220 lines
8.8 KiB
Plaintext
220 lines
8.8 KiB
Plaintext
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
|