5 Vinod Koul <vinod dot koul at intel.com>
7 .. note:: For DMA Engine usage in async_tx please see:
8 ``Documentation/crypto/async-tx-api.txt``
11 Below is a guide to device driver writers on how to use the Slave-DMA API of the
12 DMA Engine. This is applicable only for slave DMA usage only.
17 The slave DMA usage consists of following steps:
19 - Allocate a DMA slave channel
21 - Set slave and controller specific parameters
23 - Get a descriptor for transaction
25 - Submit the transaction
27 - Issue pending requests and wait for callback notification
29 The details of these operations are:
31 1. Allocate a DMA slave channel
33 Channel allocation is slightly different in the slave DMA context,
34 client drivers typically need a channel from a particular DMA
35 controller only and even in some cases a specific channel is desired.
36 To request a channel dma_request_chan() API is used.
42 struct dma_chan *dma_request_chan(struct device *dev, const char *name);
44 Which will find and return the ``name`` DMA channel associated with the 'dev'
45 device. The association is done via DT, ACPI or board file based
46 dma_slave_map matching table.
48 A channel allocated via this interface is exclusive to the caller,
49 until dma_release_channel() is called.
51 2. Set slave and controller specific parameters
53 Next step is always to pass some specific information to the DMA
54 driver. Most of the generic information which a slave DMA can use
55 is in struct dma_slave_config. This allows the clients to specify
56 DMA direction, DMA addresses, bus widths, DMA burst lengths etc
59 If some DMA controllers have more parameters to be sent then they
60 should try to embed struct dma_slave_config in their controller
61 specific structure. That gives flexibility to client to pass more
62 parameters, if required.
68 int dmaengine_slave_config(struct dma_chan *chan,
69 struct dma_slave_config *config)
71 Please see the dma_slave_config structure definition in dmaengine.h
72 for a detailed explanation of the struct members. Please note
73 that the 'direction' member will be going away as it duplicates the
74 direction given in the prepare call.
76 3. Get a descriptor for transaction
78 For slave usage the various modes of slave transfers supported by the
81 - slave_sg: DMA a list of scatter gather buffers from/to a peripheral
83 - dma_cyclic: Perform a cyclic DMA operation from/to a peripheral till the
84 operation is explicitly stopped.
86 - interleaved_dma: This is common to Slave as well as M2M clients. For slave
87 address of devices' fifo could be already known to the driver.
88 Various types of operations could be expressed by setting
89 appropriate values to the 'dma_interleaved_template' members.
91 A non-NULL return of this transfer API represents a "descriptor" for
92 the given transaction.
98 struct dma_async_tx_descriptor *dmaengine_prep_slave_sg(
99 struct dma_chan *chan, struct scatterlist *sgl,
100 unsigned int sg_len, enum dma_data_direction direction,
101 unsigned long flags);
103 struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(
104 struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
105 size_t period_len, enum dma_data_direction direction);
107 struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma(
108 struct dma_chan *chan, struct dma_interleaved_template *xt,
109 unsigned long flags);
111 The peripheral driver is expected to have mapped the scatterlist for
112 the DMA operation prior to calling dmaengine_prep_slave_sg(), and must
113 keep the scatterlist mapped until the DMA operation has completed.
114 The scatterlist must be mapped using the DMA struct device.
115 If a mapping needs to be synchronized later, dma_sync_*_for_*() must be
116 called using the DMA struct device, too.
117 So, normal setup should look like this:
121 nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
125 desc = dmaengine_prep_slave_sg(chan, sgl, nr_sg, direction, flags);
127 Once a descriptor has been obtained, the callback information can be
128 added and the descriptor must then be submitted. Some DMA engine
129 drivers may hold a spinlock between a successful preparation and
130 submission so it is important that these two operations are closely
135 Although the async_tx API specifies that completion callback
136 routines cannot submit any new operations, this is not the
137 case for slave/cyclic DMA.
139 For slave DMA, the subsequent transaction may not be available
140 for submission prior to callback function being invoked, so
141 slave DMA callbacks are permitted to prepare and submit a new
144 For cyclic DMA, a callback function may wish to terminate the
145 DMA via dmaengine_terminate_async().
147 Therefore, it is important that DMA engine drivers drop any
148 locks before calling the callback function which may cause a
151 Note that callbacks will always be invoked from the DMA
152 engines tasklet, never from interrupt context.
154 **Optional: per descriptor metadata**
156 DMAengine provides two ways for metadata support.
160 The metadata buffer is allocated/provided by the client driver and it is
161 attached to the descriptor.
165 int dmaengine_desc_attach_metadata(struct dma_async_tx_descriptor *desc,
166 void *data, size_t len);
170 The metadata buffer is allocated/managed by the DMA driver. The client
171 driver can ask for the pointer, maximum size and the currently used size of
172 the metadata and can directly update or read it.
174 Becasue the DMA driver manages the memory area containing the metadata,
175 clients must make sure that they do not try to access or get the pointer
176 after their transfer completion callback has run for the descriptor.
177 If no completion callback has been defined for the transfer, then the
178 metadata must not be accessed after issue_pending.
179 In other words: if the aim is to read back metadata after the transfer is
180 completed, then the client must use completion callback.
