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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 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);
When the optional 'filter_fn' parameter is set to NULL dma_request_channel
simply returns the first channel that satisfies the capability mask. Otherwise,
when the mask parameter is insufficient for specifying the necessary channel,
the filter_fn routine can be used to disposition the available channels in the
system. The filter_fn routine is called once for each free channel in the
system. Upon seeing a suitable channel filter_fn returns DMA_ACK which flags
that channel to be the return value from dma_request_channel. 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.
It allows the clients to specify DMA direction, DMA addresses, bus widths, DMA
burst lengths etc. 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
int dmaengine_slave_config(struct dma_chan *chan,
struct dma_slave_config *config)
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.
The non NULL return of this transfer API represents a "descriptor" for the given
struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_sg)(
struct dma_chan *chan,
struct scatterlist *dst_sg, unsigned int dst_nents,
struct scatterlist *src_sg, unsigned int src_nents,
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);
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.
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