Start the transfer if possible */
    (void) xxstart((caddr_t)xsp);
    return (0);