altos: Incremental viterbi decode

Decode radio input one interleave block at a time. This overlaps the
decode computation with the packet reception, leading to lower latency
in an attempt to keep up with the transmitter.

Signed-off-by: Keith Packard <keithp@keithp.com>

4 files changed, 39 insertions, 11 deletions

diff --git a/src/core/ao_fec.h b/src/core/ao_fec.h
index e3c55d6e..4fd398eb 100644
--- a/src/core/ao_fec.h
+++ b/src/core/ao_fec.h
@@ -66,8 +66,10 @@ ao_fec_encode(uint8_t *in, uint8_t len, uint8_t *out);
  * 'out' must be len/8 bytes long
  */
 
+#define AO_FEC_DECODE_BLOCK	(8 * 32)	/* callback must return multiples of this many bits */
+
 uint8_t
-ao_fec_decode(uint8_t *in, uint16_t in_len, uint8_t *out, uint8_t out_len);
+ao_fec_decode(uint8_t *in, uint16_t in_len, uint8_t *out, uint8_t out_len, uint16_t (*callback)());
 
 /*
  * Interleave data packed in bytes. 'out' must be 'len' bytes long.
diff --git a/src/core/ao_viterbi.c b/src/core/ao_viterbi.c
index 77681556..69e9c1f5 100644
--- a/src/core/ao_viterbi.c
+++ b/src/core/ao_viterbi.c
@@ -91,7 +91,7 @@ ao_cost(struct ao_soft_sym a, struct ao_soft_sym b)
  */
 
 uint8_t
-ao_fec_decode(uint8_t *in, uint16_t len, uint8_t *out, uint8_t out_len)
+ao_fec_decode(uint8_t *in, uint16_t len, uint8_t *out, uint8_t out_len, uint16_t (*callback)())
 {
 	static uint16_t	cost[2][NUM_STATE];		/* path cost */
 	static uint16_t	bits[2][NUM_STATE];		/* save bits to quickly output them */
@@ -105,6 +105,7 @@ ao_fec_decode(uint8_t *in, uint16_t len, uint8_t *out, uint8_t out_len)
 	const uint8_t	*whiten = ao_fec_whiten_table;
 	uint16_t	interleave;			/* input byte array index */
 	struct ao_soft_sym	s;			/* input symbol pair */
+	uint16_t	avail;
 
 	p = 0;
 	for (state = 0; state < NUM_STATE; state++) {
@@ -113,11 +114,22 @@ ao_fec_decode(uint8_t *in, uint16_t len, uint8_t *out, uint8_t out_len)
 	}
 	cost[0][0] = 0;
 
+	if (callback)
+		avail = 0;
+	else
+		avail = len;
+
 	o = 0;
 	for (i = 0; i < len; i += 2) {
 		b = i/2;
 		n = p ^ 1;
 
+		if (!avail) {
+			avail = callback();
+			if (!avail)
+				break;
+		}
+
 		/* Fetch one pair of input bytes, de-interleaving
 		 * the input.
 		 */
diff --git a/src/drivers/ao_cc1120.c b/src/drivers/ao_cc1120.c
index b42ca54c..394cf7c5 100644
--- a/src/drivers/ao_cc1120.c
+++ b/src/drivers/ao_cc1120.c
@@ -400,6 +400,7 @@ ao_radio_send(void *d, uint8_t size)
 
 static uint8_t	rx_data[2048];
 static uint16_t	rx_data_count;
+static uint16_t	rx_data_consumed;
 static uint16_t rx_data_cur;
 static uint8_t	rx_ignore;
 
@@ -408,9 +409,9 @@ ao_radio_rx_isr(void)
 {
 	if (rx_ignore == 0) {
 		rx_data[rx_data_cur++] = stm_spi2.dr;
-		if (rx_data_cur >= rx_data_count) {
+		if (rx_data_cur >= rx_data_count)
 			ao_exti_disable(&AO_CC1120_INT_PORT, AO_CC1120_INT_PIN);
-			ao_radio_wake = 1;
+		if (rx_data_cur - rx_data_consumed >= AO_FEC_DECODE_BLOCK) {
 			ao_wakeup(&ao_radio_wake);
 		}
 	} else {
@@ -420,6 +421,19 @@ ao_radio_rx_isr(void)
 	stm_spi2.dr = 0x00;
 }
 
+static uint16_t
+ao_radio_rx_wait(void)
+{
+	cli();
+	while (rx_data_cur - rx_data_consumed < AO_FEC_DECODE_BLOCK &&
+	       !ao_radio_abort)
+		ao_sleep(&ao_radio_wake);
+	sei();
+	if (ao_radio_abort)
+		return 0;
+	return AO_FEC_DECODE_BLOCK;
+}
+
 uint8_t
 ao_radio_recv(__xdata void *d, uint8_t size)
 {
@@ -433,6 +447,7 @@ ao_radio_recv(__xdata void *d, uint8_t size)
 	len *= 2;			/* 1/2 rate convolution */
 	rx_data_count = len * 8;	/* bytes to bits */
 	rx_data_cur = 0;
+	rx_data_consumed = 0;
 	rx_ignore = 2;
 
 	printf ("len %d rx_data_count %d\n", len, rx_data_count);
@@ -461,10 +476,9 @@ ao_radio_recv(__xdata void *d, uint8_t size)
 	ao_radio_strobe(CC1120_SRX);
 
 	ao_radio_burst_read_start(CC1120_SOFT_RX_DATA_OUT);
-	cli();
-	while (!ao_radio_wake && !ao_radio_abort)
-		ao_sleep(&ao_radio_wake);
-	sei();
+
+	ao_fec_decode(rx_data, rx_data_count, d, size + 2, ao_radio_rx_wait);
+
 	ao_radio_burst_read_stop();
 
 	/* Convert from 'real' rssi to cc1111-style values */
@@ -477,7 +491,7 @@ ao_radio_recv(__xdata void *d, uint8_t size)
 
 	/* Construct final packet */
 
-	ao_fec_decode(rx_data, rx_data_cur, d, size + 2);
+	ao_fec_decode(rx_data, rx_data_cur, d, size + 2, 0);
 
 	if (ao_fec_check_crc(d, size))
 		((uint8_t *) d)[size + 1] = 0x80;
diff --git a/src/test/ao_fec_test.c b/src/test/ao_fec_test.c
index 98067486..e8d3d8b1 100644
--- a/src/test/ao_fec_test.c
+++ b/src/test/ao_fec_test.c
@@ -266,7 +266,7 @@ ao_real_packet(void)
 	int off;
 
 	for (off = 0; off < sizeof (real_packet) - 576;  off++) {
-		decode_len = ao_fec_decode(real_packet+off, 576, decode);
+		decode_len = ao_fec_decode(real_packet+off, 576, decode, 34, NULL);
 
 		if (ao_fec_check_crc(decode, 32)) {
 			printf ("match at %d\n", off);
@@ -317,7 +317,7 @@ main(int argc, char **argv)
 		receive_len = transmit_len;
 		
 		/* Decode it */
-		decode_len = ao_fec_decode(receive, receive_len, decode);
+		decode_len = ao_fec_decode(receive, receive_len, decode, original_len + 2, NULL);
 
 		/* Check to see if we received the right data */
 		error = 0;