[media] cec: accept two replies for CEC_MSG_INITIATE_ARC

The CEC_MSG_INITIATE_ARC message is special since it is the ONLY
CEC message that accepts two possible valid replies:

CEC_MSG_REPORT_ARC_INITIATED and CEC_MSG_REPORT_ARC_TERMINATED.

So if the transmitted message is CEC_MSG_INITIATE_ARC and the remote
side replied with CEC_MSG_REPORT_ARC_INITIATED or CEC_MSG_REPORT_ARC_TERMINATED,
then a msg->reply value of CEC_MSG_REPORT_ARC_INITIATED or
CEC_MSG_REPORT_ARC_TERMINATED will match either reply.

I thought about either adding a second reply2 field, but that's ugly
for all other messages that have only one reply, and what if in the
future a new message is added that can have three replies?

Another option would be to add a cec_msg flag, but really, the combination
of CEC_MSG_REPORT_ARC_INITIATED and a reply value of one of the two
possible replies already functions as a flag.

Another advantage of this approach is that it is safe to re-use a
cec_msg struct. No need to zero a flags field or a reply2 field.

So since this really is an exception in the CEC specification, I
decided to implement it as an exception as well.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/media/uapi/cec/cec-ioc-receive.rst b/Documentation/media/uapi/cec/cec-ioc-receive.rst
index b4dffd2f7dfa..bdf015b1d1dc 100644
--- a/Documentation/media/uapi/cec/cec-ioc-receive.rst
+++ b/Documentation/media/uapi/cec/cec-ioc-receive.rst
@@ -142,6 +142,14 @@ result.
 	Feature Abort reply. In this case ``rx_status`` will either be set
 	to :ref:`CEC_RX_STATUS_TIMEOUT <CEC-RX-STATUS-TIMEOUT>` or
 	:ref:`CEC_RX_STATUS_FEATURE_ABORT <CEC-RX-STATUS-FEATURE-ABORT>`.
+
+	If the transmitter message is ``CEC_MSG_INITIATE_ARC`` then the ``reply``
+	values ``CEC_MSG_REPORT_ARC_INITIATED`` and ``CEC_MSG_REPORT_ARC_TERMINATED``
+	are processed differently: either value will match both possible replies.
+	The reason is that the ``CEC_MSG_INITIATE_ARC`` message is the only CEC
+	message that has two possible replies other than Feature Abort. The
+	``reply`` field will be updated with the actual reply so that it is
+	synchronized with the contents of the received message.
     * - __u8
       - ``rx_status``
       - The status bits of the received message. See