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Based on kernel version 3.16. Page generated on 2014-08-06 21:38 EST.

1				DMA Engine API Guide
2				====================
3	
4			 Vinod Koul <vinod dot koul at intel.com>
5	
6	NOTE: For DMA Engine usage in async_tx please see:
7		Documentation/crypto/async-tx-api.txt
8	
9	
10	Below is a guide to device driver writers on how to use the Slave-DMA API of the
11	DMA Engine. This is applicable only for slave DMA usage only.
12	
13	The slave DMA usage consists of following steps:
14	1. Allocate a DMA slave channel
15	2. Set slave and controller specific parameters
16	3. Get a descriptor for transaction
17	4. Submit the transaction
18	5. Issue pending requests and wait for callback notification
19	
20	1. Allocate a DMA slave channel
21	
22	   Channel allocation is slightly different in the slave DMA context,
23	   client drivers typically need a channel from a particular DMA
24	   controller only and even in some cases a specific channel is desired.
25	   To request a channel dma_request_channel() API is used.
26	
27	   Interface:
28		struct dma_chan *dma_request_channel(dma_cap_mask_t mask,
29				dma_filter_fn filter_fn,
30				void *filter_param);
31	   where dma_filter_fn is defined as:
32		typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param);
33	
34	   The 'filter_fn' parameter is optional, but highly recommended for
35	   slave and cyclic channels as they typically need to obtain a specific
36	   DMA channel.
37	
38	   When the optional 'filter_fn' parameter is NULL, dma_request_channel()
39	   simply returns the first channel that satisfies the capability mask.
40	
41	   Otherwise, the 'filter_fn' routine will be called once for each free
42	   channel which has a capability in 'mask'.  'filter_fn' is expected to
43	   return 'true' when the desired DMA channel is found.
44	
45	   A channel allocated via this interface is exclusive to the caller,
46	   until dma_release_channel() is called.
47	
48	2. Set slave and controller specific parameters
49	
50	   Next step is always to pass some specific information to the DMA
51	   driver.  Most of the generic information which a slave DMA can use
52	   is in struct dma_slave_config.  This allows the clients to specify
53	   DMA direction, DMA addresses, bus widths, DMA burst lengths etc
54	   for the peripheral.
55	
56	   If some DMA controllers have more parameters to be sent then they
57	   should try to embed struct dma_slave_config in their controller
58	   specific structure. That gives flexibility to client to pass more
59	   parameters, if required.
60	
61	   Interface:
62		int dmaengine_slave_config(struct dma_chan *chan,
63					  struct dma_slave_config *config)
64	
65	   Please see the dma_slave_config structure definition in dmaengine.h
66	   for a detailed explanation of the struct members.  Please note
67	   that the 'direction' member will be going away as it duplicates the
68	   direction given in the prepare call.
69	
70	3. Get a descriptor for transaction
71	
72	   For slave usage the various modes of slave transfers supported by the
73	   DMA-engine are:
74	
75	   slave_sg	- DMA a list of scatter gather buffers from/to a peripheral
76	   dma_cyclic	- Perform a cyclic DMA operation from/to a peripheral till the
77			  operation is explicitly stopped.
78	   interleaved_dma - This is common to Slave as well as M2M clients. For slave
79			 address of devices' fifo could be already known to the driver.
80			 Various types of operations could be expressed by setting
81			 appropriate values to the 'dma_interleaved_template' members.
82	
83	   A non-NULL return of this transfer API represents a "descriptor" for
84	   the given transaction.
85	
86	   Interface:
87		struct dma_async_tx_descriptor *(*chan->device->device_prep_slave_sg)(
88			struct dma_chan *chan, struct scatterlist *sgl,
89			unsigned int sg_len, enum dma_data_direction direction,
90			unsigned long flags);
91	
92		struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_cyclic)(
93			struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
94			size_t period_len, enum dma_data_direction direction);
95	
96		struct dma_async_tx_descriptor *(*device_prep_interleaved_dma)(
97			struct dma_chan *chan, struct dma_interleaved_template *xt,
98			unsigned long flags);
99	
100	   The peripheral driver is expected to have mapped the scatterlist for
101	   the DMA operation prior to calling device_prep_slave_sg, and must
102	   keep the scatterlist mapped until the DMA operation has completed.
103	   The scatterlist must be mapped using the DMA struct device.  So,
104	   normal setup should look like this:
105	
106		nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
107		if (nr_sg == 0)
108			/* error */
109	
110		desc = chan->device->device_prep_slave_sg(chan, sgl, nr_sg,
111				direction, flags);
112	
113	   Once a descriptor has been obtained, the callback information can be
114	   added and the descriptor must then be submitted.  Some DMA engine
115	   drivers may hold a spinlock between a successful preparation and
116	   submission so it is important that these two operations are closely
117	   paired.
118	
119	   Note:
120		Although the async_tx API specifies that completion callback
121		routines cannot submit any new operations, this is not the
122		case for slave/cyclic DMA.
123	
124		For slave DMA, the subsequent transaction may not be available
125		for submission prior to callback function being invoked, so
126		slave DMA callbacks are permitted to prepare and submit a new
127		transaction.
128	
129		For cyclic DMA, a callback function may wish to terminate the
130		DMA via dmaengine_terminate_all().
131	
132		Therefore, it is important that DMA engine drivers drop any
133		locks before calling the callback function which may cause a
134		deadlock.
135	
136		Note that callbacks will always be invoked from the DMA
137		engines tasklet, never from interrupt context.
138	
139	4. Submit the transaction
140	
141	   Once the descriptor has been prepared and the callback information
142	   added, it must be placed on the DMA engine drivers pending queue.
143	
144	   Interface:
145		dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
146	
147	   This returns a cookie can be used to check the progress of DMA engine
148	   activity via other DMA engine calls not covered in this document.
149	
150	   dmaengine_submit() will not start the DMA operation, it merely adds
151	   it to the pending queue.  For this, see step 5, dma_async_issue_pending.
152	
153	5. Issue pending DMA requests and wait for callback notification
154	
155	   The transactions in the pending queue can be activated by calling the
156	   issue_pending API. If channel is idle then the first transaction in
157	   queue is started and subsequent ones queued up.
158	
159	   On completion of each DMA operation, the next in queue is started and
160	   a tasklet triggered. The tasklet will then call the client driver
161	   completion callback routine for notification, if set.
162	
163	   Interface:
164		void dma_async_issue_pending(struct dma_chan *chan);
165	
166	Further APIs:
167	
168	1. int dmaengine_terminate_all(struct dma_chan *chan)
169	
170	   This causes all activity for the DMA channel to be stopped, and may
171	   discard data in the DMA FIFO which hasn't been fully transferred.
172	   No callback functions will be called for any incomplete transfers.
173	
174	2. int dmaengine_pause(struct dma_chan *chan)
175	
176	   This pauses activity on the DMA channel without data loss.
177	
178	3. int dmaengine_resume(struct dma_chan *chan)
179	
180	   Resume a previously paused DMA channel.  It is invalid to resume a
181	   channel which is not currently paused.
182	
183	4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
184	        dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
185	
186	   This can be used to check the status of the channel.  Please see
187	   the documentation in include/linux/dmaengine.h for a more complete
188	   description of this API.
189	
190	   This can be used in conjunction with dma_async_is_complete() and
191	   the cookie returned from 'descriptor->submit()' to check for
192	   completion of a specific DMA transaction.
193	
194	   Note:
195		Not all DMA engine drivers can return reliable information for
196		a running DMA channel.  It is recommended that DMA engine users
197		pause or stop (via dmaengine_terminate_all) the channel before
198		using this API.
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