Documentation/dmaengine.txt


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			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
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.

   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);

   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.

   Interface:
	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.

   Interface:
	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);