1.. SPDX-License-Identifier: GPL-2.0 2 3===================================== 4Asynchronous Transfers/Transforms API 5===================================== 6 7.. Contents 8 9 1. INTRODUCTION 10 11 2 GENEALOGY 12 13 3 USAGE 14 3.1 General format of the API 15 3.2 Supported operations 16 3.3 Descriptor management 17 3.4 When does the operation execute? 18 3.5 When does the operation complete? 19 3.6 Constraints 20 3.7 Example 21 22 4 DMAENGINE DRIVER DEVELOPER NOTES 23 4.1 Conformance points 24 4.2 "My application needs exclusive control of hardware channels" 25 26 5 SOURCE 27 281. Introduction 29=============== 30 31The async_tx API provides methods for describing a chain of asynchronous 32bulk memory transfers/transforms with support for inter-transactional 33dependencies. It is implemented as a dmaengine client that smooths over 34the details of different hardware offload engine implementations. Code 35that is written to the API can optimize for asynchronous operation and 36the API will fit the chain of operations to the available offload 37resources. 38 392.Genealogy 40=========== 41 42The API was initially designed to offload the memory copy and 43xor-parity-calculations of the md-raid5 driver using the offload engines 44present in the Intel(R) Xscale series of I/O processors. It also built 45on the 'dmaengine' layer developed for offloading memory copies in the 46network stack using Intel(R) I/OAT engines. The following design 47features surfaced as a result: 48 491. implicit synchronous path: users of the API do not need to know if 50 the platform they are running on has offload capabilities. The 51 operation will be offloaded when an engine is available and carried out 52 in software otherwise. 532. cross channel dependency chains: the API allows a chain of dependent 54 operations to be submitted, like xor->copy->xor in the raid5 case. The 55 API automatically handles cases where the transition from one operation 56 to another implies a hardware channel switch. 573. dmaengine extensions to support multiple clients and operation types 58 beyond 'memcpy' 59 603. Usage 61======== 62 633.1 General format of the API 64----------------------------- 65 66:: 67 68 struct dma_async_tx_descriptor * 69 async_<operation>(<op specific parameters>, struct async_submit ctl *submit) 70 713.2 Supported operations 72------------------------ 73 74======== ==================================================================== 75memcpy memory copy between a source and a destination buffer 76memset fill a destination buffer with a byte value 77xor xor a series of source buffers and write the result to a 78 destination buffer 79xor_val xor a series of source buffers and set a flag if the 80 result is zero. The implementation attempts to prevent 81 writes to memory 82pq generate the p+q (raid6 syndrome) from a series of source buffers 83pq_val validate that a p and or q buffer are in sync with a given series of 84 sources 85datap (raid6_datap_recov) recover a raid6 data block and the p block 86 from the given sources 872data (raid6_2data_recov) recover 2 raid6 data blocks from the given 88 sources 89======== ==================================================================== 90 913.3 Descriptor management 92------------------------- 93 94The return value is non-NULL and points to a 'descriptor' when the operation 95has been queued to execute asynchronously. Descriptors are recycled 96resources, under control of the offload engine driver, to be reused as 97operations complete. When an application needs to submit a chain of 98operations it must guarantee that the descriptor is not automatically recycled 99before the dependency is submitted. This requires that all descriptors be 100acknowledged by the application before the offload engine driver is allowed to 101recycle (or free) the descriptor. A descriptor can be acked by one of the 102following methods: 103 1041. setting the ASYNC_TX_ACK flag if no child operations are to be submitted 1052. submitting an unacknowledged descriptor as a dependency to another 106 async_tx call will implicitly set the acknowledged state. 1073. calling async_tx_ack() on the descriptor. 108 1093.4 When does the operation execute? 110------------------------------------ 111 112Operations do not immediately issue after return from the 113async_<operation> call. Offload engine drivers batch operations to 114improve performance by reducing the number of mmio cycles needed to 115manage the channel. Once a driver-specific threshold is met the driver 116automatically issues pending operations. An application can force this 117event by calling async_tx_issue_pending_all(). This operates on all 118channels since the application has no knowledge of channel to operation 119mapping. 120 1213.5 When does the operation complete? 122------------------------------------- 123 124There are two methods for an application to learn about the completion 125of an operation. 126 1271. Call dma_wait_for_async_tx(). This call causes the CPU to spin while 128 it polls for the completion of the operation. It handles dependency 129 chains and issuing pending operations. 