--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+Asynchronous Transfers/Transforms API
+=====================================
+
+.. Contents
+
+ 1. INTRODUCTION
+
+ 2 GENEALOGY
+
+ 3 USAGE
+ 3.1 General format of the API
+ 3.2 Supported operations
+ 3.3 Descriptor management
+ 3.4 When does the operation execute?
+ 3.5 When does the operation complete?
+ 3.6 Constraints
+ 3.7 Example
+
+ 4 DMAENGINE DRIVER DEVELOPER NOTES
+ 4.1 Conformance points
+ 4.2 "My application needs exclusive control of hardware channels"
+
+ 5 SOURCE
+
+1. Introduction
+===============
+
+The async_tx API provides methods for describing a chain of asynchronous
+bulk memory transfers/transforms with support for inter-transactional
+dependencies. It is implemented as a dmaengine client that smooths over
+the details of different hardware offload engine implementations. Code
+that is written to the API can optimize for asynchronous operation and
+the API will fit the chain of operations to the available offload
+resources.
+
+2.Genealogy
+===========
+
+The API was initially designed to offload the memory copy and
+xor-parity-calculations of the md-raid5 driver using the offload engines
+present in the Intel(R) Xscale series of I/O processors. It also built
+on the 'dmaengine' layer developed for offloading memory copies in the
+network stack using Intel(R) I/OAT engines. The following design
+features surfaced as a result:
+
+1. implicit synchronous path: users of the API do not need to know if
+ the platform they are running on has offload capabilities. The
+ operation will be offloaded when an engine is available and carried out
+ in software otherwise.
+2. cross channel dependency chains: the API allows a chain of dependent
+ operations to be submitted, like xor->copy->xor in the raid5 case. The
+ API automatically handles cases where the transition from one operation
+ to another implies a hardware channel switch.
+3. dmaengine extensions to support multiple clients and operation types
+ beyond 'memcpy'
+
+3. Usage
+========
+
+3.1 General format of the API
+-----------------------------
+
+::
+
+ struct dma_async_tx_descriptor *
+ async_<operation>(<op specific parameters>, struct async_submit ctl *submit)
+
+3.2 Supported operations
+------------------------
+
+======== ====================================================================
+memcpy memory copy between a source and a destination buffer
+memset fill a destination buffer with a byte value
+xor xor a series of source buffers and write the result to a
+ destination buffer
+xor_val xor a series of source buffers and set a flag if the
+ result is zero. The implementation attempts to prevent
+ writes to memory
+pq generate the p+q (raid6 syndrome) from a series of source buffers
+pq_val validate that a p and or q buffer are in sync with a given series of
+ sources
+datap (raid6_datap_recov) recover a raid6 data block and the p block
+ from the given sources
+2data (raid6_2data_recov) recover 2 raid6 data blocks from the given
+ sources
+======== ====================================================================
+
+3.3 Descriptor management
+-------------------------
+
+The return value is non-NULL and points to a 'descriptor' when the operation
+has been queued to execute asynchronously. Descriptors are recycled
+resources, under control of the offload engine driver, to be reused as
+operations complete. When an application needs to submit a chain of
+operations it must guarantee that the descriptor is not automatically recycled
+before the dependency is submitted. This requires that all descriptors be
+acknowledged by the application before the offload engine driver is allowed to
+recycle (or free) the descriptor. A descriptor can be acked by one of the
+following methods:
+
+1. setting the ASYNC_TX_ACK flag if no child operations are to be submitted
+2. submitting an unacknowledged descriptor as a dependency to another
+ async_tx call will implicitly set the acknowledged state.
+3. calling async_tx_ack() on the descriptor.
+
+3.4 When does the operation execute?
+------------------------------------
+
+Operations do not immediately issue after return from the
+async_<operation> call. Offload engine drivers batch operations to
+improve performance by reducing the number of mmio cycles needed to
+manage the channel. Once a driver-specific threshold is met the driver
+automatically issues pending operations. An application can force this
+event by calling async_tx_issue_pending_all(). This operates on all
+channels since the application has no knowledge of channel to operation
+mapping.
+
+3.5 When does the operation complete?
+-------------------------------------
+
+There are two methods for an application to learn about the completion
+of an operation.
+
+1. Call dma_wait_for_async_tx(). This call causes the CPU to spin while
+ it polls for the completion of the operation. It handles dependency
+ chains and issuing pending operations.
+2. Specify a completion callback. The callback routine runs in tasklet
+ context if the offload engine driver supports interrupts, or it is
+ called in application context if the operation is carried out
+ synchronously in software. The callback can be set in the call to
+ async_<operation>, or when the application needs to submit a chain of
+ unknown length it can use the async_trigger_callback() routine to set a
+ completion interrupt/callback at the end of the chain.
