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.rst`` 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. Cyclic 90 interleaved DMA transfers are also possible if supported by the channel by 91 setting the DMA_PREP_REPEAT transfer flag. 92 93 A non-NULL return of this transfer API represents a "descriptor" for 94 the given transaction. 95 96 Interface: 97 98 .. code-block:: c 99 100 struct dma_async_tx_descriptor *dmaengine_prep_slave_sg( 101 struct dma_chan *chan, struct scatterlist *sgl, 102 unsigned int sg_len, enum dma_data_direction direction, 103 unsigned long flags); 104 105 struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic( 106 struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len, 107 size_t period_len, enum dma_data_direction direction); 108 109 struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma( 110 struct dma_chan *chan, struct dma_interleaved_template *xt, 111 unsigned long flags); 112 113 The peripheral driver is expected to have mapped the scatterlist for 114 the DMA operation prior to calling dmaengine_prep_slave_sg(), and must 115 keep the scatterlist mapped until the DMA operation has completed. 116 The scatterlist must be mapped using the DMA struct device. 117 If a mapping needs to be synchronized later, dma_sync_*_for_*() must be 118 called using the DMA struct device, too. 119 So, normal setup should look like this: 120 121 .. code-block:: c 122 123 struct device *dma_dev = dmaengine_get_dma_device(chan); 124 125 nr_sg = dma_map_sg(dma_dev, sgl, sg_len); 126 if (nr_sg == 0) 127 /* error */ 128 129 desc = dmaengine_prep_slave_sg(chan, sgl, nr_sg, direction, flags); 130 131 Once a descriptor has been obtained, the callback information can be 132 added and the descriptor must then be submitted. Some DMA engine 133 drivers may hold a spinlock between a successful preparation and 134 submission so it is important that these two operations are closely 135 paired. 136 137 .. note:: 138 139 Although the async_tx API specifies that completion callback 140 routines cannot submit any new operations, this is not the 141 case for slave/cyclic DMA. 142 143 For slave DMA, the subsequent transaction may not be available 144 for submission prior to callback function being invoked, so 145 slave DMA callbacks are permitted to prepare and submit a new 146 transaction. 147 148 For cyclic DMA, a callback function may wish to terminate the 149 DMA via dmaengine_terminate_async(). 150 151 Therefore, it is important that DMA engine drivers drop any 152 locks before calling the callback function which may cause a 153 deadlock. 154 155 Note that callbacks will always be invoked from the DMA 156 engines tasklet, never from interrupt context. 157 158 **Optional: per descriptor metadata** 159 160 DMAengine provides two ways for metadata support. 161 162 DESC_METADATA_CLIENT 163 164 The metadata buffer is allocated/provided by the client driver and it is 165 attached to the descriptor. 166 167 .. code-block:: c 168 169 int dmaengine_desc_attach_metadata(struct dma_async_tx_descriptor *desc, 170 void *data, size_t len); 171 172 DESC_METADATA_ENGINE 173 174 The metadata buffer is allocated/managed by the DMA driver. The client 175 driver can ask for the pointer, maximum size and the currently used size of 176 the metadata and can directly update or read it. 177 178 Because the DMA driver manages the memory area containing the metadata, 179 clients must make sure that they do not try to access or get the pointer 180 after their transfer completion callback has run for the descriptor. 181 If no completion callback has been defined for the transfer, then the 182 metadata must not be accessed after issue_pending. 183 In other words: if the aim is to read back metadata after the transfer is 184 completed, then the client must use completion callback. 185 186 .. code-block:: c 187 188 void *dmaengine_desc_get_metadata_ptr(struct dma_async_tx_descriptor *desc, 189 size_t *payload_len, size_t *max_len); 190 191 int dmaengine_desc_set_metadata_len(struct dma_async_tx_descriptor *desc, 192 size_t payload_len); 193 194 Client drivers can query if a given mode is supported with: 195 196 .. code-block:: c 197 198 bool dmaengine_is_metadata_mode_supported(struct dma_chan *chan, 199 enum dma_desc_metadata_mode mode); 200 201 Depending on the used mode client drivers must follow different flow. 202 203 DESC_METADATA_CLIENT 204 205 - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM: 206 207 1. prepare the descriptor (dmaengine_prep_*) 208 construct the metadata in the client's buffer 209 2. use dmaengine_desc_attach_metadata() to attach the buffer to the 210 descriptor 211 3. submit the transfer 212 213 - DMA_DEV_TO_MEM: 214 215 1. prepare the descriptor (dmaengine_prep_*) 216 2. use dmaengine_desc_attach_metadata() to attach the buffer to the 217 descriptor 218 3. submit the transfer 219 4. when the transfer is completed, the metadata should be available in the 220 attached buffer 221 222 DESC_METADATA_ENGINE 223 224 - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM: 225 226 1. prepare the descriptor (dmaengine_prep_*) 227 2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the 228 engine's metadata area 229 3. update the metadata at the pointer 230 4. use dmaengine_desc_set_metadata_len() to tell the DMA engine the 231 amount of data the client has placed into the metadata buffer 232 5. submit the transfer 233 234 - DMA_DEV_TO_MEM: 235 236 1. prepare the descriptor (dmaengine_prep_*) 237 2. submit the transfer 238 3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get 239 the pointer to the engine's metadata area 240 4. read out the metadata from the pointer 241 242 .. note:: 243 244 When DESC_METADATA_ENGINE mode is used the metadata area for the descriptor 245 is no longer valid after the transfer has been completed (valid up to the 246 point when the completion callback returns if used). 