The original design and current implementation of the PFVF protocol
expects the sender to both acquire and relinquish the ownership of the
shared CSR by setting and clearing the "in use" pattern on the remote
half of the register when sending a message. This happens regardless of
the acknowledgment of the reception, to guarantee changes, including
collisions, are surely detected.

However, in the case of a request that requires a response, collisions
can also be detected by the lack of a reply. This can be exploited to
speed up and simplify the above behaviour, letting the receiver both
acknowledge the message and release the CSR in a single transaction:

1) the sender can return as soon as the message has been acknowledged
2) the receiver doesn't have to wait long before acquiring ownership
of the CSR for the response message, greatly improving the overall
throughput.

Howerver, this improvement cannot be leveraged for fire-and-forget
notifications, as it would be impossible for the sender to clearly
distinguish between a collision and an ack immediately followed by a new
message.

This patch implements this optimization in a new version of the protocol
(v3), which applies the fast-ack logic only whenever possible and
guarantees backward compatibility with older versions. For requests, a
new retry loop guarantees a correct behaviour.
Signed-off-by: Marco Chiappero <marco.chiappero@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Fiona Trahe <fiona.trahe@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>