greybus: send operation result in response message header

Define a result byte in an operation response message header.

All the protocols now define the mandatory status as the first
byte in their response message.  Assume that, for the moment,
and save that value into the header result field (until we can
get the simulator set up to handle the new protocol).

Record the result from the response header as the result of the
overall operation.

Start enforcing the rule that we ignore all response payload (in
fact, the entire message) if we see a non-zero result value.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/operation.c

@@ -34,14 +34,22 @@ static struct workqueue_struct *gb_operation_recv_workqueue;
 
 /*
  * All operation messages (both requests and responses) begin with
- * a common header that encodes the size of the data (header
- * included).  This header also contains a unique identifier, which
- * is used to keep track of in-flight operations.  Finally, the
- * header contains a operation type field, whose interpretation is
- * dependent on what type of device lies on the other end of the
- * connection.  Response messages are distinguished from request
- * messages by setting the high bit (0x80) in the operation type
- * value.
+ * a header that encodes the size of the data (header included).
+ * This header also contains a unique identifier, which is used to
+ * keep track of in-flight operations.  The header contains an
+ * operation type field, whose interpretation is dependent on what
+ * type of protocol is used over the connection.
+ *
+ * The high bit (0x80) of the operation type field is used to
+ * indicate whether the message is a request (clear) or a response
+ * (set).
+ *
+ * Response messages include an additional status byte, which
+ * communicates the result of the corresponding request.  A zero
+ * status value means the operation completed successfully.  Any
+ * other value indicates an error; in this case, the payload of the
+ * response message (if any) is ignored.  The status byte must be
+ * zero in the header for a request message.
  *
  * The wire format for all numeric fields in the header is little
  * endian.  Any operation-specific data begins immediately after the
@@ -51,7 +59,8 @@ struct gb_operation_msg_hdr {
 	__le16	size;		/* Size in bytes of header + payload */
 	__le16	operation_id;	/* Operation unique id */
 	__u8	type;		/* E.g GB_I2C_TYPE_* or GB_GPIO_TYPE_* */
-	/* 3 bytes pad, must be zero (ignore when read) */
+	__u8	result;		/* Result of request (in responses only) */
+	/* 2 bytes pad, must be zero (ignore when read) */
 } __aligned(sizeof(u64));
 
 /* XXX Could be per-host device, per-module, or even per-connection */
@@ -469,6 +478,7 @@ static void gb_connection_recv_response(struct gb_connection *connection,
 {
 	struct gb_operation *operation;
 	struct gb_message *message;
+	struct gb_operation_msg_hdr *header;
 
 	operation = gb_pending_operation_find(connection, operation_id);
 	if (!operation) {
@@ -485,8 +495,12 @@ static void gb_connection_recv_response(struct gb_connection *connection,
 		gb_connection_err(connection, "recv buffer too small");
 		return;		/* XXX Should still complete operation */
 	}
-	operation->result = GB_OP_SUCCESS;	/* XXX Maybe not yet? */
 
+	/* Hack the status from the buffer into the header */
+	header = message->buffer;
+	header->result = *(char *)message->payload;	/* Eeew. */
+	operation->result = header->result;
+	if (operation->result == GB_OP_SUCCESS)
 		memcpy(message->buffer, data, size);
 
 	/* The rest will be handled in work queue context */