Asynchronous Data Transfers (Block Drivers)

This section presents a method for performing asynchronous I/O transfers. The driver queues the I/O requests and then returns control to the caller. Again, the assumption is that the hardware is a simple disk device that allows one transfer at a time. The device interrupts when a data transfer has completed. An interrupt also takes place if an error occurs. The basic steps for performing asynchronous data transfers are:

1. Check for invalid buf(9S) requests.

2. Enqueue the request.

3. Start the first transfer.

4. Handle the interrupting device.

Checking for Invalid buf Requests

As in the synchronous case, the device driver should check the buf(9S) structure passed to strategy(9E) for validity. See Synchronous Data Transfers (Block Drivers) for more details.

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.
     */
    mutex_enter(&xsp->mu);
    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;
    }
    mutex_exit(&xsp->mu);
    /* Start the transfer if possible */
    (void) xxstart((caddr_t)xsp);
    return (0);
}

Starting the First Transfer

Device drivers that implement queuing usually have a start() routine. start() dequeues the next request and starts the data transfer to or from the device. In this example, start() processes all requests regardless of the state of the device, whether busy or free.

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Note - start() must be written to be called from any context. start() can be called by both the strategy routine in kernel context and the interrupt routine in interrupt context.

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start() is called by strategy(9E) every time strategy() queues a request so that an idle device can be started. If the device is busy, start() returns immediately.

start() is also called by the interrupt handler before the handler returns from a claimed interrupt so that a nonempty queue can be serviced. If the queue is empty, start() returns immediately.

Because start() is a private driver routine, start() can take any arguments and can return any type. The following code sample is written to be used as a DMA callback, although that portion is not shown. Accordingly, the example must take a caddr_t as an argument and return an int. See Handling Resource Allocation Failures for more information about DMA callback routines.

Example 16-6 Starting the First Data Request for a Block Driver

static int
xxstart(caddr_t arg)
{
    struct xxstate *xsp = (struct xxstate *)arg;
    struct buf *bp;

    mutex_enter(&xsp->mu);
    /*
     * If there is nothing more to do, or the device is
     * busy, return.
     */
    if (xsp->list_head == NULL || xsp->busy) {
       mutex_exit(&xsp->mu);
       return (0);
    }
    xsp->busy = 1;
    /* Get the first buffer off the transfer list */
    bp = xsp->list_head;
    /* Update the head and tail pointer */
    xsp->list_head = xsp->list_head->av_forw;
    if (xsp->list_head == NULL)
       xsp->list_tail = NULL;
    bp->av_forw = NULL;
    mutex_exit(&xsp->mu);
    /* 
     * If the device has power manageable components, 
     * mark the device busy with pm_busy_components(9F),
     * and then ensure that the device 
     * is powered up by calling pm_raise_power(9F).
     *
     * Set up DMA resources with ddi_dma_alloc_handle(9F) and
     * ddi_dma_buf_bind_handle(9F).
     */
    xsp->bp = bp;
    ddi_put32(xsp->data_access_handle, &xsp->regp->dma_addr,
        cookie.dmac_address);
    ddi_put32(xsp->data_access_handle, &xsp->regp->dma_size,
        (uint32_t)cookie.dmac_size);
    ddi_put8(xsp->data_access_handle, &xsp->regp->csr,
        ENABLE_INTERRUPTS | START_TRANSFER);
    return (0);
}

Handling the Interrupting Device

The interrupt routine is similar to the asynchronous version, with the addition of the call to start() and the removal of the call to cv_signal(9F).

Example 16-7 Block Driver Routine for Asynchronous Interrupts

static u_int
xxintr(caddr_t arg)
{
    struct xxstate *xsp = (struct xxstate *)arg;
    struct buf *bp;
    uint8_t status;
    mutex_enter(&xsp->mu);
    status = ddi_get8(xsp->data_access_handle, &xsp->regp->csr);
    if (!(status & INTERRUPTING)) {
        mutex_exit(&xsp->mu);
        return (DDI_INTR_UNCLAIMED);
    }
    /* Get the buf responsible for this interrupt */
    bp = xsp->bp;
    xsp->bp = NULL;
    /*
     * This example is for a simple device which either
     * succeeds or fails the data transfer, indicated in the
     * command/status register.
     */
    if (status & DEVICE_ERROR) {
        /* failure */
        bp->b_resid = bp->b_bcount;
        bioerror(bp, EIO);
    } else {
        /* success */
        bp->b_resid = 0;
    }
    ddi_put8(xsp->data_access_handle, &xsp->regp->csr,
        CLEAR_INTERRUPT);
    /* The transfer has finished, successfully or not */
    biodone(bp);
    /*
     * If the device has power manageable components that were
     * marked busy in strategy(9F), mark them idle now with
     * pm_idle_component(9F)
     * Release any resources used in the transfer, such as DMA
     * resources (ddi_dma_unbind_handle(9F) and
     * ddi_dma_free_handle(9F)).
     *
     * Let the next I/O thread have access to the device.
     */
    xsp->busy = 0;
    mutex_exit(&xsp->mu);
    (void) xxstart((caddr_t)xsp);
    return (DDI_INTR_CLAIMED);
}