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Vinod Koul46b29032011-05-25 14:49:20 -07001 DMA Engine API Guide
2 ====================
3
4 Vinod Koul <vinod dot koul at intel.com>
5
6NOTE: For DMA Engine usage in async_tx please see:
7 Documentation/crypto/async-tx-api.txt
8
9
10Below is a guide to device driver writers on how to use the Slave-DMA API of the
11DMA Engine. This is applicable only for slave DMA usage only.
12
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010013The slave DMA usage consists of following steps:
Vinod Koul46b29032011-05-25 14:49:20 -0700141. Allocate a DMA slave channel
152. Set slave and controller specific parameters
163. Get a descriptor for transaction
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +0100174. Submit the transaction
185. Issue pending requests and wait for callback notification
Vinod Koul46b29032011-05-25 14:49:20 -070019
201. Allocate a DMA slave channel
Vinod Koul46b29032011-05-25 14:49:20 -070021
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010022 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.
Vinod Koul46b29032011-05-25 14:49:20 -070026
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010027 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.
Vinod Koul46b29032011-05-25 14:49:20 -070047
482. Set slave and controller specific parameters
Vinod Koul46b29032011-05-25 14:49:20 -070049
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010050 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
Masanari Iida40e47122012-03-04 23:16:11 +090066 for a detailed explanation of the struct members. Please note
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010067 that the 'direction' member will be going away as it duplicates the
68 direction given in the prepare call.
Vinod Koul46b29032011-05-25 14:49:20 -070069
703. Get a descriptor for transaction
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010071
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
Vinod Koul46b29032011-05-25 14:49:20 -070077 operation is explicitly stopped.
Jassi Brarb14dab72011-10-13 12:33:30 +053078 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.
Vinod Koul46b29032011-05-25 14:49:20 -070082
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010083 A non-NULL return of this transfer API represents a "descriptor" for
84 the given transaction.
85
86 Interface:
Geert Uytterhoeven96cb9892014-06-16 15:13:24 +020087 struct dma_async_tx_descriptor *dmaengine_prep_slave_sg(
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010088 struct dma_chan *chan, struct scatterlist *sgl,
89 unsigned int sg_len, enum dma_data_direction direction,
Vinod Koul46b29032011-05-25 14:49:20 -070090 unsigned long flags);
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +010091
Geert Uytterhoeven96cb9892014-06-16 15:13:24 +020092 struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(
Vinod Koul46b29032011-05-25 14:49:20 -070093 struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
94 size_t period_len, enum dma_data_direction direction);
95
Geert Uytterhoeven96cb9892014-06-16 15:13:24 +020096 struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma(
Jassi Brarb14dab72011-10-13 12:33:30 +053097 struct dma_chan *chan, struct dma_interleaved_template *xt,
98 unsigned long flags);
99
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +0100100 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.
Geert Uytterhoeven70c9b8b2014-07-11 17:56:21 +0200103 The scatterlist must be mapped using the DMA struct device.
104 If a mapping needs to be synchronized later, dma_sync_*_for_*() must be
105 called using the DMA struct device, too.
106 So, normal setup should look like this:
Vinod Koul46b29032011-05-25 14:49:20 -0700107
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +0100108 nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
109 if (nr_sg == 0)
110 /* error */
Vinod Koul46b29032011-05-25 14:49:20 -0700111
Geert Uytterhoeven96cb9892014-06-16 15:13:24 +0200112 desc = dmaengine_prep_slave_sg(chan, sgl, nr_sg, direction, flags);
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +0100113
114 Once a descriptor has been obtained, the callback information can be
115 added and the descriptor must then be submitted. Some DMA engine
116 drivers may hold a spinlock between a successful preparation and
117 submission so it is important that these two operations are closely
118 paired.
119
120 Note:
121 Although the async_tx API specifies that completion callback
122 routines cannot submit any new operations, this is not the
123 case for slave/cyclic DMA.
124
125 For slave DMA, the subsequent transaction may not be available
126 for submission prior to callback function being invoked, so
127 slave DMA callbacks are permitted to prepare and submit a new
128 transaction.
129
130 For cyclic DMA, a callback function may wish to terminate the
131 DMA via dmaengine_terminate_all().
132
133 Therefore, it is important that DMA engine drivers drop any
134 locks before calling the callback function which may cause a
135 deadlock.
136
137 Note that callbacks will always be invoked from the DMA
138 engines tasklet, never from interrupt context.
139
1404. Submit the transaction
141
142 Once the descriptor has been prepared and the callback information
143 added, it must be placed on the DMA engine drivers pending queue.
144
145 Interface:
146 dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
147
148 This returns a cookie can be used to check the progress of DMA engine
149 activity via other DMA engine calls not covered in this document.
150
151 dmaengine_submit() will not start the DMA operation, it merely adds
152 it to the pending queue. For this, see step 5, dma_async_issue_pending.
153
1545. Issue pending DMA requests and wait for callback notification
155
156 The transactions in the pending queue can be activated by calling the
157 issue_pending API. If channel is idle then the first transaction in
158 queue is started and subsequent ones queued up.
159
160 On completion of each DMA operation, the next in queue is started and
161 a tasklet triggered. The tasklet will then call the client driver
162 completion callback routine for notification, if set.
163
164 Interface:
165 void dma_async_issue_pending(struct dma_chan *chan);
166
167Further APIs:
168
1691. int dmaengine_terminate_all(struct dma_chan *chan)
170
171 This causes all activity for the DMA channel to be stopped, and may
172 discard data in the DMA FIFO which hasn't been fully transferred.
173 No callback functions will be called for any incomplete transfers.
174
1752. int dmaengine_pause(struct dma_chan *chan)
176
177 This pauses activity on the DMA channel without data loss.
178
1793. int dmaengine_resume(struct dma_chan *chan)
180
181 Resume a previously paused DMA channel. It is invalid to resume a
182 channel which is not currently paused.
183
1844. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
185 dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
186
187 This can be used to check the status of the channel. Please see
188 the documentation in include/linux/dmaengine.h for a more complete
189 description of this API.
190
191 This can be used in conjunction with dma_async_is_complete() and
Geert Uytterhoeven96cb9892014-06-16 15:13:24 +0200192 the cookie returned from dmaengine_submit() to check for
Russell King - ARM Linux5a42fb92011-07-26 14:25:10 +0100193 completion of a specific DMA transaction.
194
195 Note:
196 Not all DMA engine drivers can return reliable information for
197 a running DMA channel. It is recommended that DMA engine users
198 pause or stop (via dmaengine_terminate_all) the channel before
199 using this API.