gdb: allow gdb stub to automatically start when a crash occurs

The user can still gdb_start() + gdb_main(NULL) manually, which allows
e.g. early setup of breakpoints. The start_on_except mechanism is the
lazier method where we just run the add-in normally and start fxsdk gdb
on the PC *after* a crash occurs.

include/gint/gdb.h

@@ -11,19 +11,12 @@ extern "C" {
 
 #include <stdint.h>
 
-/* Error codes for GDB functions */
-enum {
-	GDB_NO_INTERFACE = -1,
-	GDB_ALREADY_STARTED = -2,
-	GDB_USB_ERROR = -3,
-};
-
 /* gdb_cpu_state_t: State of the CPU when breaking
    This struct keep the same register indices as those declared by GDB to allow
    easy R/W without needing a "translation" table.
    See : https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gdb/sh-tdep.c;
          h=c402961b80a0b4589243023ea5362d43f644a9ec;hb=4f3e26ac6ee31f7bc4b04abd
-         8bdb944e7f1fc5d2#l327
+         8bdb944e7f1fc5d2#l361
  */
 // TODO : Should we expose r*b*, ssr, spc ? are they double-saved when breaking
 //        inside an interrupt handler ?
@@ -60,17 +53,27 @@ typedef struct {
 
 /* gdb_start(): Start the GDB remote serial protocol server
 
-   This function will start the GDB remote serial protocol implementation and
-   block until the program is resumed from the connected debugger.
-   It currently only supports USB communication and will fail if USB is already
-   in use.*/
+   This function starts the GDB stub (program that communicates with GDB). It
+   takes over USB, and returns once communication is established with a GDB
+   instance on the computer. Normally this is called by the panic handler after
+   a crash. It can also be called manually, in which case it should be followed
+   by a call to gdb_main() with the current state (NULL if you don't have any).
+
+   If the GDB stub is already started, this is a no-op. Returns 0 on success
+   and a negative value if the USB setup fails. */
 int gdb_start(void);
 
 /* gdb_main(): Main GDB loop
+   Main loop for when the program is in a paused state. Communicates with GDB
+   over USB and mutates the cpu_state struct, memory and the UBC configuration
+   accordingly. Returns once the program should resume. */
+void gdb_main(gdb_cpu_state_t *cpu_state);
 
-   This function handles GDB messages sent over USB and will mutate the cpu_state
-   struct, memory and the UBC configuration accordingly. */
-void gdb_main(gdb_cpu_state_t* cpu_state);
+/* gdb_start_on_exception(): Set the GDB stub to autostart on crashes
+   This function sets a crash handler that starts the GDB stub whenever a
+   System ERROR occurs. */
+// TODO: Get some visible signal on-screen when that happens
+void gdb_start_on_exception(void);
 
 #ifdef __cplusplus
 }

include/gint/usb.h

@@ -90,6 +90,9 @@ void usb_open_wait(void);
 /* usb_is_open(): Check whether the USB link is active */
 bool usb_is_open(void);
 
+/* usb_is_open_interface(): Check whether a particular interface is open */
+bool usb_is_open_interface(usb_interface_t const *interface);
+
 /* usb_close(): Close the USB link
 
    This function closes the link opened by usb_open(), and notifies the host of

src/gdb/gdb.c

@@ -38,11 +38,7 @@ static uint32_t gdb_unhexlify(const char* input_string)
 	return gdb_unhexlify_sized(input_string, strlen(input_string));
 }
 
-static enum {
-	GDB_STATE_STOPPED,
-	GDB_STATE_FIRST_BREAK,
-	GDB_STATE_STARTED,
-} GPACKEDENUM gdb_state = GDB_STATE_STOPPED;
+static bool gdb_started = false;
 
 static void gdb_send(const char *data, size_t size)
 {
@@ -55,6 +51,16 @@ static void gdb_send(const char *data, size_t size)
 	usb_commit_sync(pipe);
 }
 
