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DMA Engine API Guide

Vinod Koul <vinod dot koul at>

NOTE: For DMA Engine usage in async_tx please see:

Below is a guide to device driver writers on how to use the Slave-DMA API of the
DMA Engine. This is applicable only for slave DMA usage only.

The slave DMA usage consists of following steps:
1. Allocate a DMA slave channel
2. Set slave and controller specific parameters
3. Get a descriptor for transaction
4. Submit the transaction
5. Issue pending requests and wait for callback notification

1. Allocate a DMA slave channel

   Channel allocation is slightly different in the slave DMA context,
   client drivers typically need a channel from a particular DMA
   controller only and even in some cases a specific channel is desired.
   To request a channel dma_request_channel() API is used.

struct dma_chan *dma_request_channel(dma_cap_mask_t mask,
dma_filter_fn filter_fn,
void *filter_param);
   where dma_filter_fn is defined as:
typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param);

   The 'filter_fn' parameter is optional, but highly recommended for
   slave and cyclic channels as they typically need to obtain a specific
   DMA channel.

   When the optional 'filter_fn' parameter is NULL, dma_request_channel()
   simply returns the first channel that satisfies the capability mask.

   Otherwise, the 'filter_fn' routine will be called once for each free
   channel which has a capability in 'mask'. 'filter_fn' is expected to
   return 'true' when the desired DMA channel is found.

   A channel allocated via this interface is exclusive to the caller,
   until dma_release_channel() is called.

2. Set slave and controller specific parameters

   Next step is always to pass some specific information to the DMA
   driver. Most of the generic information which a slave DMA can use
   is in struct dma_slave_config. This allows the clients to specify
   DMA direction, DMA addresses, bus widths, DMA burst lengths etc
   for the peripheral.

   If some DMA controllers have more parameters to be sent then they
   should try to embed struct dma_slave_config in their controller
   specific structure. That gives flexibility to client to pass more
   parameters, if required.

int dmaengine_slave_config(struct dma_chan *chan,
struct dma_slave_config *config)

   Please see the dma_slave_config structure definition in dmaengine.h
   for a detailed explanation of the struct members. Please note
   that the 'direction' member will be going away as it duplicates the
   direction given in the prepare call.

3. Get a descriptor for transaction

   For slave usage the various modes of slave transfers supported by the
   DMA-engine are:

   slave_sg - DMA a list of scatter gather buffers from/to a peripheral
   dma_cyclic - Perform a cyclic DMA operation from/to a peripheral till the
operation is explicitly stopped.
   interleaved_dma - This is common to Slave as well as M2M clients. For slave
address of devices' fifo could be already known to the driver.
Various types of operations could be expressed by setting
appropriate values to the 'dma_interleaved_template' members.

   A non-NULL return of this transfer API represents a "descriptor" for
   the given transaction.

struct dma_async_tx_descriptor *(*chan->device->device_prep_slave_sg)(
struct dma_chan *chan, struct scatterlist *sgl,
unsigned int sg_len, enum dma_data_direction direction,
unsigned long flags);

struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_cyclic)(
struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
size_t period_len, enum dma_data_direction direction);

struct dma_async_tx_descriptor *(*device_prep_interleaved_dma)(
struct dma_chan *chan, struct dma_interleaved_template *xt,
unsigned long flags);

   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

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

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

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.

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.

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.

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.
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