Add a second counter to keep track of bytes transferred by the M4.

With both counters in place, the number of bytes in the buffer is now
indicated by the difference between the M0 and M4 counts.

The M4 count needs to be increased whenever the M4 produces or consumes
data in the USB bulk buffer, so that the two counts remain correctly
synchronised.

There are three places where this is done:

1. When a USB bulk transfer in or out of the buffer completes, the count
   is increased by the number of bytes transferred. This is the most
   common case.

2. At TX startup, the M4 effectively sends the M0 16K of zeroes to
   transmit, before the first host-provided data.

   This is done by zeroing the whole 32K buffer area, and then setting
   up the first bulk transfer to write to the second 16K, whilst the M0
   begins transmission of the first 16K.

   The count is therefore increased by 16K during TX startup, to account
   for the initial 16K of zeros.

3. In sweep mode, some data is discarded. When this is done, the count
   is incremented by the size of the discarded data.

   The USB IRQ is masked whilst doing this, since a read-modify-write is
   required, and the bulk transfer completion callback may be called at
   any point, which also increases the count.

firmware/hackrf_usb/m0_state.h

@@ -27,6 +27,7 @@
 
 struct m0_state {
 	uint32_t m0_count;
+	uint32_t m4_count;
 	uint32_t tx;
 };
 

firmware/hackrf_usb/sgpio_m0.s

@@ -106,7 +106,8 @@ registers and fixed memory addresses.
 
 // Offsets into the state structure.
 .equ M0_COUNT,                             0x00
-.equ TX,                                   0x04
+.equ M4_COUNT,                             0x04
+.equ TX,                                   0x08
 
 // Our slice chain is set up as follows (ascending data age; arrows are reversed for flow):
 //     L  -> F  -> K  -> C -> J  -> E  -> I  -> A
@@ -153,6 +154,7 @@ main:
 	zero .req r0
 	mov zero, #0                                    // zero = 0                             // 1
 	str zero, [state, #M0_COUNT]                    // state.m0_count = zero                // 2
+	str zero, [state, #M4_COUNT]                    // state.m4_count = zero                // 2
 	str zero, [state, #TX]                          // state.tx = zero                      // 2
 
 loop:

firmware/hackrf_usb/usb_api_sweep.c

@@ -87,6 +87,12 @@ usb_request_status_t usb_vendor_request_init_sweep(
 	return USB_REQUEST_STATUS_OK;
 }
 
+void sweep_bulk_transfer_complete(void *user_data, unsigned int bytes_transferred)
+{
+	(void) user_data;
+	m0_state.m4_count += bytes_transferred;
+}
+
 void sweep_mode(uint32_t seq) {
 	unsigned int blocks_queued = 0;
 	unsigned int phase = 1;
@@ -134,10 +140,21 @@ void sweep_mode(uint32_t seq) {
 					&usb_endpoint_bulk_in,
 					buffer,
 					0x4000,
-					NULL, NULL
+					sweep_bulk_transfer_complete,
+					NULL
 				);
 			}
 			transfer = false;
+		} else {
+			// Account for having discarded a buffer.
+
+			// Disable USB IRQ whilst doing so, since this requires
+			// a read-modify-write, and sweep_bulk_transfer_complete()
+			// might be called from the USB ISR while we are changing
+			// this count.
+			nvic_disable_irq(NVIC_USB0_IRQ);
+			m0_state.m4_count += 0x4000;
+			nvic_enable_irq(NVIC_USB0_IRQ);
 		}
 
 		if ((dwell_blocks + THROWAWAY_BUFFERS) <= blocks_queued) {

firmware/hackrf_usb/usb_api_transceiver.c

@@ -297,6 +297,7 @@ void transceiver_startup(const transceiver_mode_t mode) {
 	activate_best_clock_source();
 	hw_sync_enable(_hw_sync_mode);
 	m0_state.m0_count = 0;
+	m0_state.m4_count = 0;
 }
 
 usb_request_status_t usb_vendor_request_set_transceiver_mode(
@@ -334,6 +335,12 @@ usb_request_status_t usb_vendor_request_set_hw_sync_mode(
 	}
 }
 
+void transceiver_bulk_transfer_complete(void *user_data, unsigned int bytes_transferred)
+{
+	(void) user_data;
+	m0_state.m4_count += bytes_transferred;
+}
+
 void rx_mode(uint32_t seq) {
 	unsigned int phase = 1;
 
@@ -349,7 +356,8 @@ void rx_mode(uint32_t seq) {
 				&usb_endpoint_bulk_in,
 				&usb_bulk_buffer[0x0000],
 				0x4000,
-				NULL, NULL
+				transceiver_bulk_transfer_complete,
+				NULL
 				);
 			phase = 0;
 		}
@@ -359,7 +367,8 @@ void rx_mode(uint32_t seq) {
 				&usb_endpoint_bulk_in,
 				&usb_bulk_buffer[0x4000],
 				0x4000,
-				NULL, NULL
+				transceiver_bulk_transfer_complete,
+				NULL
 				);
 			phase = 1;
 		}
@@ -374,12 +383,15 @@ void tx_mode(uint32_t seq) {
 	transceiver_startup(TRANSCEIVER_MODE_TX);
 
 	memset(&usb_bulk_buffer[0x0000], 0, 0x8000);
+	// Account for having filled buffer 0.
+	m0_state.m4_count += 0x4000;
 	// Set up OUT transfer of buffer 1.
 	usb_transfer_schedule_block(
 		&usb_endpoint_bulk_out,
 		&usb_bulk_buffer[0x4000],
 		0x4000,
-		NULL, NULL
+		transceiver_bulk_transfer_complete,
+		NULL
 		);
 	// Start transmitting zeros while the host fills buffer 1.
 	baseband_streaming_enable(&sgpio_config);
@@ -392,7 +404,8 @@ void tx_mode(uint32_t seq) {
 				&usb_endpoint_bulk_out,
 				&usb_bulk_buffer[0x0000],
 				0x4000,
-				NULL, NULL
+				transceiver_bulk_transfer_complete,
+				NULL
 				);
 			phase = 0;
 		}
@@ -402,7 +415,8 @@ void tx_mode(uint32_t seq) {
 				&usb_endpoint_bulk_out,
 				&usb_bulk_buffer[0x4000],
 				0x4000,
-				NULL, NULL
+				transceiver_bulk_transfer_complete,
+				NULL
 				);
 			phase = 1;
 		}

host/hackrf-tools/src/hackrf_debug.c

@@ -380,6 +380,7 @@ int write_register(hackrf_device* device, uint8_t part,
 static void print_state(hackrf_m0_state *state) {
 	printf("M0 state:\n");
 	printf("M0 count: %u bytes\n", state->m0_count);
+	printf("M4 count: %u bytes\n", state->m4_count);
 	printf("TX: %u\n", state->tx);
 }
 

host/libhackrf/src/hackrf.h

@@ -159,6 +159,8 @@ typedef struct {
 typedef struct {
 	/** Number of bytes transferred by the M0. */
 	uint32_t m0_count;
+	/** Number of bytes transferred by the M4. */
+	uint32_t m4_count;
 	/** TX flag. */
 	uint32_t tx;
 } hackrf_m0_state;