3.6 Constraints
3.7 Example
-4 DRIVER DEVELOPER NOTES
+4 DMAENGINE DRIVER DEVELOPER NOTES
4.1 Conformance points
-4.2 "My application needs finer control of hardware channels"
+4.2 "My application needs exclusive control of hardware channels"
5 SOURCE
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
+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
3.3 Descriptor management:
The return value is non-NULL and points to a 'descriptor' when the operation
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.
+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?
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_memcpy(copy_dest, copy_src, 0, 0, copy_len, 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,
+ ASYNC_TX_XOR_DROP_DST | ASYNC_TX_ACK,
tx, complete_xor_copy_xor, NULL);
async_tx_issue_pending_all();
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:
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;
-
- . . .
-}
+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 clients
+
+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
git.openpandora.org / pandora-kernel.git / blobdiff