device_tx_status /* poll for transaction completion */
device_issue_pending /* push pending transactions to hardware */
and,
device_prep_slave_sg /* prepares a slave dma operation */
device_terminate_all /* manipulate all pending operations on a channel, returns zero or
error code */
The first four functions are used by the DMA Engine Framework, the last two are used
by the DMA slave (DMA client). Notably, device_issue_pending is used to trigger the
start of a PxP operation.
The PxP DMA driver also implements the interface tx_submit in struct
dma_async_tx_descriptor
, which is used to prepare the descriptor(s) which will be
executed by the engine. When tasks are received in pxp_tx_submit, they are not
configured and executed immediately. Rather, they are added to a task queue and the
function call is allowed to return immediately.
10.3.2 Channel Management
Although ePxP does not have multiple channels in hardware, 16 virtual channels are
supported in the driver; this provides flexibility in the multiple instance/client design. At
any time, a user can call dma_request_channel() to get a free channel, and then configure
this channel with several descriptors (a descriptor is required for each input plane and for
the output plane). When the PxP is no longer being used, the channel should be released
by calling dma_release_channel(). Detailed elements of channel management are
handled by the driver and are transparent to the client.
10.3.3 Descriptor Management
The DMA Engine processes the task based on the descriptor. One DMA channel is
usually associated with several descriptors. Descriptors are recycled resources, under
control of the offload engine driver, to be reused as operations complete. The extended
TX descriptor packet (pxp_tx_desc), allows the user to pass PxP configuration
information to the driver. This includes everything that the PxP needs to execute a
processing task.
Software Operation
i.MX 6Solo/6DualLite Linux Reference Manual, Rev. L3.0.35_4.1.0, 09/2013
98
Freescale Semiconductor, Inc.
