DMA Callback Example

In Example 9–1, xxstart() is used as the callback function. The per-device state structure is used as the argument to xxstart(). The xxstart() function attempts to start the command. If the command cannot be started because resources are not available, xxstart() is scheduled to be called later when resources are available.

Because xxstart() is used as a DMA callback, xxstart() must adhere to the following rules, which are imposed on DMA callbacks:

• Resources cannot be assumed to be available. The callback must try to allocate resources again.

• The callback must indicate to the system whether allocation succeeded. DDI_DMA_CALLBACK_RUNOUT should be returned if the callback fails to allocate resources, in which case xxstart() needs to be called again later. DDI_DMA_CALLBACK_DONE indicates success, so that no further callback is necessary.

Example 9–1 DMA Callback Example

static int
xxstart(caddr_t arg)
{
    struct xxstate *xsp = (struct xxstate *)arg;
    struct device_reg *regp;
    int flags;
    mutex_enter(&xsp->mu);
    if (xsp->busy) {
        /* transfer in progress */
        mutex_exit(&xsp->mu);
        return (DDI_DMA_CALLBACK_RUNOUT);
    }
    xsp->busy = 1;
    regp = xsp->regp;
    if ( /* transfer is a read */ ) {
        flags = DDI_DMA_READ;
    } else {
        flags = DDI_DMA_WRITE;
    }
    mutex_exit(&xsp->mu);
    if (ddi_dma_buf_bind_handle(xsp->handle,xsp->bp,flags, xxstart,
        (caddr_t)xsp, &cookie, &ccount) != DDI_DMA_MAPPED) {
        /* really should check all return values in a switch */
        mutex_enter(&xsp->mu);
        xsp->busy=0;
        mutex_exit(&xsp->mu);
        return (DDI_DMA_CALLBACK_RUNOUT);
    }
    /* Program the DMA engine. */
    return (DDI_DMA_CALLBACK_DONE);
}