247 248 Mixed use of DESC_METADATA_CLIENT / DESC_METADATA_ENGINE is not allowed, 249 client drivers must use either of the modes per descriptor. 250 2514. Submit the transaction 252 253 Once the descriptor has been prepared and the callback information 254 added, it must be placed on the DMA engine drivers pending queue. 255 256 Interface: 257 258 .. code-block:: c 259 260 dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc) 261 262 This returns a cookie can be used to check the progress of DMA engine 263 activity via other DMA engine calls not covered in this document. 264 265 dmaengine_submit() will not start the DMA operation, it merely adds 266 it to the pending queue. For this, see step 5, dma_async_issue_pending. 267 268 .. note:: 269 270 After calling ``dmaengine_submit()`` the submitted transfer descriptor 271 (``struct dma_async_tx_descriptor``) belongs to the DMA engine. 272 Consequently, the client must consider invalid the pointer to that 273 descriptor. 274 2755. Issue pending DMA requests and wait for callback notification 276 277 The transactions in the pending queue can be activated by calling the 278 issue_pending API. If channel is idle then the first transaction in 279 queue is started and subsequent ones queued up. 280 281 On completion of each DMA operation, the next in queue is started and 282 a tasklet triggered. The tasklet will then call the client driver 283 completion callback routine for notification, if set. 284 285 Interface: 286 287 .. code-block:: c 288 289 void dma_async_issue_pending(struct dma_chan *chan); 290 291Further APIs 292------------ 293 2941. Terminate APIs 295 296 .. code-block:: c 297 298 int dmaengine_terminate_sync(struct dma_chan *chan) 299 int dmaengine_terminate_async(struct dma_chan *chan) 300 int dmaengine_terminate_all(struct dma_chan *chan) /* DEPRECATED */ 301 302 This causes all activity for the DMA channel to be stopped, and may 303 discard data in the DMA FIFO which hasn't been fully transferred. 304 No callback functions will be called for any incomplete transfers. 305 306 Two variants of this function are available. 307 308 dmaengine_terminate_async() might not wait until the DMA has been fully 309 stopped or until any running complete callbacks have finished. But it is 310 possible to call dmaengine_terminate_async() from atomic context or from 311 within a complete callback. dmaengine_synchronize() must be called before it 312 is safe to free the memory accessed by the DMA transfer or free resources 313 accessed from within the complete callback. 314 315 dmaengine_terminate_sync() will wait for the transfer and any running 316 complete callbacks to finish before it returns. But the function must not be 317 called from atomic context or from within a complete callback. 318 319 dmaengine_terminate_all() is deprecated and should not be used in new code. 320 3212. Pause API 322 323 .. code-block:: c 324 325 int dmaengine_pause(struct dma_chan *chan) 326 327 This pauses activity on the DMA channel without data loss. 328 3293. Resume API 330 331 .. code-block:: c 332 333 int dmaengine_resume(struct dma_chan *chan) 334 335 Resume a previously paused DMA channel. It is invalid to resume a 336 channel which is not currently paused. 337 3384. Check Txn complete 339 340 .. code-block:: c 341 342 enum dma_status dma_async_is_tx_complete(struct dma_chan *chan, 343 dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used) 344 345 This can be used to check the status of the channel. Please see 346 the documentation in include/linux/dmaengine.h for a more complete 347 description of this API. 348 349 This can be used in conjunction with dma_async_is_complete() and 350 the cookie returned from dmaengine_submit() to check for 351 completion of a specific DMA transaction. 352 353 .. note:: 354 355 Not all DMA engine drivers can return reliable information for 356 a running DMA channel. It is recommended that DMA engine users 357 pause or stop (via dmaengine_terminate_all()) the channel before 358 using this API. 359 3605. Synchronize termination API 361 362 .. code-block:: c 363 364 void dmaengine_synchronize(struct dma_chan *chan) 365 366 Synchronize the termination of the DMA channel to the current context. 367 368 This function should be used after dmaengine_terminate_async() to synchronize 369 the termination of the DMA channel to the current context. The function will 370 wait for the transfer and any running complete callbacks to finish before it 371 returns. 372 373 If dmaengine_terminate_async() is used to stop the DMA channel this function 374 must be called before it is safe to free memory accessed by previously 375 submitted descriptors or to free any resources accessed within the complete 376 callback of previously submitted descriptors. 377 378 The behavior of this function is undefined if dma_async_issue_pending() has 379 been called between dmaengine_terminate_async() and this function. 380