sdr: properly flush the transmit buffers

impl/src/main.c

@@ -264,16 +264,13 @@ int main(void)
 				if(!on_air) {
 					RESULT_CHECK(sdr_stop_rx(&sdr));
 					RESULT_CHECK(sdr_start_tx(&sdr, burst_len * SDR_OVERSAMPLING));
-
-					// hopefully no longer needed with latest libhackrf
-					//RESULT_CHECK(sdr_flush_tx_buffer(&sdr));
 				}
 
 				RESULT_CHECK(transmit_in_chunks(&sdr, whole_burst, burst_len));
 
 				on_air = true;
 			} else if(on_air) { // ret == 0
-				//RESULT_CHECK(sdr_flush_tx_buffer(&sdr));
+				RESULT_CHECK(sdr_flush_tx_buffer(&sdr));
 
 				RESULT_CHECK(sdr_stop_tx(&sdr));
 				RESULT_CHECK(sdr_start_rx(&sdr));

impl/src/sdr/sdr.c

@@ -67,6 +67,7 @@ static inline int8_t clamp_float2int8(float s)
 static int tx_callback(hackrf_transfer *transfer)
 {
 	int result;
+	int retcode = 0;
 
 	sdr_ctx_t *sdr_ctx = (sdr_ctx_t*)transfer->tx_ctx;
 
@@ -86,9 +87,15 @@ static int tx_callback(hackrf_transfer *transfer)
 	result = cbuffercf_read(sdr_ctx->tx_buf, samples_requested, &samples, &samples_read);
 	if(result != LIQUID_OK) {
 		fprintf(stderr, "cbuffercf_read failed: %d. Samples are lost.\n", result);
+		sem_post(&sdr_ctx->buf_sem);
 		return HACKRF_ERROR_OTHER;
 	}
 
+	if(samples_read == 0) {
+		// buffer has run empty, so this is the end of the transmission
+		sdr_ctx->tx_done = true;
+	}
+
 	for(size_t i = 0; i < samples_read; i++) {
 		transfer->buffer[2*i + 0] = clamp_float2int8(creal(samples[i]));
 		transfer->buffer[2*i + 1] = clamp_float2int8(cimag(samples[i]));
@@ -111,7 +118,7 @@ static int tx_callback(hackrf_transfer *transfer)
 		return HACKRF_ERROR_OTHER;
 	}
 
-	return 0;
+	return retcode;
 }
 
 static result_t stop_streaming(sdr_ctx_t *ctx)
@@ -282,12 +289,13 @@ result_t sdr_transmit(sdr_ctx_t *ctx, const float complex *samples, size_t nsamp
 		return ERR_LIQUID;
 	}
 
+	ctx->tx_done = false;
+
 	if(sem_post(&ctx->buf_sem) < 0) {
 		perror("sem_post");
 		return ERR_SDR;
 	}
 
-
 	return OK;
 }
 
@@ -334,8 +342,10 @@ result_t sdr_receive(sdr_ctx_t *ctx, float complex *samples, size_t *nsamples, l
 
 result_t sdr_flush_tx_buffer(sdr_ctx_t *ctx)
 {
-	// currently not implemented
-	(void)ctx;
+	while(!ctx->tx_done) {
+		struct timespec ts_1ms = {0, 1000000};
+		nanosleep(&ts_1ms, NULL);
+	}
 
 	return OK;
 }

impl/src/sdr/sdr.h

@@ -3,6 +3,7 @@
 
 #include <complex.h>
 #include <semaphore.h>
+#include <stdbool.h>
 
 #include <libhackrf/hackrf.h>
 
@@ -30,6 +31,7 @@ typedef struct {
 	sem_t          buf_sem;
 
 	sdr_status_t   status;
+	bool           tx_done;
 } sdr_ctx_t;
 
 result_t sdr_init(sdr_ctx_t *ctx);