Linux Device Drivers Development

Issuing pending DMA requests and waiting for callback notification

Starting the transaction is the last step in the DMA transfer setup. One activates transactions in the pending queue of a channel by calling dma_async_issue_pending() on that channel. If the channel is idle, then the first transaction in the queue is started and subsequent ones are queued up. On completion of a DMA operation, the next one in the queue is started and a tasklet triggered. This tasklet is in charge of calling the client driver completion callback routine for notification, if set:

void dma_async_issue_pending(struct dma_chan *chan); 

An example would look like this:

dma_async_issue_pending(my_dma_chan); wait_for_completion(&transfer_ok); dma_unmap_single(my_dma_chan->device->dev, ...