dmaengine: Add API documentation for slave dma usage

Signed-off-by: NVinod Koul <vinod.koul@intel.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

Documentation/dmaengine.txt

-See Documentation/crypto/async-tx-api.txt
+			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