iio: buffer-dmaengine: Add support for cyclic DMA transfers

Allow buffer blocks flagged as cyclic to be submitted as repeating DMA
transfers. For cyclic blocks, use DMA_PREP_REPEAT so the engine keeps
replaying the descriptor.

This is useful for output buffers where the same data should be driven
continuously without userspace having to requeue it. Examples include
continuous RF transmit paths replaying a calibration, test or beacon
pattern.

Skip installing the completion callback for cyclic blocks. Since the
transfer is continuously replayed, the callback would fire on every
period, throwing off the block refcount.

Because nothing prevents a new cyclic transfer from replacing an
already active cyclic one, always set DMA_PREP_LOAD_EOT so the engine
correctly terminates the active transfer before loading the new
descriptor.

Limit the DMA buffer queue to one cyclic DMABUF at a time. There is
currently no known use case for queueing multiple cyclic blocks, and
cyclic blocks stay referenced until the buffer is disabled.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

0 files changed, 0 insertions, 0 deletions