firmware: arm_scmi: Fix base agent discover response

According to scmi specification, the response of the discover agent request
is made of:
- int32 status
- uint32 agent_id
- uint8 name[16]

but the current implementation doesn't take into account the agent_id field
and only allocates a rx buffer of SCMI_MAX_STR_SIZE length

Allocate the correct length for rx buffer and copy the name from the
correct offset in the response.

While no error were returned until v5.15, v5.16-rc1 fails with virtio_scmi
transport channel:

 | arm-scmi firmware:scmi0: SCMI Notifications - Core Enabled.
 | arm-scmi firmware:scmi0: SCMI Protocol v2.0 'Linaro:PMWG' Firmware version 0x2090000
 | scmi-virtio virtio0: tx:used len 28 is larger than in buflen 24

Link: https://lore.kernel.org/r/20211117081856.9932-1-vincent.guittot@linaro.org
Fixes: b6f20ff8 ("firmware: arm_scmi: add common infrastructure and support for base protocol")
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/base.c

@@ -34,6 +34,12 @@ struct scmi_msg_resp_base_attributes {
 	__le16 reserved;
 };
 
+struct scmi_msg_resp_base_discover_agent {
+	__le32 agent_id;
+	u8 name[SCMI_MAX_STR_SIZE];
+};
+
+
 struct scmi_msg_base_error_notify {
 	__le32 event_control;
 #define BASE_TP_NOTIFY_ALL	BIT(0)
@@ -225,18 +231,21 @@ static int scmi_base_discover_agent_get(const struct scmi_protocol_handle *ph,
 					int id, char *name)
 {
 	int ret;
+	struct scmi_msg_resp_base_discover_agent *agent_info;
 	struct scmi_xfer *t;
 
 	ret = ph->xops->xfer_get_init(ph, BASE_DISCOVER_AGENT,
-				      sizeof(__le32), SCMI_MAX_STR_SIZE, &t);
+				      sizeof(__le32), sizeof(*agent_info), &t);
 	if (ret)
 		return ret;
 
 	put_unaligned_le32(id, t->tx.buf);
 
 	ret = ph->xops->do_xfer(ph, t);
-	if (!ret)
-		strlcpy(name, t->rx.buf, SCMI_MAX_STR_SIZE);
+	if (!ret) {
+		agent_info = t->rx.buf;
+		strlcpy(name, agent_info->name, SCMI_MAX_STR_SIZE);
+	}
 
 	ph->xops->xfer_put(ph, t);