Starting the First Transfer

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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.

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.

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);
}