276 lines
9.4 KiB
ReStructuredText
276 lines
9.4 KiB
ReStructuredText
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====================
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DMA Engine API Guide
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====================
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Vinod Koul <vinod dot koul at intel.com>
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.. note:: For DMA Engine usage in async_tx please see:
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``Documentation/crypto/async-tx-api.txt``
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Below is a guide to device driver writers on how to use the Slave-DMA API of the
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DMA Engine. This is applicable only for slave DMA usage only.
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DMA usage
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=========
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The slave DMA usage consists of following steps:
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- Allocate a DMA slave channel
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- Set slave and controller specific parameters
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- Get a descriptor for transaction
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- Submit the transaction
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- Issue pending requests and wait for callback notification
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The details of these operations are:
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1. Allocate a DMA slave channel
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Channel allocation is slightly different in the slave DMA context,
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client drivers typically need a channel from a particular DMA
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controller only and even in some cases a specific channel is desired.
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To request a channel dma_request_chan() API is used.
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Interface:
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.. code-block:: c
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struct dma_chan *dma_request_chan(struct device *dev, const char *name);
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Which will find and return the ``name`` DMA channel associated with the 'dev'
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device. The association is done via DT, ACPI or board file based
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dma_slave_map matching table.
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A channel allocated via this interface is exclusive to the caller,
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until dma_release_channel() is called.
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2. Set slave and controller specific parameters
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Next step is always to pass some specific information to the DMA
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driver. Most of the generic information which a slave DMA can use
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is in struct dma_slave_config. This allows the clients to specify
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DMA direction, DMA addresses, bus widths, DMA burst lengths etc
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for the peripheral.
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If some DMA controllers have more parameters to be sent then they
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should try to embed struct dma_slave_config in their controller
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specific structure. That gives flexibility to client to pass more
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parameters, if required.
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Interface:
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.. code-block:: c
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int dmaengine_slave_config(struct dma_chan *chan,
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struct dma_slave_config *config)
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Please see the dma_slave_config structure definition in dmaengine.h
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for a detailed explanation of the struct members. Please note
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that the 'direction' member will be going away as it duplicates the
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direction given in the prepare call.
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3. Get a descriptor for transaction
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For slave usage the various modes of slave transfers supported by the
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DMA-engine are:
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- slave_sg: DMA a list of scatter gather buffers from/to a peripheral
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- dma_cyclic: Perform a cyclic DMA operation from/to a peripheral till the
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operation is explicitly stopped.
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- interleaved_dma: This is common to Slave as well as M2M clients. For slave
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address of devices' fifo could be already known to the driver.
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Various types of operations could be expressed by setting
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appropriate values to the 'dma_interleaved_template' members.
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A non-NULL return of this transfer API represents a "descriptor" for
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the given transaction.
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Interface:
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.. code-block:: c
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struct dma_async_tx_descriptor *dmaengine_prep_slave_sg(
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struct dma_chan *chan, struct scatterlist *sgl,
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unsigned int sg_len, enum dma_data_direction direction,
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unsigned long flags);
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struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(
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struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
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size_t period_len, enum dma_data_direction direction);
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struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma(
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struct dma_chan *chan, struct dma_interleaved_template *xt,
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unsigned long flags);
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The peripheral driver is expected to have mapped the scatterlist for
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the DMA operation prior to calling dmaengine_prep_slave_sg(), and must
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keep the scatterlist mapped until the DMA operation has completed.
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The scatterlist must be mapped using the DMA struct device.
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If a mapping needs to be synchronized later, dma_sync_*_for_*() must be
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called using the DMA struct device, too.
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So, normal setup should look like this:
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.. code-block:: c
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nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
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if (nr_sg == 0)
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/* error */
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desc = dmaengine_prep_slave_sg(chan, sgl, nr_sg, direction, flags);
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Once a descriptor has been obtained, the callback information can be
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added and the descriptor must then be submitted. Some DMA engine
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drivers may hold a spinlock between a successful preparation and
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submission so it is important that these two operations are closely
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paired.
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.. note::
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Although the async_tx API specifies that completion callback
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routines cannot submit any new operations, this is not the
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case for slave/cyclic DMA.