+static void gdb_send_start(void)
+{
+	usb_fxlink_header_t header;
+	usb_fxlink_fill_header(&header, "gdb", "start", 0);
+
+	int pipe = usb_ff_bulk_output();
+	usb_write_sync(pipe, &header, sizeof(header), false);
+	usb_commit_sync(pipe);
+}
+
 static char *gdb_recv_buffer = NULL;
 static const size_t gdb_recv_buffer_capacity = 256;
 static size_t gdb_recv_buffer_size = 0;
@@ -488,6 +494,13 @@ static void gdb_handle_single_step(uint32_t pc, ubc_break_mode_t break_mode)
 
 void gdb_main(gdb_cpu_state_t* cpu_state)
 {
+	if (!gdb_started && gdb_start()) {
+		// TODO: Visual signal "GDB stub error"
+		return;
+	}
+
+	// TODO: Visual signal "GDB stub idle"
+
 	if (gdb_single_step_backup.single_stepped) {
 		if (gdb_single_step_backup.channel0_used) {
 			ubc_set_breakpoint(0, gdb_single_step_backup.channel0_addr, UBC_BREAK_BEFORE);
@@ -508,6 +521,8 @@ void gdb_main(gdb_cpu_state_t* cpu_state)
 	}
 
 	while (1) {
+		// TODO: Visual signal "GDB stub communicating"
+
 		char packet_buffer[256];
 		ssize_t packet_size = gdb_recv_packet(packet_buffer, sizeof(packet_buffer));
 		if (packet_size <= 0) {
@@ -515,6 +530,8 @@ void gdb_main(gdb_cpu_state_t* cpu_state)
 			continue;
 		}
 
+		// TODO: Visual signal "GDB stub working"
+
 		switch (packet_buffer[0]) {
 			case '?': // Halt reason
 				gdb_send_stop_reply();
@@ -544,7 +561,6 @@ void gdb_main(gdb_cpu_state_t* cpu_state)
 
 			case 'k': // Kill request
 				abort();
-				return;
 
 			case 'Z':
 				gdb_handle_insert_hardware_breakpoint(packet_buffer);
@@ -555,21 +571,27 @@ void gdb_main(gdb_cpu_state_t* cpu_state)
 
 			case 's':
 				gdb_handle_single_step(cpu_state->reg.pc, UBC_BREAK_AFTER);
-				return;
+				goto ret;
 			case 'c': // Continue
-				return;
+				goto ret;
 
 			default: // Unsupported packet
 				gdb_send_packet(NULL, 0);
 				break;
 		}
+
+		// TODO: Visual signal "GDB stub idle"
 	}
+
+ret:
+	// We're started after the first round of exchanges
+	gdb_started = true;
 }
 
 static void gdb_notifier_function(void)
 {
 	// We ignore fxlink notifications when we're already inside GDB code.
-	if (ubc_dbh_lock || gdb_state != GDB_STATE_STARTED)
+	if (ubc_dbh_lock || !gdb_started)
 		return;
 
 	// We make sure we are called during a USB interrupt.
@@ -585,7 +607,7 @@ static void gdb_notifier_function(void)
 
 static int gdb_panic_handler(uint32_t code)
 {
-	// If we are currently expecting to handle TLB misses
+	// Catch memory access errors from GDB trying to read/print stuff
 	if (gdb_tlbh_enable) {
 		// We only handle TLB miss reads (0x040) and writes (0x060)
 		if (code != 0x040 && code != 0x060)
@@ -598,11 +620,12 @@ static int gdb_panic_handler(uint32_t code)
 		return 0;
 	}
 	// If we are in user code, let's break
-	else if (!ubc_dbh_lock && gdb_state == GDB_STATE_STARTED) {
+	else if (!ubc_dbh_lock) {
 		// We make sure an other step break is not already set up
 		if (gdb_single_step_backup.single_stepped)
 			return 1;
 
