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Based on kernel version 3.2. Page generated on 2012-01-05 23:28 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 explaination 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	
79	   A non-NULL return of this transfer API represents a "descriptor" for
80	   the given transaction.
81	
82	   Interface:
83		struct dma_async_tx_descriptor *(*chan->device->device_prep_slave_sg)(
84			struct dma_chan *chan, struct scatterlist *sgl,
85			unsigned int sg_len, enum dma_data_direction direction,
86			unsigned long flags);
87	
88		struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_cyclic)(
89			struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
90			size_t period_len, enum dma_data_direction direction);
91	
92	   The peripheral driver is expected to have mapped the scatterlist for
93	   the DMA operation prior to calling device_prep_slave_sg, and must
94	   keep the scatterlist mapped until the DMA operation has completed.
95	   The scatterlist must be mapped using the DMA struct device.  So,
96	   normal setup should look like this:
97	
98		nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
99		if (nr_sg == 0)
100			/* error */
101	
102		desc = chan->device->device_prep_slave_sg(chan, sgl, nr_sg,
103				direction, flags);
104	
105	   Once a descriptor has been obtained, the callback information can be
106	   added and the descriptor must then be submitted.  Some DMA engine
107	   drivers may hold a spinlock between a successful preparation and
108	   submission so it is important that these two operations are closely
109	   paired.
110	
111	   Note:
112		Although the async_tx API specifies that completion callback
113		routines cannot submit any new operations, this is not the
114		case for slave/cyclic DMA.
115	
116		For slave DMA, the subsequent transaction may not be available
117		for submission prior to callback function being invoked, so
118		slave DMA callbacks are permitted to prepare and submit a new
119		transaction.
120	
121		For cyclic DMA, a callback function may wish to terminate the
122		DMA via dmaengine_terminate_all().
123	
124		Therefore, it is important that DMA engine drivers drop any
125		locks before calling the callback function which may cause a
126		deadlock.
127	
128		Note that callbacks will always be invoked from the DMA
129		engines tasklet, never from interrupt context.
130	
131	4. Submit the transaction
132	
133	   Once the descriptor has been prepared and the callback information
134	   added, it must be placed on the DMA engine drivers pending queue.
135	
136	   Interface:
137		dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
138	
139	   This returns a cookie can be used to check the progress of DMA engine
140	   activity via other DMA engine calls not covered in this document.
141	
142	   dmaengine_submit() will not start the DMA operation, it merely adds
143	   it to the pending queue.  For this, see step 5, dma_async_issue_pending.
144	
145	5. Issue pending DMA requests and wait for callback notification
146	
147	   The transactions in the pending queue can be activated by calling the
148	   issue_pending API. If channel is idle then the first transaction in
149	   queue is started and subsequent ones queued up.
150	
151	   On completion of each DMA operation, the next in queue is started and
152	   a tasklet triggered. The tasklet will then call the client driver
153	   completion callback routine for notification, if set.
154	
155	   Interface:
156		void dma_async_issue_pending(struct dma_chan *chan);
157	
158	Further APIs:
159	
160	1. int dmaengine_terminate_all(struct dma_chan *chan)
161	
162	   This causes all activity for the DMA channel to be stopped, and may
163	   discard data in the DMA FIFO which hasn't been fully transferred.
164	   No callback functions will be called for any incomplete transfers.
165	
166	2. int dmaengine_pause(struct dma_chan *chan)
167	
168	   This pauses activity on the DMA channel without data loss.
169	
170	3. int dmaengine_resume(struct dma_chan *chan)
171	
172	   Resume a previously paused DMA channel.  It is invalid to resume a
173	   channel which is not currently paused.
174	
175	4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
176	        dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
177	
178	   This can be used to check the status of the channel.  Please see
179	   the documentation in include/linux/dmaengine.h for a more complete
180	   description of this API.
181	
182	   This can be used in conjunction with dma_async_is_complete() and
183	   the cookie returned from 'descriptor->submit()' to check for
184	   completion of a specific DMA transaction.
185	
186	   Note:
187		Not all DMA engine drivers can return reliable information for
188		a running DMA channel.  It is recommended that DMA engine users
189		pause or stop (via dmaengine_terminate_all) the channel before
190		using this API.
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