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For slave DMA, the subsequent transaction may not be available
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for submission prior to callback function being invoked, so
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slave DMA callbacks are permitted to prepare and submit a new
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transaction.
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For cyclic DMA, a callback function may wish to terminate the
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DMA via dmaengine_terminate_async().
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Therefore, it is important that DMA engine drivers drop any
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locks before calling the callback function which may cause a
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deadlock.
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Note that callbacks will always be invoked from the DMA
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engines tasklet, never from interrupt context.
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4. Submit the transaction
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Once the descriptor has been prepared and the callback information
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added, it must be placed on the DMA engine drivers pending queue.
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Interface:
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.. code-block:: c
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dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
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This returns a cookie can be used to check the progress of DMA engine
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activity via other DMA engine calls not covered in this document.
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dmaengine_submit() will not start the DMA operation, it merely adds
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it to the pending queue. For this, see step 5, dma_async_issue_pending.
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5. Issue pending DMA requests and wait for callback notification
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The transactions in the pending queue can be activated by calling the
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issue_pending API. If channel is idle then the first transaction in
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queue is started and subsequent ones queued up.
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On completion of each DMA operation, the next in queue is started and
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a tasklet triggered. The tasklet will then call the client driver
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completion callback routine for notification, if set.
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Interface:
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.. code-block:: c
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void dma_async_issue_pending(struct dma_chan *chan);
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Further APIs:
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-------------
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1. Terminate APIs
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.. code-block:: c
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int dmaengine_terminate_sync(struct dma_chan *chan)
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int dmaengine_terminate_async(struct dma_chan *chan)
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int dmaengine_terminate_all(struct dma_chan *chan) /* DEPRECATED */
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This causes all activity for the DMA channel to be stopped, and may
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discard data in the DMA FIFO which hasn't been fully transferred.
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No callback functions will be called for any incomplete transfers.
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Two variants of this function are available.
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dmaengine_terminate_async() might not wait until the DMA has been fully
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stopped or until any running complete callbacks have finished. But it is
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possible to call dmaengine_terminate_async() from atomic context or from
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within a complete callback. dmaengine_synchronize() must be called before it
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is safe to free the memory accessed by the DMA transfer or free resources
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accessed from within the complete callback.
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dmaengine_terminate_sync() will wait for the transfer and any running
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complete callbacks to finish before it returns. But the function must not be
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called from atomic context or from within a complete callback.
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dmaengine_terminate_all() is deprecated and should not be used in new code.
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2. Pause API
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.. code-block:: c
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int dmaengine_pause(struct dma_chan *chan)
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This pauses activity on the DMA channel without data loss.
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3. Resume API
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.. code-block:: c
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int dmaengine_resume(struct dma_chan *chan)
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Resume a previously paused DMA channel. It is invalid to resume a
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channel which is not currently paused.
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4. Check Txn complete
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.. code-block:: c
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enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
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dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
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This can be used to check the status of the channel. Please see
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the documentation in include/linux/dmaengine.h for a more complete
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description of this API.
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This can be used in conjunction with dma_async_is_complete() and
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the cookie returned from dmaengine_submit() to check for
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completion of a specific DMA transaction.
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.. note::
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Not all DMA engine drivers can return reliable information for
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a running DMA channel. It is recommended that DMA engine users
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pause or stop (via dmaengine_terminate_all()) the channel before
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using this API.
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5. Synchronize termination API
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.. code-block:: c
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void dmaengine_synchronize(struct dma_chan *chan)
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Synchronize the termination of the DMA channel to the current context.
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This function should be used after dmaengine_terminate_async() to synchronize
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the termination of the DMA channel to the current context. The function will
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wait for the transfer and any running complete callbacks to finish before it
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returns.
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If dmaengine_terminate_async() is used to stop the DMA channel this function
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must be called before it is safe to free memory accessed by previously
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submitted descriptors or to free any resources accessed within the complete
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callback of previously submitted descriptors.
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The behavior of this function is undefined if dma_async_issue_pending() has
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been called between dmaengine_terminate_async() and this function.
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