firmware: arm_scmi: Add support for v3.2 NEGOTIATE_PROTOCOL_VERSION

Freshly introduced NEGOTIATE_PROTOCOL_VERSION allows the agent to ascertain
upfront if a specific protocol(usually older) version is supported by the
platform.

It is used by the agent in case the platform has advertised the support of
a newer protocol version than the latest version supported by the agent,
since backward compatibility cannot be automatically assumed.

Emit a warning about possible incompatibility when negotiation was not
possible or just print the successfully negotiated protocol.
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20240214183006.3403207-3-cristian.marussi@arm.comSigned-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/driver.c

@@ -86,6 +86,12 @@ struct scmi_xfers_info {
  * @users: A refcount to track effective users of this protocol.
  * @priv: Reference for optional protocol private data.
  * @version: Protocol version supported by the platform as detected at runtime.
+ * @negotiated_version: When the platform supports a newer protocol version,
+ *			the agent will try to negotiate with the platform the
+ *			usage of the newest version known to it, since
+ *			backward compatibility is NOT automatically assured.
+ *			This field is NON-zero when a successful negotiation
+ *			has completed.
  * @ph: An embedded protocol handle that will be passed down to protocol
  *	initialization code to identify this instance.
  *
@@ -99,6 +105,7 @@ struct scmi_protocol_instance {
 	refcount_t			users;
 	void				*priv;
 	unsigned int			version;
+	unsigned int			negotiated_version;
 	struct scmi_protocol_handle	ph;
 };
 
@@ -1815,6 +1822,44 @@ scmi_revision_area_get(const struct scmi_protocol_handle *ph)
 	return pi->handle->version;
 }
 
+/**
+ * scmi_protocol_version_negotiate  - Negotiate protocol version
+ *
+ * @ph: A reference to the protocol handle.
+ *
+ * An helper to negotiate a protocol version different from the latest
+ * advertised as supported from the platform: on Success backward
+ * compatibility is assured by the platform.
+ *
+ * Return: 0 on Success
+ */
+static int scmi_protocol_version_negotiate(struct scmi_protocol_handle *ph)
+{
+	int ret;
+	struct scmi_xfer *t;
+	struct scmi_protocol_instance *pi = ph_to_pi(ph);
+
+	/* At first check if NEGOTIATE_PROTOCOL_VERSION is supported ... */
+	ret = scmi_protocol_msg_check(ph, NEGOTIATE_PROTOCOL_VERSION, NULL);
+	if (ret)
+		return ret;
+
+	/* ... then attempt protocol version negotiation */
+	ret = xfer_get_init(ph, NEGOTIATE_PROTOCOL_VERSION,
+			    sizeof(__le32), 0, &t);
+	if (ret)
+		return ret;
+
+	put_unaligned_le32(pi->proto->supported_version, t->tx.buf);
+	ret = do_xfer(ph, t);
+	if (!ret)
+		pi->negotiated_version = pi->proto->supported_version;
+
+	xfer_put(ph, t);
+
+	return ret;
+}
+
 /**
  * scmi_alloc_init_protocol_instance  - Allocate and initialize a protocol
  * instance descriptor.
@@ -1887,11 +1932,21 @@ scmi_alloc_init_protocol_instance(struct scmi_info *info,
 	devres_close_group(handle->dev, pi->gid);
 	dev_dbg(handle->dev, "Initialized protocol: 0x%X\n", pi->proto->id);
 
-	if (pi->version > proto->supported_version)
-		dev_warn(handle->dev,
-			 "Detected UNSUPPORTED higher version 0x%X for protocol 0x%X."
-			 "Backward compatibility is NOT assured.\n",
-			 pi->version, pi->proto->id);
+	if (pi->version > proto->supported_version) {
+		ret = scmi_protocol_version_negotiate(&pi->ph);
+		if (!ret) {
+			dev_info(handle->dev,
+				 "Protocol 0x%X successfully negotiated version 0x%X\n",
+				 proto->id, pi->negotiated_version);
+		} else {
+			dev_warn(handle->dev,
+				 "Detected UNSUPPORTED higher version 0x%X for protocol 0x%X.\n",
+				 pi->version, pi->proto->id);
+			dev_warn(handle->dev,
+				 "Trying version 0x%X. Backward compatibility is NOT assured.\n",
+				 pi->proto->supported_version);
+		}
+	}
 
 	return pi;
 

drivers/firmware/arm_scmi/protocols.h

@@ -33,6 +33,7 @@ enum scmi_common_cmd {
 	PROTOCOL_VERSION = 0x0,
 	PROTOCOL_ATTRIBUTES = 0x1,
 	PROTOCOL_MESSAGE_ATTRIBUTES = 0x2,
+	NEGOTIATE_PROTOCOL_VERSION = 0x10,
 };
 
 /**