The DMA API has several expectations for drivers and callers that were
underdocumented or undocumented. Better clarify the driver expectations
and caller expectations.
The DMA API from the caller side is not a portable API and really cannot
be as each DMA has unique properties and expectations of memory,
peripheral interaction, and features. The API in effect provides a union
of all useful DMA functionality drivers have needed in the tree. It can
still be a good abstraction, with care, for peripheral devices for
vendors where the DMA IP might be very similar but have slight
variances.
From the driver implementation side expectations around synchronization,
state transitions, and memory management for transfer descriptors is now
described in documentation rather than solely from me in github review
comments.
Signed-off-by: Tom Burdick <thomas.burdick@intel.com>
344d24bcb7