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 1544. Submit the transaction 155 156 Once the descriptor has been prepared and the callback information 157 added, it must be placed on the DMA engine drivers pending queue. 158 159 Interface: 160 161 .. code-block:: c 162 163 dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc) 164 165 This returns a cookie can be used to check the progress of DMA engine 166 activity via other DMA engine calls not covered in this document. 167 168 dmaengine_submit() will not start the DMA operation, it merely adds 169 it to the pending queue. For this, see step 5, dma_async_issue_pending. 170 1715. Issue pending DMA requests and wait for callback notification 172 173 The transactions in the pending queue can be activated by calling the 174 issue_pending API. If channel is idle then the first transaction in 175 queue is started and subsequent ones queued up. 176 177 On completion of each DMA operation, the next in queue is started and 178 a tasklet triggered. The tasklet will then call the client driver 179 completion callback routine for notification, if set. 180 181 Interface: 182 183 .. code-block:: c 184 185 void dma_async_issue_pending(struct dma_chan *chan); 186 187Further APIs: 188------------- 189 1901. Terminate APIs 191 192 .. code-block:: c 193 194 int dmaengine_terminate_sync(struct dma_chan *chan) 195 int dmaengine_terminate_async(struct dma_chan *chan) 196 int dmaengine_terminate_all(struct dma_chan *chan) /* DEPRECATED */ 197 198 This causes all activity for the DMA channel to be stopped, and may 199 discard data in the DMA FIFO which hasn't been fully transferred. 200 No callback functions will be called for any incomplete transfers. 201 202 Two variants of this function are available. 203 204 dmaengine_terminate_async() might not wait until the DMA has been fully 205 stopped or until any running complete callbacks have finished. But it is 206 possible to call dmaengine_terminate_async() from atomic context or from 207 within a complete callback. dmaengine_synchronize() must be called before it 208 is safe to free the memory accessed by the DMA transfer or free resources 209 accessed from within the complete callback. 210 211 dmaengine_terminate_sync() will wait for the transfer and any running 212 complete callbacks to finish before it returns. But the function must not be 213 called from atomic context or from within a complete callback. 214 215 dmaengine_terminate_all() is deprecated and should not be used in new code. 216 2172. Pause API 218 219 .. code-block:: c 220 221 int dmaengine_pause(struct dma_chan *chan) 222 223 This pauses activity on the DMA channel without data loss. 224 2253. Resume API 226 227 .. code-block:: c 228 229 int dmaengine_resume(struct dma_chan *chan) 230 231 Resume a previously paused DMA channel. It is invalid to resume a 232 channel which is not currently paused. 233 2344. Check Txn complete 235 236 .. code-block:: c 237 238 enum dma_status dma_async_is_tx_complete(struct dma_chan *chan, 239 dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used) 240 241 This can be used to check the status of the channel. Please see 242 the documentation in include/linux/dmaengine.h for a more complete 243 description of this API. 244 245 This can be used in conjunction with dma_async_is_complete() and 246 the cookie returned from dmaengine_submit() to check for 247 completion of a specific DMA transaction. 248 249 .. note:: 250 251 Not all DMA engine drivers can return reliable information for 252 a running DMA channel. It is recommended that DMA engine users 253 pause or stop (via dmaengine_terminate_all()) the channel before 254 using this API. 255 2565. Synchronize termination API 257 258 .. code-block:: c 259 260 void dmaengine_synchronize(struct dma_chan *chan) 261 262 Synchronize the termination of the DMA channel to the current context. 263 264 This function should be used after dmaengine_terminate_async() to synchronize 265 the termination of the DMA channel to the current context. The function will 266 wait for the transfer and any running complete callbacks to finish before it 267 returns. 268 269 If dmaengine_terminate_async() is used to stop the DMA channel this function 270 must be called before it is safe to free memory accessed by previously 271 submitted descriptors or to free any resources accessed within the complete 272 callback of previously submitted descriptors. 273 274 The behavior of this function is undefined if dma_async_issue_pending() has 275 been called between dmaengine_terminate_async() and this function. 276