1==================== 2DMA Engine API Guide 3==================== 4 5Vinod Koul <vinod dot koul at intel.com> 6 7.. note:: For DMA Engine usage in async_tx please see: 8 ``Documentation/crypto/async-tx-api.txt`` 9 10 11Below is a guide to device driver writers on how to use the Slave-DMA API of the 12DMA Engine. This is applicable only for slave DMA usage only. 13 14DMA usage 15========= 16 17The slave DMA usage consists of following steps: 18 19- Allocate a DMA slave channel 20 21- Set slave and controller specific parameters 22 23- Get a descriptor for transaction 24 25- Submit the transaction 26 27- Issue pending requests and wait for callback notification 28 29The details of these operations are: 30 311. Allocate a DMA slave channel 32 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. 37 38 Interface: 39 40 .. code-block:: c 41 42 struct dma_chan *dma_request_chan(struct device *dev, const char *name); 43 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. 47 48 A channel allocated via this interface is exclusive to the caller, 49 until dma_release_channel() is called. 50 512. Set slave and controller specific parameters 52 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 57 for the peripheral. 58 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. 63 64 Interface: 65 66 .. code-block:: c 67 68 int dmaengine_slave_config(struct dma_chan *chan, 69 struct dma_slave_config *config) 70 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. 75 763. Get a descriptor for transaction 77 78 For slave usage the various modes of slave transfers supported by the 79 DMA-engine are: 80 81 - slave_sg: DMA a list of scatter gather buffers from/to a peripheral 82 83 - dma_cyclic: Perform a cyclic DMA operation from/to a peripheral till the 84 operation is explicitly stopped. 85 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. 90 91 A non-NULL return of this transfer API represents a "descriptor" for 92 the given transaction. 93 94 Interface: 95 96 .. code-block:: c 97 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); 102 103 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); 106 107 struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma( 108 struct dma_chan *chan, struct dma_interleaved_template *xt, 109 unsigned long flags); 110 111 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: 118 119 .. code-block:: c 120 121 nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len); 122 if (nr_sg == 0) 123 /* error */ 124 125 desc = dmaengine_prep_slave_sg(chan, sgl, nr_sg, direction, flags); 126 127 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 131 paired. 132 133 .. note:: 134 135 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. 138 139 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 142 transaction. 143 144 For cyclic DMA, a callback function may wish to terminate the 145 DMA via dmaengine_terminate_async(). 146 147 Therefore, it is important that DMA engine drivers drop any 148 locks before calling the callback function which may cause a 149 deadlock. 150 151 Note that callbacks will always be invoked from the DMA 152 engines tasklet, never from interrupt context. 153 154Optional: per descriptor metadata 155--------------------------------- 156 DMAengine provides two ways for metadata support. 157 158 DESC_METADATA_CLIENT 159 160 The metadata buffer is allocated/provided by the client driver and it is 161 attached to the descriptor. 162 163 .. code-block:: c 164 165 int dmaengine_desc_attach_metadata(struct dma_async_tx_descriptor *desc, 166 void *data, size_t len); 167 168 DESC_METADATA_ENGINE 169 170 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. 173 174 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. 181 182 .. code-block:: c 183 184 void *dmaengine_desc_get_metadata_ptr(struct dma_async_tx_descriptor *desc, 185 size_t *payload_len, size_t *max_len); 186 187 int dmaengine_desc_set_metadata_len(struct dma_async_tx_descriptor *desc, 188 size_t payload_len); 189 190 Client drivers can query if a given mode is supported with: 191 192 .. code-block:: c 193 194 bool dmaengine_is_metadata_mode_supported(struct dma_chan *chan, 195 enum dma_desc_metadata_mode mode); 196 197 Depending on the used mode client drivers must follow different flow. 198 199 DESC_METADATA_CLIENT 200 201 - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM: 202 1. prepare the descriptor (dmaengine_prep_*) 203 construct the metadata in the client's buffer 204 2. use dmaengine_desc_attach_metadata() to attach the buffer to the 205 descriptor 206 3. submit the transfer 207 - DMA_DEV_TO_MEM: 208 1. prepare the descriptor (dmaengine_prep_*) 209 2. use dmaengine_desc_attach_metadata() to attach the buffer to the 210 descriptor 211 3. submit the transfer 212 4. when the transfer is completed, the metadata should be available in the 213 attached buffer 214 215 DESC_METADATA_ENGINE 216 217 - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM: 218 1. prepare the descriptor (dmaengine_prep_*) 219 2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the 220 engine's metadata area 221 3. update the metadata at the pointer 222 4. use dmaengine_desc_set_metadata_len() to tell the DMA engine the 223 amount of data the client has placed into the metadata buffer 224 5. submit the transfer 225 - DMA_DEV_TO_MEM: 226 1. prepare the descriptor (dmaengine_prep_*) 227 2. submit the transfer 228 3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get 229 the pointer to the engine's metadata area 230 4. read out the metadata from the pointer 231 232 .. note:: 233 234 When DESC_METADATA_ENGINE mode is used the metadata area for the descriptor 235 is no longer valid after the transfer has been completed (valid up to the 236 point when the completion callback returns if used). 