![Vinod Koul](/assets/img/avatar_default.png)
Signed-off-by: Vinod Koul <vinod.koul@intel.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
97 lines
4.4 KiB
Plaintext
97 lines
4.4 KiB
Plaintext
DMA Engine API Guide
|
|
====================
|
|
|
|
Vinod Koul <vinod dot koul at intel.com>
|
|
|
|
NOTE: For DMA Engine usage in async_tx please see:
|
|
Documentation/crypto/async-tx-api.txt
|
|
|
|
|
|
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.
|
|
|
|
Interface:
|
|
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
|
|
required.
|
|
|
|
Interface:
|
|
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
|
|
transaction.
|
|
|
|
Interface:
|
|
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.
|
|
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
|