1                 Asynchronous Transfers/Transforms API
   73 USAGE
   83.1 General format of the API
   93.2 Supported operations
  103.3 Descriptor management
  113.4 When does the operation execute?
  123.5 When does the operation complete?
  133.6 Constraints
  143.7 Example
  174.1 Conformance points
  184.2 "My application needs exclusive control of hardware channels"
  205 SOURCE
  26The async_tx API provides methods for describing a chain of asynchronous
  27bulk memory transfers/transforms with support for inter-transactional
  28dependencies.  It is implemented as a dmaengine client that smooths over
  29the details of different hardware offload engine implementations.  Code
  30that is written to the API can optimize for asynchronous operation and
  31the API will fit the chain of operations to the available offload
  36The API was initially designed to offload the memory copy and
  37xor-parity-calculations of the md-raid5 driver using the offload engines
  38present in the Intel(R) Xscale series of I/O processors.  It also built
  39on the 'dmaengine' layer developed for offloading memory copies in the
  40network stack using Intel(R) I/OAT engines.  The following design
  41features surfaced as a result:
  421/ implicit synchronous path: users of the API do not need to know if
  43   the platform they are running on has offload capabilities.  The
  44   operation will be offloaded when an engine is available and carried out
  45   in software otherwise.
  462/ cross channel dependency chains: the API allows a chain of dependent
  47   operations to be submitted, like xor->copy->xor in the raid5 case.  The
  48   API automatically handles cases where the transition from one operation
  49   to another implies a hardware channel switch.
  503/ dmaengine extensions to support multiple clients and operation types
  51   beyond 'memcpy'
  533 USAGE
  553.1 General format of the API:
  56struct dma_async_tx_descriptor *
  57async_<operation>(<op specific parameters>, struct async_submit ctl *submit)
  593.2 Supported operations:
  60memcpy  - memory copy between a source and a destination buffer
  61memset  - fill a destination buffer with a byte value
  62xor     - xor a series of source buffers and write the result to a
  63          destination buffer
  64xor_val - xor a series of source buffers and set a flag if the
  65          result is zero.  The implementation attempts to prevent
  66          writes to memory
  67pq      - generate the p+q (raid6 syndrome) from a series of source buffers
  68pq_val  - validate that a p and or q buffer are in sync with a given series of
  69          sources
  70datap   - (raid6_datap_recov) recover a raid6 data block and the p block
  71          from the given sources
  722data   - (raid6_2data_recov) recover 2 raid6 data blocks from the given
  73          sources
  753.3 Descriptor management:
  76The return value is non-NULL and points to a 'descriptor' when the operation
  77has been queued to execute asynchronously.  Descriptors are recycled
  78resources, under control of the offload engine driver, to be reused as
  79operations complete.  When an application needs to submit a chain of
  80operations it must guarantee that the descriptor is not automatically recycled
  81before the dependency is submitted.  This requires that all descriptors be
  82acknowledged by the application before the offload engine driver is allowed to
  83recycle (or free) the descriptor.  A descriptor can be acked by one of the
  84following methods:
  851/ setting the ASYNC_TX_ACK flag if no child operations are to be submitted
  862/ submitting an unacknowledged descriptor as a dependency to another
  87   async_tx call will implicitly set the acknowledged state.
  883/ calling async_tx_ack() on the descriptor.
  903.4 When does the operation execute?
  91Operations do not immediately issue after return from the
  92async_<operation> call.  Offload engine drivers batch operations to
  93improve performance by reducing the number of mmio cycles needed to
  94manage the channel.  Once a driver-specific threshold is met the driver
  95automatically issues pending operations.  An application can force this
  96event by calling async_tx_issue_pending_all().  This operates on all
  97channels since the application has no knowledge of channel to operation
 1003.5 When does the operation complete?
 101There are two methods for an application to learn about the completion
 102of an operation.
 1031/ Call dma_wait_for_async_tx().  This call causes the CPU to spin while
 104   it polls for the completion of the operation.  It handles dependency
 105   chains and issuing pending operations.
 1062/ Specify a completion callback.  The callback routine runs in tasklet
 107   context if the offload engine driver supports interrupts, or it is
 108   called in application context if the operation is carried out
 109   synchronously in software.  The callback can be set in the call to
 110   async_<operation>, or when the application needs to submit a chain of
 111   unknown length it can use the async_trigger_callback() routine to set a
 112   completion interrupt/callback at the end of the chain.
 1143.6 Constraints:
 1151/ Calls to async_<operation> are not permitted in IRQ context.  Other
 116   contexts are permitted provided constraint #2 is not violated.
