Add hackrf_transfer option to display buffer stats.

This adds the `hackrf_transfer -B` option, which displays the number of
bytes currently in the buffer along with the existing per-second stats.

The number of bytes in the buffer is indicated by the difference between
the M0 and M4 byte counters. In TX, the M4 count should lead the M0 count.
In RX, the M0 count should lead the M4 count.

host/hackrf-tools/src/hackrf_transfer.c

@@ -377,6 +377,8 @@ bool limit_num_samples = false;
 uint64_t samples_to_xfer = 0;
 size_t bytes_to_xfer = 0;
 
+bool display_stats = false;
+
 bool baseband_filter_bw = false;
 uint32_t baseband_filter_bw_hz = 0;
 
@@ -533,6 +535,7 @@ static void usage() {
 /* The required atomic load/store functions aren't available when using C with MSVC */
 	printf("\t[-S buf_size] # Enable receive streaming with buffer size buf_size.\n");
 #endif
+	printf("\t[-B] # Print buffer statistics during transfer\n");
 	printf("\t[-c amplitude] # CW signal source mode, amplitude 0-127 (DC value to DAC).\n");
         printf("\t[-R] # Repeat TX mode (default is off) \n");
 	printf("\t[-b baseband_filter_bw_hz] # Set baseband filter bandwidth in Hz.\n\tPossible values: 1.75/2.5/3.5/5/5.5/6/7/8/9/10/12/14/15/20/24/28MHz, default <= 0.75 * sample_rate_hz.\n" );
@@ -580,7 +583,7 @@ int main(int argc, char** argv) {
 	float time_diff;
 	unsigned int lna_gain=8, vga_gain=20, txvga_gain=0;
   
-	while( (opt = getopt(argc, argv, "H:wr:t:f:i:o:m:a:p:s:n:b:l:g:x:c:d:C:RS:h?")) != EOF )
+	while( (opt = getopt(argc, argv, "H:wr:t:f:i:o:m:a:p:s:n:b:l:g:x:c:d:C:RS:Bh?")) != EOF )
 	{
 		result = HACKRF_SUCCESS;
 		switch( opt ) 
@@ -668,6 +671,10 @@ int main(int argc, char** argv) {
 			bytes_to_xfer = samples_to_xfer * 2ull;
 			break;
 
+		case 'B':
+			display_stats = true;
+			break;
+
 		case 'b':
 			result = parse_frequency_u32(optarg, endptr, &baseband_filter_bw_hz);
 			baseband_filter_bw = true;
@@ -1097,12 +1104,25 @@ int main(int argc, char** argv) {
 			    double	dB_full_scale_ratio = 10*log10(full_scale_ratio);
 			    if (dB_full_scale_ratio > 1)
 			    	dB_full_scale_ratio = NAN;	// Guard against ridiculous reports
-			    fprintf(stderr, "%4.1f MiB / %5.3f sec = %4.1f MiB/second, amplitude %3.1f dBfs\n",
+			    fprintf(stderr, "%4.1f MiB / %5.3f sec = %4.1f MiB/second, amplitude %3.1f dBfs",
 				    (byte_count_now / 1e6f),
 				    time_difference,
 				    (rate / 1e6f),
 				    dB_full_scale_ratio
 			    );
+			    if (display_stats) {
+				    bool tx = transmit || signalsource;
+				    hackrf_m0_state state;
+				    result = hackrf_get_m0_state(device, &state);
+				    if (result != HACKRF_SUCCESS)
+					    fprintf(stderr, "\nhackrf_get_m0_state() failed: %s (%d)\n", hackrf_error_name(result), result);
+				    else
+					    fprintf(stderr, ", %d bytes %s in buffer\n",
+						    tx ? state.m4_count - state.m0_count : state.m0_count - state.m4_count,
+						    tx ? "filled" : "free");
+			    } else {
+				    fprintf(stderr, "\n");
+			    }
 			}
 
 			time_start = time_now;