Improve slave/cyclic DMA engine documentation

Improve the documentation for the slave and cyclic DMA engine support
reformatting it for easier reading, adding further APIs, splitting it
into five steps, and including references to the documentation in
dmaengine.h.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[Fixed the index title to reflect new changes]
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/dmaengine.txt

@@ -10,87 +10,181 @@ 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
+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
+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);
-
-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.
+
+   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.
-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)
+
+   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 explaination 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
+
+   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,
+
+   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_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
+   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
+   paired.
+
+   Note:
+	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
+	transaction.
+
+	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
+	deadlock.
+
+	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.
+
+   Interface:
+	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.
+
+   Interface:
+	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.
+
+   Note:
+	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.