-4. Submit the transaction and wait for callback notification
-To schedule the transaction to be scheduled by dma device, the "descriptor"
-returned in above (3) needs to be submitted.
-To tell the dma driver that a transaction is ready to be serviced, the
-descriptor->submit() callback needs to be invoked. This chains the descriptor to
-the pending queue.
-The transactions in the pending queue can be activated by calling the
-issue_pending API. If channel is idle then the first transaction in queue is
-started and subsequent ones queued up.
-On completion of the DMA operation the next in queue is submitted and a tasklet
-triggered. The tasklet would then call the client driver completion callback
-routine for notification, if set.
-Interface:
-void dma_async_issue_pending(struct dma_chan *chan);
-
-==============================================================================
-
-Additional usage notes for dma driver writers
-1/ Although DMA engine specifies that completion callback routines cannot submit
-any new operations, but typically for slave DMA subsequent transaction may not
-be available for submit prior to callback routine being called. This requirement
-is not a requirement for DMA-slave devices. But they should take care to drop
-the spin-lock they might be holding before calling the callback routine
+ The peripheral driver is expected to have mapped the scatterlist for
+ the DMA operation prior to calling device_prep_slave_sg, and must
+ keep the scatterlist mapped until the DMA operation has completed.
+ The scatterlist must be mapped using the DMA struct device. So,
+ normal setup should look like this:
+
+ nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
+ if (nr_sg == 0)
+ /* error */
+
+ desc = chan->device->device_prep_slave_sg(chan, sgl, nr_sg,
+ direction, flags);
+
+ Once a descriptor has been obtained, the callback information can be
+ added and the descriptor must then be submitted. Some DMA engine
+ drivers may hold a spinlock between a successful preparation and
+ submission so it is important that these two operations are closely
+ paired.
+
+ Note:
+ Although the async_tx API specifies that completion callback
+ routines cannot submit any new operations, this is not the
+ case for slave/cyclic DMA.
+
+ For slave DMA, the subsequent transaction may not be available
+ for submission prior to callback function being invoked, so
+ slave DMA callbacks are permitted to prepare and submit a new
+ transaction.
+
+ For cyclic DMA, a callback function may wish to terminate the
+ DMA via dmaengine_terminate_all().
+
+ Therefore, it is important that DMA engine drivers drop any
+ locks before calling the callback function which may cause a
+ deadlock.
+
+ Note that callbacks will always be invoked from the DMA
+ engines tasklet, never from interrupt context.
+
+4. Submit the transaction
+
+ Once the descriptor has been prepared and the callback information
+ added, it must be placed on the DMA engine drivers pending queue.
+
+ Interface:
+ dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
+
+ This returns a cookie can be used to check the progress of DMA engine
+ activity via other DMA engine calls not covered in this document.
+
+ dmaengine_submit() will not start the DMA operation, it merely adds
+ it to the pending queue. For this, see step 5, dma_async_issue_pending.
+
+5. Issue pending DMA requests and wait for callback notification
+
+ The transactions in the pending queue can be activated by calling the
+ issue_pending API. If channel is idle then the first transaction in
+ queue is started and subsequent ones queued up.
+
+ On completion of each DMA operation, the next in queue is started and
+ a tasklet triggered. The tasklet will then call the client driver
+ completion callback routine for notification, if set.
+
+ Interface:
+ void dma_async_issue_pending(struct dma_chan *chan);
+
+Further APIs:
+
+1. int dmaengine_terminate_all(struct dma_chan *chan)
+
+ This causes all activity for the DMA channel to be stopped, and may
+ discard data in the DMA FIFO which hasn't been fully transferred.
+ No callback functions will be called for any incomplete transfers.
+
+2. int dmaengine_pause(struct dma_chan *chan)
+
+ This pauses activity on the DMA channel without data loss.
+
+3. int dmaengine_resume(struct dma_chan *chan)
+
+ Resume a previously paused DMA channel. It is invalid to resume a
+ channel which is not currently paused.
+
+4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
+ dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
+
+ This can be used to check the status of the channel. Please see
+ the documentation in include/linux/dmaengine.h for a more complete
+ description of this API.
+
+ This can be used in conjunction with dma_async_is_complete() and
+ the cookie returned from 'descriptor->submit()' to check for
+ completion of a specific DMA transaction.
+
+ Note:
+ Not all DMA engine drivers can return reliable information for
+ a running DMA channel. It is recommended that DMA engine users
+ pause or stop (via dmaengine_terminate_all) the channel before
+ using this API.