Keep track of longest shortfall.

This adds six cycles to the TX and RX shortfall paths.

firmware/hackrf_usb/m0_state.h

@@ -30,6 +30,7 @@ struct m0_state {
 	uint32_t m0_count;
 	uint32_t m4_count;
 	uint32_t num_shortfalls;
+	uint32_t longest_shortfall;
 };
 
 enum m0_mode {

firmware/hackrf_usb/sgpio_m0.s

@@ -67,9 +67,9 @@ shadow registers.
 There are four key code paths, with the following worst-case timings:
 
 RX, normal:     147 cycles
-RX, overrun:    65 cycles
+RX, overrun:    71 cycles
 TX, normal:     133 cycles
-TX, underrun:   129 cycles
+TX, underrun:   135 cycles
 
 Design
 ======
@@ -112,6 +112,7 @@ registers and fixed memory addresses.
 .equ M0_COUNT,                             0x04
 .equ M4_COUNT,                             0x08
 .equ NUM_SHORTFALLS,                       0x0C
+.equ LONGEST_SHORTFALL,                    0x10
 
 // Operating modes.
 .equ MODE_IDLE,                            0
@@ -172,6 +173,7 @@ main:
 	str zero, [state, #M0_COUNT]                    // state.m0_count = zero                // 2
 	str zero, [state, #M4_COUNT]                    // state.m4_count = zero                // 2
 	str zero, [state, #NUM_SHORTFALLS]              // state.num_shortfalls = zero          // 2
+	str zero, [state, #LONGEST_SHORTFALL]           // state.longest_shortfall = zero       // 2
 
 idle:
 	// Wait for RX or TX mode to be set.
@@ -185,6 +187,7 @@ idle:
 	str zero, [state, #M0_COUNT]                    // state.m0_count = zero                // 2
 	str zero, [state, #M4_COUNT]                    // state.m4_count = zero                // 2
 	str zero, [state, #NUM_SHORTFALLS]              // state.num_shortfalls = zero          // 2
+	str zero, [state, #LONGEST_SHORTFALL]           // state.longest_shortfall = zero       // 2
 	mov shortfall_length, zero                      // shortfall_length = zero              // 1
 
 loop:
@@ -299,6 +302,13 @@ extend_shortfall:
 	add length, #32                                 // length += 32                         // 1
 	mov shortfall_length, length                    // shortfall_length = length            // 1
 
+	// Is this now the longest shortfall?
+	longest .req r1
+	ldr longest, [state, #LONGEST_SHORTFALL]        // longest = state.longest_shortfall    // 2
+	cmp length, longest                             // if length <= longest:                // 1
+	blt loop                                        //      goto loop                       // 1 thru, 3 taken
+	str length, [state, #LONGEST_SHORTFALL]         // state.longest_shortfall = length     // 2
+
 	// Return to main loop.
 	b loop                                          // goto loop                            // 3
 

host/hackrf-tools/src/hackrf_debug.c

@@ -389,6 +389,7 @@ static void print_state(hackrf_m0_state *state) {
 	printf("M0 count: %u bytes\n", state->m0_count);
 	printf("M4 count: %u bytes\n", state->m4_count);
 	printf("Number of shortfalls: %u\n", state->num_shortfalls);
+	printf("Longest shortfall: %u bytes\n", state->longest_shortfall);
 }
 
 static void usage() {

host/hackrf-tools/src/hackrf_transfer.c

@@ -1159,11 +1159,12 @@ int main(int argc, char** argv) {
 				    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, %u %s\n",
+					    fprintf(stderr, ", %d bytes %s in buffer, %u %s, longest %u bytes\n",
 						    tx ? state.m4_count - state.m0_count : state.m0_count - state.m4_count,
 						    tx ? "filled" : "free",
 						    state.num_shortfalls,
-						    tx ? "underruns" : "overruns");
+						    tx ? "underruns" : "overruns",
+						    state.longest_shortfall);
 			    } else {
 				    fprintf(stderr, "\n");
 			    }
@@ -1219,11 +1220,12 @@ int main(int argc, char** argv) {
 					"Transfer statistics:\n"
 					"%lu bytes transferred by M0\n"
 					"%lu bytes transferred by M4\n"
-					"%u %s\n",
+					"%u %s, longest %u bytes\n",
 					stats.m0_total,
 					stats.m4_total,
 					state.num_shortfalls,
-					(transmit || signalsource) ? "underruns" : "overruns");
+					(transmit || signalsource) ? "underruns" : "overruns",
+					state.longest_shortfall);
 			}
 		}
 

host/libhackrf/src/hackrf.h

@@ -165,6 +165,8 @@ typedef struct {
 	uint32_t m4_count;
 	/** Number of shortfalls. */
 	uint32_t num_shortfalls;
+	/** Longest shortfall. */
+	uint32_t longest_shortfall;
 } hackrf_m0_state;
 
 struct hackrf_device_list {