simplified message protocol

This simplifies the messaging protocol. Messages will now only have
the raw content plus a checksum byte at the end. We will worry about
needs of the transport layer (e.g. preamble/postamble bytes) on the
host driver side, and will not support variable-length commands for
now.

There is also a protocol version command, which is now command number
0x00, which returns a 4-byte protocol version followed by a 1-byte
checksum.

BUG=none
TEST=tested on daisy using mkbp kernel driver
Signed-off-by: David Hendricks <dhendrix@chromium.org>

Change-Id: I8fcc693cd50bc2b515164ea7a2a941cdd8333e73

2 files changed, 23 insertions, 40 deletions

diff --git a/common/message.c b/common/message.c
index 6828273e78..0040f479f4 100644
--- a/common/message.c
+++ b/common/message.c
@@ -13,9 +13,12 @@
 #include "keyboard_scan.h"
 #include "util.h"
 
-/* Our ID message - Matrix KeyBoard Protocol */
-static const char proto_id[] = "Google Chrome MKBP v1";
+/* EC ID */
+/* (TODO(dhendrix): Define this in board-specific code */
+static const char ec_id[] = "Google Chrome EC";
 
+/* Protocol version (least significant byte in lowest byte position) */
+static const uint8_t proto_ver[] = { 1, 0, 0, 0 };
 
 /**
  * Get the response to a given command
@@ -34,12 +37,15 @@ static int message_get_response(int cmd, uint8_t **buffp, int max_len)
 	 * Invalid commands are ignored, just returning a stream of 0xff
 	 * bytes.
 	 */
-	switch (cmd & MSG_CMD_MASK) {
+	switch (cmd) {
+	case CMDC_PROTO_VER:
+		*buffp = (uint8_t *)proto_ver;
+		return sizeof(proto_ver);
 	case CMDC_NOP:
 		return 0;
 	case CMDC_ID:
-		*buffp = (char *)proto_id;
-		return sizeof(proto_id) - 1;
+		*buffp = (char *)ec_id;
+		return sizeof(ec_id) - 1;
 	case CMDC_KEY_STATE:
 		return keyboard_get_scan(buffp, max_len);
 	default:
@@ -55,10 +61,9 @@ int message_process_cmd(int cmd, uint8_t *out_msg, int max_len)
 	int msg_len;
 	int need_copy;
 	int sum = 0;
-	int len;
 	int i;
 
-	msg = out_msg + MSG_HEADER_BYTES;
+	msg = out_msg;
 	msg_len = message_get_response(cmd, &msg, max_len - MSG_PROTO_BYTES);
 	if (msg_len < 0)
 		return msg_len;
@@ -69,25 +74,18 @@ int message_process_cmd(int cmd, uint8_t *out_msg, int max_len)
 	 */
 	if (msg_len + MSG_PROTO_BYTES > max_len)
 		msg_len = max_len - MSG_PROTO_BYTES;
-	len = msg_len + MSG_PROTO_BYTES;
 	ASSERT(msg_len >= 0 && msg_len < 0xffff);
-	need_copy = msg != out_msg + MSG_HEADER_BYTES;
+	need_copy = msg != out_msg;
 	ASSERT(!need_copy ||
 		msg + msg_len < out_msg ||
 		msg > out_msg + sizeof(out_msg));
 
-	out_msg[0] = MSG_HEADER;
-	out_msg[1] = len & 0xff;
-	out_msg[2] = (len >> 8) & 0xff;
-	sum += MSG_HEADER + len + (len >> 8);
-
 	for (i = 0; i < msg_len; i++) {
 		if (need_copy)
-			out_msg[i + 3] = msg[i];
+			out_msg[i] = msg[i];
 		sum += msg[i];
 	}
-	out_msg[i + 3] = sum & 0xff;
-	out_msg[i + 4] = MSG_PREAMBLE;
+	out_msg[i] = sum;
 
 	return msg_len + MSG_PROTO_BYTES;
 }
diff --git a/include/message.h b/include/message.h
index 0930eb24f1..439c6df67e 100644
--- a/include/message.h
+++ b/include/message.h
@@ -11,28 +11,19 @@
 /* Command interface between EC and AP */
 
 enum {
-	/* Mask to convert a command byte into a command */
-	MSG_CMD_MASK	= 0x7f,
-
-	/* The bytes which appear before the header in a message */
-	MSG_PREAMBLE	= 0xff,
-
-	/* The header byte, which follows the preamble */
-	MSG_HEADER	= 0xec,
-
-	MSG_HEADER_BYTES	= 3,
-	MSG_TRAILER_BYTES	= 2,
-	MSG_PROTO_BYTES		= MSG_HEADER_BYTES + MSG_TRAILER_BYTES,
+	MSG_TRAILER_BYTES	= 1,
+	MSG_PROTO_BYTES		= MSG_TRAILER_BYTES,
 };
 
 /* The command codes that we understand */
 enum message_cmd_t {
-	/* control / status messages */
+	/* EC control/status messages */
+	CMDC_PROTO_VER	= 0x00,		/* Protocol version */
 	CMDC_NOP,			/* No operation / ping */
 	CMDC_ID,			/* Read EC ID */
 
-	/* functional messages */
-	CMDC_KEY_STATE = 0x20,		/* Read key state */
+	/* Functional messages */
+	CMDC_KEY_STATE	= 0x20,		/* Read key state */
 };
 
 
@@ -44,15 +35,9 @@ enum message_cmd_t {
  *
  * The format of a reply is a sequence of bytes:
  *
- * <hdr> <len_lo> <len_hi> <msg bytes> <sum> <preamble bytes>
- *
- * The hdr byte is just a tag to indicate that the real message follows. It
- * signals the end of any preamble required by the interface.
- *
- * The 16-bit length is the entire packet size, including the header, length
- * bytes, message payload, checksum, and postamble byte.
+ * <msg bytes> <sum>
  *
- * The checksum is calculated as the sum of the header, len byte and message.
+ * The checksum is calculated as the sum of all message bytes
  *
  * @param cmd		Command to process (CMD_...)
  * @param buff		Pointer to buffer to store reponse