1302. Specify a completion callback. The callback routine runs in tasklet 131 context if the offload engine driver supports interrupts, or it is 132 called in application context if the operation is carried out 133 synchronously in software. The callback can be set in the call to 134 async_<operation>, or when the application needs to submit a chain of 135 unknown length it can use the async_trigger_callback() routine to set a 136 completion interrupt/callback at the end of the chain. 137 1383.6 Constraints 139--------------- 140 1411. Calls to async_<operation> are not permitted in IRQ context. Other 142 contexts are permitted provided constraint #2 is not violated. 1432. Completion callback routines cannot submit new operations. This 144 results in recursion in the synchronous case and spin_locks being 145 acquired twice in the asynchronous case. 146 1473.7 Example 148----------- 149 150Perform a xor->copy->xor operation where each operation depends on the 151result from the previous operation:: 152 153 void callback(void *param) 154 { 155 struct completion *cmp = param; 156 157 complete(cmp); 158 } 159 160 void run_xor_copy_xor(struct page **xor_srcs, 161 int xor_src_cnt, 162 struct page *xor_dest, 163 size_t xor_len, 164 struct page *copy_src, 165 struct page *copy_dest, 166 size_t copy_len) 167 { 168 struct dma_async_tx_descriptor *tx; 169 addr_conv_t addr_conv[xor_src_cnt]; 170 struct async_submit_ctl submit; 171 addr_conv_t addr_conv[NDISKS]; 172 struct completion cmp; 173 174 init_async_submit(&submit, ASYNC_TX_XOR_DROP_DST, NULL, NULL, NULL, 175 addr_conv); 176 tx = async_xor(xor_dest, xor_srcs, 0, xor_src_cnt, xor_len, &submit) 177 178 submit->depend_tx = tx; 179 tx = async_memcpy(copy_dest, copy_src, 0, 0, copy_len, &submit); 180 181 init_completion(&cmp); 182 init_async_submit(&submit, ASYNC_TX_XOR_DROP_DST | ASYNC_TX_ACK, tx, 183 callback, &cmp, addr_conv); 184 tx = async_xor(xor_dest, xor_srcs, 0, xor_src_cnt, xor_len, &submit); 185 186 async_tx_issue_pending_all(); 187 188 wait_for_completion(&cmp); 189 } 190 191See include/linux/async_tx.h for more information on the flags. See the 192ops_run_* and ops_complete_* routines in drivers/md/raid5.c for more 193implementation examples. 194 1954. Driver Development Notes 196=========================== 197 1984.1 Conformance points 199---------------------- 200 201There are a few conformance points required in dmaengine drivers to 202accommodate assumptions made by applications using the async_tx API: 203 2041. Completion callbacks are expected to happen in tasklet context 2052. dma_async_tx_descriptor fields are never manipulated in IRQ context 2063. Use async_tx_run_dependencies() in the descriptor clean up path to 207 handle submission of dependent operations 208 2094.2 "My application needs exclusive control of hardware channels" 210----------------------------------------------------------------- 211 212Primarily this requirement arises from cases where a DMA engine driver 213is being used to support device-to-memory operations. A channel that is 214performing these operations cannot, for many platform specific reasons, 215be shared. For these cases the dma_request_channel() interface is 216provided. 217 218The interface is:: 219 220 struct dma_chan *dma_request_channel(dma_cap_mask_t mask, 221 dma_filter_fn filter_fn, 222 void *filter_param); 223 224Where dma_filter_fn is defined as:: 225 226 typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param); 227 228When the optional 'filter_fn' parameter is set to NULL 229dma_request_channel simply returns the first channel that satisfies the 230capability mask. Otherwise, when the mask parameter is insufficient for 231specifying the necessary channel, the filter_fn routine can be used to 232disposition the available channels in the system. The filter_fn routine 233is called once for each free channel in the system. Upon seeing a 234suitable channel filter_fn returns DMA_ACK which flags that channel to 235be the return value from dma_request_channel. A channel allocated via 236this interface is exclusive to the caller, until dma_release_channel() 237is called. 238 239The DMA_PRIVATE capability flag is used to tag dma devices that should 240not be used by the general-purpose allocator. It can be set at 241initialization time if it is known that a channel will always be 242private. Alternatively, it is set when dma_request_channel() finds an 243unused "public" channel. 244 245A couple caveats to note when implementing a driver and consumer: 246 2471. Once a channel has been privately allocated it will no longer be 248 considered by the general-purpose allocator even after a call to 249 dma_release_channel(). 2502. Since capabilities are specified at the device level a dma_device 251 with multiple channels will either have all channels public, or all 252 channels private. 253 2545. Source 255--------- 256 257include/linux/dmaengine.h: 258 core header file for DMA drivers and api users 259drivers/dma/dmaengine.c: 260 offload engine channel management routines 261drivers/dma/: 262 location for offload engine drivers 263include/linux/async_tx.h: 264 core header file for the async_tx api 265crypto/async_tx/async_tx.c: 266 async_tx interface to dmaengine and common code 267crypto/async_tx/async_memcpy.c: 268 copy offload 269crypto/async_tx/async_xor.c: 270 xor and xor zero sum offload 271