+		// TODO: This only works for re-execution type exceptions
 		uint32_t spc;
 		__asm__("stc spc, %0" : "=r"(spc));
 		gdb_handle_single_step(spc, UBC_BREAK_BEFORE);
@@ -613,32 +636,35 @@ static int gdb_panic_handler(uint32_t code)
 
 int gdb_start(void)
 {
-	if (gdb_state != GDB_STATE_STOPPED) {
-		return GDB_ALREADY_STARTED;
+	if (gdb_started)
+		return 0;
+
+	if(usb_is_open() && !usb_is_open_interface(&usb_ff_bulk))
+		usb_close();
+
+	if(!usb_is_open()) {
+		usb_interface_t const *interfaces[] = { &usb_ff_bulk, NULL };
+		if(usb_open(interfaces, GINT_CALL_NULL) < 0)
+			return -1;
+		usb_open_wait();
 	}
 
-	if (usb_is_open()) {
-		return GDB_NO_INTERFACE;
-	}
+	usb_fxlink_set_notifier(gdb_notifier_function);
+	gdb_send_start();
 
 	if (!gdb_recv_buffer) {
 		gdb_recv_buffer = malloc(gdb_recv_buffer_capacity);
 	}
 
-	usb_interface_t const *interfaces[] = { &usb_ff_bulk, NULL };
-	if (usb_open(interfaces, GINT_CALL_NULL) < 0) {
-		return GDB_USB_ERROR;
-	}
-	usb_open_wait();
-	usb_fxlink_set_notifier(gdb_notifier_function);
-
 	// TODO : Should we detect if other panic or debug handlers are setup ?
 	gint_exc_catch(gdb_panic_handler);
-
 	ubc_set_debug_handler(gdb_main);
 
-	gdb_state = GDB_STATE_FIRST_BREAK;
-	gdb_main(NULL);
-	gdb_state = GDB_STATE_STARTED;
 	return 0;
 }
+
+void gdb_start_on_exception(void)
+{
+	gint_exc_catch(gdb_panic_handler);
+	ubc_set_debug_handler(gdb_main);
+}

src/usb/configure.c

@@ -8,7 +8,8 @@
 //---
 
 /* Current configuration: list of interfaces and endpoint assignments */
-static usb_interface_t const *conf_if[16];
+#define MAX_INTERFACES 16 /* actually 15 */
+static usb_interface_t const *conf_if[MAX_INTERFACES];
 static endpoint_t *conf_ep;
 
 GCONSTRUCTOR static void usb_alloc(void)
@@ -86,7 +87,7 @@ static int find_pipe(int type)
 int usb_configure_solve(usb_interface_t const **interfaces)
 {
 	/* Reset the previous configuration */
-	memset(conf_if, 0, 16 * sizeof *conf_if);
+	memset(conf_if, 0, MAX_INTERFACES * sizeof *conf_if);
 	memset(conf_ep, 0, 32 * sizeof *conf_ep);
 
 	/* Next interface number to assign */
@@ -98,7 +99,8 @@ int usb_configure_solve(usb_interface_t const **interfaces)
 
 	for(int i = 0; interfaces[i]; i++)
 	{
-		if(i == 16) return USB_OPEN_TOO_MANY_INTERFACES;
+		if(i == MAX_INTERFACES - 1)
+			return USB_OPEN_TOO_MANY_INTERFACES;
 		usb_interface_t const *intf = interfaces[i];
 
 		conf_if[next_interface++] = intf;
@@ -180,7 +182,7 @@ void usb_configure_log(void)
 {
 #ifdef GINT_USB_DEBUG
 	/* Log the final configuration */
-	for(int i = 0; i < 16 && conf_if[i]; i++)
+	for(int i = 0; conf_if[i]; i++)
 		USB_LOG("Interface #%d: %p\n", i, conf_if[i]);
 
 	for(int i = 0; i < 32; i++)

src/usb/usb.c

@@ -229,6 +229,19 @@ bool usb_is_open(void)
 	return usb_open_status;
 }
 
+bool usb_is_open_interface(usb_interface_t const *interface)
+{
+	if(!usb_is_open())
+		return false;
+
+	usb_interface_t const * const *open = usb_configure_interfaces();
+	for(int i = 0; open[i]; i++) {
+		if(open[i] == interface)
+			return true;
+	}
+	return false;
+}
+
 void usb_open_wait(void)
 {
 	while(!usb_open_status) sleep();