 1172/ Completion callback routines cannot submit new operations.  This
 118   results in recursion in the synchronous case and spin_locks being
 119   acquired twice in the asynchronous case.
 1213.7 Example:
 122Perform a xor->copy->xor operation where each operation depends on the
 123result from the previous operation:
 125void callback(void *param)
 127        struct completion *cmp = param;
 129        complete(cmp);
 132void run_xor_copy_xor(struct page **xor_srcs,
 133                      int xor_src_cnt,
 134                      struct page *xor_dest,
 135                      size_t xor_len,
 136                      struct page *copy_src,
 137                      struct page *copy_dest,
 138                      size_t copy_len)
 140        struct dma_async_tx_descriptor *tx;
 141        addr_conv_t addr_conv[xor_src_cnt];
 142        struct async_submit_ctl submit;
 143        addr_conv_t addr_conv[NDISKS];
 144        struct completion cmp;
 146        init_async_submit(&submit, ASYNC_TX_XOR_DROP_DST, NULL, NULL, NULL,
 147                          addr_conv);
 148        tx = async_xor(xor_dest, xor_srcs, 0, xor_src_cnt, xor_len, &submit)
 150        submit->depend_tx = tx;
 151        tx = async_memcpy(copy_dest, copy_src, 0, 0, copy_len, &submit);
 153        init_completion(&cmp);
 154        init_async_submit(&submit, ASYNC_TX_XOR_DROP_DST | ASYNC_TX_ACK, tx,
 155                          callback, &cmp, addr_conv);
 156        tx = async_xor(xor_dest, xor_srcs, 0, xor_src_cnt, xor_len, &submit);
 158        async_tx_issue_pending_all();
 160        wait_for_completion(&cmp);
 163See include/linux/async_tx.h for more information on the flags.  See the
 164ops_run_* and ops_complete_* routines in drivers/md/raid5.c for more
 165implementation examples.
 1694.1 Conformance points:
 170There are a few conformance points required in dmaengine drivers to
 171accommodate assumptions made by applications using the async_tx API:
 1721/ Completion callbacks are expected to happen in tasklet context
 1732/ dma_async_tx_descriptor fields are never manipulated in IRQ context
 1743/ Use async_tx_run_dependencies() in the descriptor clean up path to
 175   handle submission of dependent operations
 1774.2 "My application needs exclusive control of hardware channels"
 178Primarily this requirement arises from cases where a DMA engine driver
 179is being used to support device-to-memory operations.  A channel that is
 180performing these operations cannot, for many platform specific reasons,
 181be shared.  For these cases the dma_request_channel() interface is
 184The interface is:
 185struct dma_chan *dma_request_channel(dma_cap_mask_t mask,
 186                                     dma_filter_fn filter_fn,
 187                                     void *filter_param);
 189Where dma_filter_fn is defined as:
 190typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param);
 192When the optional 'filter_fn' parameter is set to NULL
 193dma_request_channel simply returns the first channel that satisfies the
 194capability mask.  Otherwise, when the mask parameter is insufficient for
 195specifying the necessary channel, the filter_fn routine can be used to
 196disposition the available channels in the system. The filter_fn routine
 197is called once for each free channel in the system.  Upon seeing a
 198suitable channel filter_fn returns DMA_ACK which flags that channel to
 199be the return value from dma_request_channel.  A channel allocated via
 200this interface is exclusive to the caller, until dma_release_channel()
 201is called.
 203The DMA_PRIVATE capability flag is used to tag dma devices that should
 204not be used by the general-purpose allocator.  It can be set at
 205initialization time if it is known that a channel will always be
 206private.  Alternatively, it is set when dma_request_channel() finds an
 207unused "public" channel.
 209A couple caveats to note when implementing a driver and consumer:
 2101/ Once a channel has been privately allocated it will no longer be
 211   considered by the general-purpose allocator even after a call to
 212   dma_release_channel().
 2132/ Since capabilities are specified at the device level a dma_device
 214   with multiple channels will either have all channels public, or all
 215   channels private.
 2175 SOURCE
 219include/linux/dmaengine.h: core header file for DMA drivers and api users
 220drivers/dma/dmaengine.c: offload engine channel management routines
 221drivers/dma/: location for offload engine drivers
 222include/linux/async_tx.h: core header file for the async_tx api
 223crypto/async_tx/async_tx.c: async_tx interface to dmaengine and common code
 224crypto/async_tx/async_memcpy.c: copy offload
 225crypto/async_tx/async_memset.c: memory fill offload
 226crypto/async_tx/async_xor.c: xor and xor zero sum offload
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