+
+3.6 Constraints
+---------------
+
+1. Calls to async_<operation> are not permitted in IRQ context. Other
+ contexts are permitted provided constraint #2 is not violated.
+2. Completion callback routines cannot submit new operations. This
+ results in recursion in the synchronous case and spin_locks being
+ acquired twice in the asynchronous case.
+
+3.7 Example
+-----------
+
+Perform a xor->copy->xor operation where each operation depends on the
+result from the previous operation::
+
+ void callback(void *param)
+ {
+ struct completion *cmp = param;
+
+ complete(cmp);
+ }
+
+ void run_xor_copy_xor(struct page **xor_srcs,
+ int xor_src_cnt,
+ struct page *xor_dest,
+ size_t xor_len,
+ struct page *copy_src,
+ struct page *copy_dest,
+ size_t copy_len)
+ {
+ struct dma_async_tx_descriptor *tx;
+ addr_conv_t addr_conv[xor_src_cnt];
+ struct async_submit_ctl submit;
+ addr_conv_t addr_conv[NDISKS];
+ struct completion cmp;
+
+ init_async_submit(&submit, ASYNC_TX_XOR_DROP_DST, NULL, NULL, NULL,
+ addr_conv);
+ tx = async_xor(xor_dest, xor_srcs, 0, xor_src_cnt, xor_len, &submit)
+
+ submit->depend_tx = tx;
+ tx = async_memcpy(copy_dest, copy_src, 0, 0, copy_len, &submit);
+
+ init_completion(&cmp);
+ init_async_submit(&submit, ASYNC_TX_XOR_DROP_DST | ASYNC_TX_ACK, tx,
+ callback, &cmp, addr_conv);
+ tx = async_xor(xor_dest, xor_srcs, 0, xor_src_cnt, xor_len, &submit);
+
+ async_tx_issue_pending_all();
+
+ wait_for_completion(&cmp);
+ }
+
+See include/linux/async_tx.h for more information on the flags. See the
+ops_run_* and ops_complete_* routines in drivers/md/raid5.c for more
+implementation examples.
+
+4. Driver Development Notes
+===========================
+
+4.1 Conformance points
+----------------------
+
+There are a few conformance points required in dmaengine drivers to
+accommodate assumptions made by applications using the async_tx API:
+
+1. Completion callbacks are expected to happen in tasklet context
+2. dma_async_tx_descriptor fields are never manipulated in IRQ context
+3. Use async_tx_run_dependencies() in the descriptor clean up path to
+ handle submission of dependent operations
+
+4.2 "My application needs exclusive control of hardware channels"
+-----------------------------------------------------------------
+
+Primarily this requirement arises from cases where a DMA engine driver
+is being used to support device-to-memory operations. A channel that is
+performing these operations cannot, for many platform specific reasons,
+be shared. For these cases the dma_request_channel() interface is
+provided.
+
+The interface is::
+
+ struct dma_chan *dma_request_channel(dma_cap_mask_t mask,
+ dma_filter_fn filter_fn,
+ void *filter_param);
+
+Where dma_filter_fn is defined as::
+
+ typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param);
+
+When the optional 'filter_fn' parameter is set to NULL
+dma_request_channel simply returns the first channel that satisfies the
+capability mask. Otherwise, when the mask parameter is insufficient for
+specifying the necessary channel, the filter_fn routine can be used to
+disposition the available channels in the system. The filter_fn routine
+is called once for each free channel in the system. Upon seeing a
+suitable channel filter_fn returns DMA_ACK which flags that channel to
+be the return value from dma_request_channel. A channel allocated via
+this interface is exclusive to the caller, until dma_release_channel()
+is called.
+
+The DMA_PRIVATE capability flag is used to tag dma devices that should
+not be used by the general-purpose allocator. It can be set at
+initialization time if it is known that a channel will always be
+private. Alternatively, it is set when dma_request_channel() finds an
+unused "public" channel.
+
+A couple caveats to note when implementing a driver and consumer:
+
+1. Once a channel has been privately allocated it will no longer be
+ considered by the general-purpose allocator even after a call to
+ dma_release_channel().
+2. Since capabilities are specified at the device level a dma_device
+ with multiple channels will either have all channels public, or all
+ channels private.
+
+5. Source
+---------
+
+include/linux/dmaengine.h:
+ core header file for DMA drivers and api users
+drivers/dma/dmaengine.c:
+ offload engine channel management routines
+drivers/dma/:
+ location for offload engine drivers
+include/linux/async_tx.h:
+ core header file for the async_tx api
+crypto/async_tx/async_tx.c:
+ async_tx interface to dmaengine and common code
+crypto/async_tx/async_memcpy.c:
+ copy offload
+crypto/async_tx/async_xor.c:
+ xor and xor zero sum offload
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