237 238 Mixed use of DESC_METADATA_CLIENT / DESC_METADATA_ENGINE is not allowed, 239 client drivers must use either of the modes per descriptor. 240 2414. Submit the transaction 242 243 Once the descriptor has been prepared and the callback information 244 added, it must be placed on the DMA engine drivers pending queue. 245 246 Interface: 247 248 .. code-block:: c 249 250 dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc) 251 252 This returns a cookie can be used to check the progress of DMA engine 253 activity via other DMA engine calls not covered in this document. 254 255 dmaengine_submit() will not start the DMA operation, it merely adds 256 it to the pending queue. For this, see step 5, dma_async_issue_pending. 257 258 .. note:: 259 260 After calling ``dmaengine_submit()`` the submitted transfer descriptor 261 (``struct dma_async_tx_descriptor``) belongs to the DMA engine. 262 Consequently, the client must consider invalid the pointer to that 263 descriptor. 264 2655. Issue pending DMA requests and wait for callback notification 266 267 The transactions in the pending queue can be activated by calling the 268 issue_pending API. If channel is idle then the first transaction in 269 queue is started and subsequent ones queued up. 270 271 On completion of each DMA operation, the next in queue is started and 272 a tasklet triggered. The tasklet will then call the client driver 273 completion callback routine for notification, if set. 274 275 Interface: 276 277 .. code-block:: c 278 279 void dma_async_issue_pending(struct dma_chan *chan); 280 281Further APIs: 282------------- 283 2841. Terminate APIs 285 286 .. code-block:: c 287 288 int dmaengine_terminate_sync(struct dma_chan *chan) 289 int dmaengine_terminate_async(struct dma_chan *chan) 290 int dmaengine_terminate_all(struct dma_chan *chan) /* DEPRECATED */ 291 292 This causes all activity for the DMA channel to be stopped, and may 293 discard data in the DMA FIFO which hasn't been fully transferred. 294 No callback functions will be called for any incomplete transfers. 295 296 Two variants of this function are available. 297 298 dmaengine_terminate_async() might not wait until the DMA has been fully 299 stopped or until any running complete callbacks have finished. But it is 300 possible to call dmaengine_terminate_async() from atomic context or from 301 within a complete callback. dmaengine_synchronize() must be called before it 302 is safe to free the memory accessed by the DMA transfer or free resources 303 accessed from within the complete callback. 304 305 dmaengine_terminate_sync() will wait for the transfer and any running 306 complete callbacks to finish before it returns. But the function must not be 307 called from atomic context or from within a complete callback. 308 309 dmaengine_terminate_all() is deprecated and should not be used in new code. 310 3112. Pause API 312 313 .. code-block:: c 314 315 int dmaengine_pause(struct dma_chan *chan) 316 317 This pauses activity on the DMA channel without data loss. 318 3193. Resume API 320 321 .. code-block:: c 322 323 int dmaengine_resume(struct dma_chan *chan) 324 325 Resume a previously paused DMA channel. It is invalid to resume a 326 channel which is not currently paused. 327 3284. Check Txn complete 329 330 .. code-block:: c 331 332 enum dma_status dma_async_is_tx_complete(struct dma_chan *chan, 333 dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used) 334 335 This can be used to check the status of the channel. Please see 336 the documentation in include/linux/dmaengine.h for a more complete 337 description of this API. 338 339 This can be used in conjunction with dma_async_is_complete() and 340 the cookie returned from dmaengine_submit() to check for 341 completion of a specific DMA transaction. 342 343 .. note:: 344 345 Not all DMA engine drivers can return reliable information for 346 a running DMA channel. It is recommended that DMA engine users 347 pause or stop (via dmaengine_terminate_all()) the channel before 348 using this API. 349 3505. Synchronize termination API 351 352 .. code-block:: c 353 354 void dmaengine_synchronize(struct dma_chan *chan) 355 356 Synchronize the termination of the DMA channel to the current context. 357 358 This function should be used after dmaengine_terminate_async() to synchronize 359 the termination of the DMA channel to the current context. The function will 360 wait for the transfer and any running complete callbacks to finish before it 361 returns. 362 363 If dmaengine_terminate_async() is used to stop the DMA channel this function 364 must be called before it is safe to free memory accessed by previously 365 submitted descriptors or to free any resources accessed within the complete 366 callback of previously submitted descriptors. 367 368 The behavior of this function is undefined if dma_async_issue_pending() has 369 been called between dmaengine_terminate_async() and this function. 370