hdlc: Simplify + Fix HDLC implementation

* remove the notion of states, as there is really only one state
* implement zero removal / bit stuffing for synchronous links

2 files changed, 25 insertions, 53 deletions

diff --git a/src/hdlc.c b/src/hdlc.c
index 0a53a27..19ba900 100644
--- a/src/hdlc.c
+++ b/src/hdlc.c
@@ -18,27 +18,27 @@
 #define DEBUGP(x, args ...) do {} while (0)
 #endif
 
-static const ubit_t five_ones[] = { 1,1,1,1,1 };
+static const ubit_t flag_octet[] = { 0,1,1,1,1,1,1,0 };
+static const ubit_t five_ones_zero[] = { 0,1,1,1,1,1 };
 
-static int append_bit(struct hdlc_proc *hdlc, uint8_t bit, int ignore)
+static void append_bit_history(struct hdlc_proc *hdlc, uint8_t bit)
 {
 	/* we always add the bit to the history */
 	memmove(hdlc->history+1, hdlc->history, sizeof(hdlc->history)-1);
 	hdlc->history[0] = bit;
+}
 
-	/* if it is a to-be-discarded bit, we bail out */
-	if (ignore)
-		return -1;
-
+static int append_bit_out(struct hdlc_proc *hdlc, uint8_t bit)
+{
 	memmove(hdlc->next_outbyte+1, hdlc->next_outbyte, sizeof(hdlc->next_outbyte)-1);
 	hdlc->next_outbyte[0] = bit;
 	hdlc->num_bits++;
 
 	if (hdlc->num_bits == 8) {
+		hdlc->num_bits = 0;
 		pbit_t out;
 		/* generate one output byte */
 		osmo_ubit2pbit_ext(&out, 0, hdlc->next_outbyte, 0, 8, 0);
-		hdlc->num_bits = 0;
 		return out;
 	}
 
@@ -47,50 +47,27 @@ static int append_bit(struct hdlc_proc *hdlc, uint8_t bit, int ignore)
 
 static int process_hdlc_bit(struct hdlc_proc *hdlc, uint8_t bit)
 {
-	int ignore = 0;
-	int out, flag = 0;
+	int out = -1, flag = 0;
 
 	DEBUGP("bit=%u, history_in = %s, ", bit, osmo_ubit_dump(hdlc->history, sizeof(hdlc->history)));
 
-	switch (hdlc->state) {
-	case STATE_FLAG_WAIT_ZERO:
-		/* we've received 6 consecutive '1' just before and are
-		 * waiting for the final '0' to end the flag character */
-		if (bit == 0) {
-			/* it was a zero, flag character detected */
-			DEBUGP("F ");
-			flag = 1;
-			/* we're now inside the payload state */
-			hdlc->state = STATE_PAYLOAD;
-		} else {
-			/* if we received yet another '1', we re-start
-			 * from the beginning */
-			hdlc->state = STATE_INIT;
-		}
-		ignore = 1;
+	/* always append bit to history */
+	append_bit_history(hdlc, bit);
+
+	if (!memcmp(flag_octet, hdlc->history, sizeof(flag_octet))) {
 		hdlc->num_bits = 0;
-		break;
-	case STATE_PAYLOAD:
-	case STATE_INIT:
-		if (!memcmp(five_ones, hdlc->history, sizeof(five_ones))) {
-			/* five consecutive ones in the history */
-			if (bit == 1) {
-				/* one more '1' was received -> we wait
-				 * for a zero at the end of the flag
-				 * character 0x7E */
-				hdlc->state = STATE_FLAG_WAIT_ZERO;
-				/* discard bit */
-				ignore = 1;
-			} else {
-				/* discard bit */
-				ignore = 1;
-			}
-		}
-		break;
+		DEBUGP("S ");
+		flag = 1;
+	} else if (!memcmp(five_ones_zero, hdlc->history, sizeof(five_ones_zero))) {
+		/* 4.3.1 Synchronous transmission: receiver shall
+		 * discard any "0" bit after five contiguous ones */
+		DEBUGP("I ");
+	} else {
+		out = append_bit_out(hdlc, bit);
 	}
-	out = append_bit(hdlc, bit, ignore);
+
 	DEBUGP("history_out = %s", osmo_ubit_dump(hdlc->history, sizeof(hdlc->history)));
-	if (out > 0)
+	if (out >= 0)
 		DEBUGP(", out 0x%02x\n", out);
 	else
 		DEBUGP("\n");
@@ -107,6 +84,8 @@ int process_raw_hdlc(struct hdlc_proc *hdlc, uint8_t *data, unsigned int len)
 	int out;
 	static int last_out;
 
+	DEBUGP("process_raw_hdlc(%s)\n", osmo_hexdump(data,len));
+
 	for (i = 0; i < len; i ++) {
 		out = process_hdlc_bit(hdlc, data[i]);
 		if (out == -123) {
@@ -116,7 +95,7 @@ int process_raw_hdlc(struct hdlc_proc *hdlc, uint8_t *data, unsigned int len)
 			last_out = out;
 		} else if (out >= 0) {
 			/* suppress 0xAA and 0x55 bit pattern */
-			if (out != 0xaa && out != 0x55)
+			//if (out != 0xaa && out != 0x55)
 				printf("%02x ", out);
 			last_out = out;
 		}
diff --git a/src/hdlc.h b/src/hdlc.h
index 10ec8e2..2d44e75 100644
--- a/src/hdlc.h
+++ b/src/hdlc.h
@@ -2,16 +2,9 @@
 
 #include <osmocom/core/bits.h>
 
-enum hdlc_proc_state {
-	STATE_INIT,
-	STATE_FLAG_WAIT_ZERO,
-	STATE_PAYLOAD,
-};
-
 struct hdlc_proc {
 	ubit_t history[8];
 	ubit_t next_outbyte[8];
-	enum hdlc_proc_state state;
 	uint8_t num_bits;
 };