mbox: Clarify protocol version negotion
The specification intended that the mbox daemon return its highest
supported protocol version less-than-or-equal to the version requested
by the host. This was not clear in the documentation and was not the
behaviour implemented by the daemon.
Fix both the documentation and the daemon to perform to this
expectation.
Change-Id: I6f9cf0b16d2b9319d69656eb26defb46dccd4ddf
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/Documentation/mbox_protocol.md b/Documentation/mbox_protocol.md
index 5a0879e..70728e9 100644
--- a/Documentation/mbox_protocol.md
+++ b/Documentation/mbox_protocol.md
@@ -210,16 +210,17 @@
multiple of block size it is necessary for the host and BMC to agree on a
protocol version as this determines the block size. In V1 it is hard coded at
4K and in V2 the BMC chooses and specifies this to the host as a response
-argument to MBOX_GET_INFO. Thus the host must always call MBOX_GET_INFO before
-any other command which specifies an argument in block size.
+argument to `MBOX_GET_INFO`. Thus the host must always call `MBOX_GET_INFO`
+before any other command which specifies an argument in block size.
-The host must tell the BMC the highest protocol level which it supports. The
-BMC will then respond with a protocol level. If the host doesn't understand
-the protocol level specified by the BMC then it must not continue to
-communicate with the BMC. Otherwise the protocol level specified by the
-BMC is taken to be the protocol level used for further communication and can
-only be changed by another call to MBOX_GET_INFO. The BMC should use the
-request from the host to influence its protocol version choice.
+When invoking `MBOX_GET_INFO` the host must provide the BMC its highest
+supported version of the protocol. The BMC must respond with a protocol version
+less than or equal to that requested by the host, or in the event that there is
+no such value, an error code. In the event that an error is returned the host
+must not continue to communicate with the BMC. Otherwise, the protocol version
+returned by the BMC is the agreed protocol version for all further
+communication. The host may at a future point request a change in protocol
+version by issuing a subsequent `MBOX_GET_INFO` command.
### Window Management
diff --git a/mboxd_msg.c b/mboxd_msg.c
index 179af60..84e8d9a 100644
--- a/mboxd_msg.c
+++ b/mboxd_msg.c
@@ -176,10 +176,12 @@
int rc;
/* Check we support the version requested */
- if (mbox_api_version < API_MIN_VERSION ||
- mbox_api_version > API_MAX_VERSION) {
+ if (mbox_api_version < API_MIN_VERSION)
return -MBOX_R_PARAM_ERROR;
- }
+
+ if (mbox_api_version > API_MAX_VERSION)
+ mbox_api_version = API_MAX_VERSION;
+
context->version = mbox_api_version;
MSG_OUT("Using Protocol Version: %d\n", context->version);