Based on kernel version 3.15.4. Page generated on 2014-07-07 09:02 EST.
1 DMA Engine API Guide 2 ==================== 3 4 Vinod Koul <vinod dot koul at intel.com> 5 6 NOTE: For DMA Engine usage in async_tx please see: 7 Documentation/crypto/async-tx-api.txt 8 9 10 Below is a guide to device driver writers on how to use the Slave-DMA API of the 11 DMA Engine. This is applicable only for slave DMA usage only. 12 13 The slave DMA usage consists of following steps: 14 1. Allocate a DMA slave channel 15 2. Set slave and controller specific parameters 16 3. Get a descriptor for transaction 17 4. Submit the transaction 18 5. Issue pending requests and wait for callback notification 19 20 1. Allocate a DMA slave channel 21 22 Channel allocation is slightly different in the slave DMA context, 23 client drivers typically need a channel from a particular DMA 24 controller only and even in some cases a specific channel is desired. 25 To request a channel dma_request_channel() API is used. 26 27 Interface: 28 struct dma_chan *dma_request_channel(dma_cap_mask_t mask, 29 dma_filter_fn filter_fn, 30 void *filter_param); 31 where dma_filter_fn is defined as: 32 typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param); 33 34 The 'filter_fn' parameter is optional, but highly recommended for 35 slave and cyclic channels as they typically need to obtain a specific 36 DMA channel. 37 38 When the optional 'filter_fn' parameter is NULL, dma_request_channel() 39 simply returns the first channel that satisfies the capability mask. 40 41 Otherwise, the 'filter_fn' routine will be called once for each free 42 channel which has a capability in 'mask'. 'filter_fn' is expected to 43 return 'true' when the desired DMA channel is found. 44 45 A channel allocated via this interface is exclusive to the caller, 46 until dma_release_channel() is called. 47 48 2. Set slave and controller specific parameters 49 50 Next step is always to pass some specific information to the DMA 51 driver. Most of the generic information which a slave DMA can use 52 is in struct dma_slave_config. This allows the clients to specify 53 DMA direction, DMA addresses, bus widths, DMA burst lengths etc 54 for the peripheral. 55 56 If some DMA controllers have more parameters to be sent then they 57 should try to embed struct dma_slave_config in their controller 58 specific structure. That gives flexibility to client to pass more 59 parameters, if required. 60 61 Interface: 62 int dmaengine_slave_config(struct dma_chan *chan, 63 struct dma_slave_config *config) 64 65 Please see the dma_slave_config structure definition in dmaengine.h 66 for a detailed explanation of the struct members. Please note 67 that the 'direction' member will be going away as it duplicates the 68 direction given in the prepare call. 69 70 3. Get a descriptor for transaction 71 72 For slave usage the various modes of slave transfers supported by the 73 DMA-engine are: 74 75 slave_sg - DMA a list of scatter gather buffers from/to a peripheral 76 dma_cyclic - Perform a cyclic DMA operation from/to a peripheral till the 77 operation is explicitly stopped. 78 interleaved_dma - This is common to Slave as well as M2M clients. For slave 79 address of devices' fifo could be already known to the driver. 80 Various types of operations could be expressed by setting 81 appropriate values to the 'dma_interleaved_template' members. 82 83 A non-NULL return of this transfer API represents a "descriptor" for 84 the given transaction. 85 86 Interface: 87 struct dma_async_tx_descriptor *(*chan->device->device_prep_slave_sg)( 88 struct dma_chan *chan, struct scatterlist *sgl, 89 unsigned int sg_len, enum dma_data_direction direction, 90 unsigned long flags); 91 92 struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_cyclic)( 93 struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len, 94 size_t period_len, enum dma_data_direction direction); 95 96 struct dma_async_tx_descriptor *(*device_prep_interleaved_dma)( 97 struct dma_chan *chan, struct dma_interleaved_template *xt, 98 unsigned long flags); 99 100 The peripheral driver is expected to have mapped the scatterlist for 101 the DMA operation prior to calling device_prep_slave_sg, and must 102 keep the scatterlist mapped until the DMA operation has completed. 103 The scatterlist must be mapped using the DMA struct device. So, 104 normal setup should look like this: 105 106 nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len); 107 if (nr_sg == 0) 108 /* error */ 109 110 desc = chan->device->device_prep_slave_sg(chan, sgl, nr_sg, 111 direction, flags); 112 113 Once a descriptor has been obtained, the callback information can be 114 added and the descriptor must then be submitted. Some DMA engine 115 drivers may hold a spinlock between a successful preparation and 116 submission so it is important that these two operations are closely 117 paired. 118 119 Note: 120 Although the async_tx API specifies that completion callback 121 routines cannot submit any new operations, this is not the 122 case for slave/cyclic DMA. 123 124 For slave DMA, the subsequent transaction may not be available 125 for submission prior to callback function being invoked, so 126 slave DMA callbacks are permitted to prepare and submit a new 127 transaction. 128 129 For cyclic DMA, a callback function may wish to terminate the 130 DMA via dmaengine_terminate_all(). 131 132 Therefore, it is important that DMA engine drivers drop any 133 locks before calling the callback function which may cause a 134 deadlock. 135 136 Note that callbacks will always be invoked from the DMA 137 engines tasklet, never from interrupt context. 138 139 4. Submit the transaction 140 141 Once the descriptor has been prepared and the callback information 142 added, it must be placed on the DMA engine drivers pending queue. 143 144 Interface: 145 dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc) 146 147 This returns a cookie can be used to check the progress of DMA engine 148 activity via other DMA engine calls not covered in this document. 149 150 dmaengine_submit() will not start the DMA operation, it merely adds 151 it to the pending queue. For this, see step 5, dma_async_issue_pending. 152 153 5. Issue pending DMA requests and wait for callback notification 154 155 The transactions in the pending queue can be activated by calling the 156 issue_pending API. If channel is idle then the first transaction in 157 queue is started and subsequent ones queued up. 158 159 On completion of each DMA operation, the next in queue is started and 160 a tasklet triggered. The tasklet will then call the client driver 161 completion callback routine for notification, if set. 162 163 Interface: 164 void dma_async_issue_pending(struct dma_chan *chan); 165 166 Further APIs: 167 168 1. int dmaengine_terminate_all(struct dma_chan *chan) 169 170 This causes all activity for the DMA channel to be stopped, and may 171 discard data in the DMA FIFO which hasn't been fully transferred. 172 No callback functions will be called for any incomplete transfers. 173 174 2. int dmaengine_pause(struct dma_chan *chan) 175 176 This pauses activity on the DMA channel without data loss. 177 178 3. int dmaengine_resume(struct dma_chan *chan) 179 180 Resume a previously paused DMA channel. It is invalid to resume a 181 channel which is not currently paused. 182 183 4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan, 184 dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used) 185 186 This can be used to check the status of the channel. Please see 187 the documentation in include/linux/dmaengine.h for a more complete 188 description of this API. 189 190 This can be used in conjunction with dma_async_is_complete() and 191 the cookie returned from 'descriptor->submit()' to check for 192 completion of a specific DMA transaction. 193 194 Note: 195 Not all DMA engine drivers can return reliable information for 196 a running DMA channel. It is recommended that DMA engine users 197 pause or stop (via dmaengine_terminate_all) the channel before 198 using this API.