Canceling DMA Callbacks

DMA callbacks cannot be canceled. Canceling a DMA callback requires some additional code in the driver's detach(9E) entry point. The detach() routine must not return DDI_SUCCESS if any outstanding callbacks exist. See Example 9–6. When DMA callbacks occur, the detach() routine must wait for the callback to run. When the callback has finished, detach() must prevent the callback from rescheduling itself. Callbacks can be prevented from rescheduling through additional fields in the state structure, as shown in the following example.

Example 9–6 Canceling DMA Callbacks

static int
xxdetach(dev_info_t *dip, ddi_detach_cmd_t cmd)
{
     /* ... */
     mutex_enter(&xsp->callback_mutex);
     xsp->cancel_callbacks = 1;
     while (xsp->callback_count > 0) {
        cv_wait(&xsp->callback_cv, &xsp->callback_mutex);
     }
     mutex_exit(&xsp->callback_mutex);
     /* ... */
 }

static int
xxstrategy(struct buf *bp)
{
     /* ... */
     mutex_enter(&xsp->callback_mutex);
       xsp->bp = bp;
     error = ddi_dma_buf_bind_handle(xsp->handle, xsp->bp, flags,
         xxdmacallback, (caddr_t)xsp, &cookie, &ccount);
     if (error == DDI_DMA_NORESOURCES)
       xsp->callback_count++;
     mutex_exit(&xsp->callback_mutex);
     /* ... */
}

static int
xxdmacallback(caddr_t callbackarg)
{
     struct xxstate *xsp = (struct xxstate *)callbackarg;
     /* ... */
     mutex_enter(&xsp->callback_mutex);
     if (xsp->cancel_callbacks) {
        /* do not reschedule, in process of detaching */
        xsp->callback_count--;
        if (xsp->callback_count == 0)
           cv_signal(&xsp->callback_cv);
        mutex_exit(&xsp->callback_mutex);
        return (DDI_DMA_CALLBACK_DONE);    /* don't reschedule it */
     }
     /*
      * Presumably at this point the device is still active
      * and will not be detached until the DMA has completed.
      * A return of 0 means try again later
      */
     error = ddi_dma_buf_bind_handle(xsp->handle, xsp->bp, flags,
         DDI_DMA_DONTWAIT, NULL, &cookie, &ccount);
     if (error == DDI_DMA_MAPPED) {
        /* Program the DMA engine. */
        xsp->callback_count--;
        mutex_exit(&xsp->callback_mutex);
        return (DDI_DMA_CALLBACK_DONE);
     }
     if (error != DDI_DMA_NORESOURCES) {
        xsp->callback_count--;
        mutex_exit(&xsp->callback_mutex);
        return (DDI_DMA_CALLBACK_DONE);
     }
     mutex_exit(&xsp->callback_mutex);
     return (DDI_DMA_CALLBACK_RUNOUT);
}