184 void *dmaengine_desc_get_metadata_ptr(struct dma_async_tx_descriptor *desc,
185 size_t *payload_len, size_t *max_len);
187 int dmaengine_desc_set_metadata_len(struct dma_async_tx_descriptor *desc,
190 Client drivers can query if a given mode is supported with:
194 bool dmaengine_is_metadata_mode_supported(struct dma_chan *chan,
195 enum dma_desc_metadata_mode mode);
197 Depending on the used mode client drivers must follow different flow.
201 - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM:
203 1. prepare the descriptor (dmaengine_prep_*)
204 construct the metadata in the client's buffer
205 2. use dmaengine_desc_attach_metadata() to attach the buffer to the
207 3. submit the transfer
211 1. prepare the descriptor (dmaengine_prep_*)
212 2. use dmaengine_desc_attach_metadata() to attach the buffer to the
214 3. submit the transfer
215 4. when the transfer is completed, the metadata should be available in the
220 - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM:
222 1. prepare the descriptor (dmaengine_prep_*)
223 2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the
224 engine's metadata area
225 3. update the metadata at the pointer
226 4. use dmaengine_desc_set_metadata_len() to tell the DMA engine the
227 amount of data the client has placed into the metadata buffer
228 5. submit the transfer
232 1. prepare the descriptor (dmaengine_prep_*)
233 2. submit the transfer
234 3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get
235 the pointer to the engine's metadata area
236 4. read out the metadata from the pointer
240 When DESC_METADATA_ENGINE mode is used the metadata area for the descriptor
241 is no longer valid after the transfer has been completed (valid up to the
242 point when the completion callback returns if used).
244 Mixed use of DESC_METADATA_CLIENT / DESC_METADATA_ENGINE is not allowed,
245 client drivers must use either of the modes per descriptor.
247 4. Submit the transaction
249 Once the descriptor has been prepared and the callback information
250 added, it must be placed on the DMA engine drivers pending queue.
256 dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
258 This returns a cookie can be used to check the progress of DMA engine
259 activity via other DMA engine calls not covered in this document.
261 dmaengine_submit() will not start the DMA operation, it merely adds
262 it to the pending queue. For this, see step 5, dma_async_issue_pending.
266 After calling ``dmaengine_submit()`` the submitted transfer descriptor
267 (``struct dma_async_tx_descriptor``) belongs to the DMA engine.
268 Consequently, the client must consider invalid the pointer to that
271 5. Issue pending DMA requests and wait for callback notification
273 The transactions in the pending queue can be activated by calling the
274 issue_pending API. If channel is idle then the first transaction in
275 queue is started and subsequent ones queued up.
277 On completion of each DMA operation, the next in queue is started and
278 a tasklet triggered. The tasklet will then call the client driver
279 completion callback routine for notification, if set.
285 void dma_async_issue_pending(struct dma_chan *chan);
294 int dmaengine_terminate_sync(struct dma_chan *chan)
295 int dmaengine_terminate_async(struct dma_chan *chan)
296 int dmaengine_terminate_all(struct dma_chan *chan) /* DEPRECATED */
298 This causes all activity for the DMA channel to be stopped, and may
299 discard data in the DMA FIFO which hasn't been fully transferred.
300 No callback functions will be called for any incomplete transfers.
302 Two variants of this function are available.
304 dmaengine_terminate_async() might not wait until the DMA has been fully
305 stopped or until any running complete callbacks have finished. But it is
306 possible to call dmaengine_terminate_async() from atomic context or from
307 within a complete callback. dmaengine_synchronize() must be called before it
308 is safe to free the memory accessed by the DMA transfer or free resources
309 accessed from within the complete callback.
311 dmaengine_terminate_sync() will wait for the transfer and any running
312 complete callbacks to finish before it returns. But the function must not be
313 called from atomic context or from within a complete callback.
315 dmaengine_terminate_all() is deprecated and should not be used in new code.
321 int dmaengine_pause(struct dma_chan *chan)
323 This pauses activity on the DMA channel without data loss.
329 int dmaengine_resume(struct dma_chan *chan)
331 Resume a previously paused DMA channel. It is invalid to resume a
332 channel which is not currently paused.
334 4. Check Txn complete
338 enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
339 dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
341 This can be used to check the status of the channel. Please see
342 the documentation in include/linux/dmaengine.h for a more complete
343 description of this API.
345 This can be used in conjunction with dma_async_is_complete() and
346 the cookie returned from dmaengine_submit() to check for
347 completion of a specific DMA transaction.
351 Not all DMA engine drivers can return reliable information for
352 a running DMA channel. It is recommended that DMA engine users
353 pause or stop (via dmaengine_terminate_all()) the channel before
356 5. Synchronize termination API
360 void dmaengine_synchronize(struct dma_chan *chan)
362 Synchronize the termination of the DMA channel to the current context.
364 This function should be used after dmaengine_terminate_async() to synchronize
365 the termination of the DMA channel to the current context. The function will
366 wait for the transfer and any running complete callbacks to finish before it
369 If dmaengine_terminate_async() is used to stop the DMA channel this function
370 must be called before it is safe to free memory accessed by previously
371 submitted descriptors or to free any resources accessed within the complete
372 callback of previously submitted descriptors.
374 The behavior of this function is undefined if dma_async_issue_pending() has
375 been called between dmaengine_terminate_async() and this function.