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 nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len); 124 if (nr_sg == 0) 125 /* error */ 126 127 desc = dmaengine_prep_slave_sg(chan, sgl, nr_sg, direction, flags); 128 129 Once a descriptor has been obtained, the callback information can be 130 added and the descriptor must then be submitted. Some DMA engine 131 drivers may hold a spinlock between a successful preparation and 132 submission so it is important that these two operations are closely 133 paired. 134 135 .. note:: 136 137 Although the async_tx API specifies that completion callback 138 routines cannot submit any new operations, this is not the 139 case for slave/cyclic DMA. 140 141 For slave DMA, the subsequent transaction may not be available 142 for submission prior to callback function being invoked, so 143 slave DMA callbacks are permitted to prepare and submit a new 144 transaction. 145 146 For cyclic DMA, a callback function may wish to terminate the 147 DMA via dmaengine_terminate_async(). 148 149 Therefore, it is important that DMA engine drivers drop any 150 locks before calling the callback function which may cause a 151 deadlock. 152 153 Note that callbacks will always be invoked from the DMA 154 engines tasklet, never from interrupt context. 155 156 **Optional: per descriptor metadata** 157 158 DMAengine provides two ways for metadata support. 159 160 DESC_METADATA_CLIENT 161 162 The metadata buffer is allocated/provided by the client driver and it is 163 attached to the descriptor. 164 165 .. code-block:: c 166 167 int dmaengine_desc_attach_metadata(struct dma_async_tx_descriptor *desc, 168 void *data, size_t len); 169 170 DESC_METADATA_ENGINE 171 172 The metadata buffer is allocated/managed by the DMA driver. The client 173 driver can ask for the pointer, maximum size and the currently used size of 174 the metadata and can directly update or read it. 175 176 Becasue the DMA driver manages the memory area containing the metadata, 177 clients must make sure that they do not try to access or get the pointer 178 after their transfer completion callback has run for the descriptor. 179 If no completion callback has been defined for the transfer, then the 180 metadata must not be accessed after issue_pending. 181 In other words: if the aim is to read back metadata after the transfer is 182 completed, then the client must use completion callback. 183 184 .. code-block:: c 185 186 void *dmaengine_desc_get_metadata_ptr(struct dma_async_tx_descriptor *desc, 187 size_t *payload_len, size_t *max_len); 188 189 int dmaengine_desc_set_metadata_len(struct dma_async_tx_descriptor *desc, 190 size_t payload_len); 191 192 Client drivers can query if a given mode is supported with: 193 194 .. code-block:: c 195 196 bool dmaengine_is_metadata_mode_supported(struct dma_chan *chan, 197 enum dma_desc_metadata_mode mode); 198 199 Depending on the used mode client drivers must follow different flow. 200 201 DESC_METADATA_CLIENT 202 203 - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM: 204 205 1. prepare the descriptor (dmaengine_prep_*) 206 construct the metadata in the client's buffer 207 2. use dmaengine_desc_attach_metadata() to attach the buffer to the 208 descriptor 209 3. submit the transfer 210 211 - DMA_DEV_TO_MEM: 212 213 1. prepare the descriptor (dmaengine_prep_*) 214 2. use dmaengine_desc_attach_metadata() to attach the buffer to the 215 descriptor 216 3. submit the transfer 217 4. when the transfer is completed, the metadata should be available in the 218 attached buffer 219 220 DESC_METADATA_ENGINE 221 222 - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM: 223 224 1. prepare the descriptor (dmaengine_prep_*) 225 2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the 226 engine's metadata area 227 3. update the metadata at the pointer 228 4. use dmaengine_desc_set_metadata_len() to tell the DMA engine the 229 amount of data the client has placed into the metadata buffer 230 5. submit the transfer 231 232 - DMA_DEV_TO_MEM: 233 234 1. prepare the descriptor (dmaengine_prep_*) 235 2. submit the transfer 236 3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get 237 the pointer to the engine's metadata area 238 4. read out the metadata from the pointer 239 240 .. note:: 241 242 When DESC_METADATA_ENGINE mode is used the metadata area for the descriptor 243 is no longer valid after the transfer has been completed (valid up to the 244 point when the completion callback returns if used). 245 246 Mixed use of DESC_METADATA_CLIENT / DESC_METADATA_ENGINE is not allowed, 247 client drivers must use either of the modes per descriptor. 