async_tx: usage documentation and developer notes (v2)

		 Asynchronous Transfers/Transforms API

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 DRIVER DEVELOPER NOTES

4.1 Conformance points

4.2 "My application needs finer 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>,

		  enum async_tx_flags flags,

        	  struct dma_async_tx_descriptor *dependency,

        	  dma_async_tx_callback callback_routine,

		  void *callback_parameter);

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_zero_sum - xor a series of source buffers and set a flag if the

	       result is zero.  The implementation attempts to prevent

	       writes to memory

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/ setting the ASYNC_TX_DEP_ACK flag to acknowledge the parent

   descriptor of a new operation.

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 complete_xor_copy_xor(void *param)

{

	printk("complete\n");

}

int 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;

	tx = async_xor(xor_dest, xor_srcs, 0, xor_src_cnt, xor_len,

		       ASYNC_TX_XOR_DROP_DST, NULL, NULL, NULL);

	tx = async_memcpy(copy_dest, copy_src, 0, 0, copy_len,

			  ASYNC_TX_DEP_ACK, tx, NULL, NULL);

	tx = async_xor(xor_dest, xor_srcs, 0, xor_src_cnt, xor_len,

		       ASYNC_TX_XOR_DROP_DST | ASYNC_TX_DEP_ACK | ASYNC_TX_ACK,

		       tx, complete_xor_copy_xor, NULL);

	async_tx_issue_pending_all();

}

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 finer control of hardware channels"

This requirement seems to arise from cases where a DMA engine driver is

trying to support device-to-memory DMA.  The dmaengine and async_tx

implementations were designed for offloading memory-to-memory

operations; however, there are some capabilities of the dmaengine layer

that can be used for platform-specific channel management.

Platform-specific constraints can be handled by registering the

application as a 'dma_client' and implementing a 'dma_event_callback' to

apply a filter to the available channels in the system.  Before showing

how to implement a custom dma_event callback some background of

dmaengine's client support is required.

The following routines in dmaengine support multiple clients requesting

use of a channel:

- dma_async_client_register(struct dma_client *client)

- dma_async_client_chan_request(struct dma_client *client)

dma_async_client_register takes a pointer to an initialized dma_client

structure.  It expects that the 'event_callback' and 'cap_mask' fields

are already initialized.

dma_async_client_chan_request triggers dmaengine to notify the client of

all channels that satisfy the capability mask.  It is up to the client's

event_callback routine to track how many channels the client needs and

how many it is currently using.  The dma_event_callback routine returns a

dma_state_client code to let dmaengine know the status of the

allocation.

Below is the example of how to extend this functionality for

platform-specific filtering of the available channels beyond the

standard capability mask:

static enum dma_state_client

my_dma_client_callback(struct dma_client *client,

			struct dma_chan *chan, enum dma_state state)

{

	struct dma_device *dma_dev;

	struct my_platform_specific_dma *plat_dma_dev;

	dma_dev = chan->device;

	plat_dma_dev = container_of(dma_dev,

				    struct my_platform_specific_dma,

				    dma_dev);

	if (!plat_dma_dev->platform_specific_capability)

		return DMA_DUP;

	. . .

}

5 SOURCE

include/linux/dmaengine.h: core header file for DMA drivers and clients

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_memset.c: memory fill offload

crypto/async_tx/async_xor.c: xor and xor zero sum offload