248 2494. Submit the transaction 250 251 Once the descriptor has been prepared and the callback information 252 added, it must be placed on the DMA engine drivers pending queue. 253 254 Interface: 255 256 .. code-block:: c 257 258 dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc) 259 260 This returns a cookie can be used to check the progress of DMA engine 261 activity via other DMA engine calls not covered in this document. 262 263 dmaengine_submit() will not start the DMA operation, it merely adds 264 it to the pending queue. For this, see step 5, dma_async_issue_pending. 265 266 .. note:: 267 268 After calling ``dmaengine_submit()`` the submitted transfer descriptor 269 (``struct dma_async_tx_descriptor``) belongs to the DMA engine. 270 Consequently, the client must consider invalid the pointer to that 271 descriptor. 272 2735. Issue pending DMA requests and wait for callback notification 274 275 The transactions in the pending queue can be activated by calling the 276 issue_pending API. If channel is idle then the first transaction in 277 queue is started and subsequent ones queued up. 278 279 On completion of each DMA operation, the next in queue is started and 280 a tasklet triggered. The tasklet will then call the client driver 281 completion callback routine for notification, if set. 282 283 Interface: 284 285 .. code-block:: c 286 287 void dma_async_issue_pending(struct dma_chan *chan); 288 289Further APIs 290------------ 291 2921. Terminate APIs 293 294 .. code-block:: c 295 296 int dmaengine_terminate_sync(struct dma_chan *chan) 297 int dmaengine_terminate_async(struct dma_chan *chan) 298 int dmaengine_terminate_all(struct dma_chan *chan) /* DEPRECATED */ 299 300 This causes all activity for the DMA channel to be stopped, and may 301 discard data in the DMA FIFO which hasn't been fully transferred. 302 No callback functions will be called for any incomplete transfers. 303 304 Two variants of this function are available. 305 306 dmaengine_terminate_async() might not wait until the DMA has been fully 307 stopped or until any running complete callbacks have finished. But it is 308 possible to call dmaengine_terminate_async() from atomic context or from 309 within a complete callback. dmaengine_synchronize() must be called before it 310 is safe to free the memory accessed by the DMA transfer or free resources 311 accessed from within the complete callback. 312 313 dmaengine_terminate_sync() will wait for the transfer and any running 314 complete callbacks to finish before it returns. But the function must not be 315 called from atomic context or from within a complete callback. 316 317 dmaengine_terminate_all() is deprecated and should not be used in new code. 318 3192. Pause API 320 321 .. code-block:: c 322 323 int dmaengine_pause(struct dma_chan *chan) 324 325 This pauses activity on the DMA channel without data loss. 326 3273. Resume API 328 329 .. code-block:: c 330 331 int dmaengine_resume(struct dma_chan *chan) 332 333 Resume a previously paused DMA channel. It is invalid to resume a 334 channel which is not currently paused. 335 3364. Check Txn complete 337 338 .. code-block:: c 339 340 enum dma_status dma_async_is_tx_complete(struct dma_chan *chan, 341 dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used) 342 343 This can be used to check the status of the channel. Please see 344 the documentation in include/linux/dmaengine.h for a more complete 345 description of this API. 346 347 This can be used in conjunction with dma_async_is_complete() and 348 the cookie returned from dmaengine_submit() to check for 349 completion of a specific DMA transaction. 350 351 .. note:: 352 353 Not all DMA engine drivers can return reliable information for 354 a running DMA channel. It is recommended that DMA engine users 355 pause or stop (via dmaengine_terminate_all()) the channel before 356 using this API. 357 3585. Synchronize termination API 359 360 .. code-block:: c 361 362 void dmaengine_synchronize(struct dma_chan *chan) 363 364 Synchronize the termination of the DMA channel to the current context. 365 366 This function should be used after dmaengine_terminate_async() to synchronize 367 the termination of the DMA channel to the current context. The function will 368 wait for the transfer and any running complete callbacks to finish before it 369 returns. 370 371 If dmaengine_terminate_async() is used to stop the DMA channel this function 372 must be called before it is safe to free memory accessed by previously 373 submitted descriptors or to free any resources accessed within the complete 374 callback of previously submitted descriptors. 375 376 The behavior of this function is undefined if dma_async_issue_pending() has 377 been called between dmaengine_terminate_async() and this function. 378