usb: disable USB_LOG_TR() when no log is configured

For some reason this has a *massive* impact on read performance, like
78 ms -> 20 ms on a full-screen read (177 kB) on the CG-50.

include/gint/usb.h

@@ -391,6 +391,8 @@ void usb_read_cancel(int pipe);
 
 /* usb_set_log(): Set the logging function for the USB driver */
 void usb_set_log(void (*logger)(char const *format, va_list args));
+/* usb_get_log(): Get the logging function */
+void (*usb_get_log(void))(char const *format, va_list args);
 /* usb_log(): Send a message to the USB log */
 void usb_log(char const *format, ...);
 
@@ -398,6 +400,8 @@ void usb_log(char const *format, ...);
    The function is called atomically, thus cannot be interrupted, therefore it
    is safe to call usb_trace() in interrupt handlers. */
 void usb_set_trace(void (*tracer)(char const *message));
+/* usb_get_trace(): Get the tracing function */
+void (*usb_get_trace(void))(char const *message);
 /* usb_trace(): Trace the current state of the driver */
 void usb_trace(char const *message);
 

src/usb/pipes.c

@@ -529,6 +529,9 @@ void usb_pipe_write_bemp(int pipe)
 #ifdef GINT_USB_DEBUG
 static void USB_LOG_TR(char const *p, asyncio_op_t *t, char const *fmt, ...)
 {
+	if(!usb_get_log())
+		return;
+
 	int E = USB.INTENB0.BRDYE;
 	USB.INTENB0.BRDYE = 0;
 

src/usb/usb.c

@@ -40,6 +40,11 @@ void usb_set_log(void (*logger)(char const *format, va_list args))
 	usb_logger = logger;
 }
 
+void (*usb_get_log(void))(char const *format, va_list args)
+{
+	return usb_logger;
+}
+
 void usb_log(char const *format, ...)
 {
 	if(!usb_logger) return;
@@ -54,6 +59,11 @@ void usb_set_trace(void (*tracer)(char const *message))
 	usb_tracer = tracer;
 }
 
+void (*usb_get_trace(void))(char const *message)
+{
+	return usb_tracer;
+}
+
 void usb_trace(char const *message)
 {
 	if(usb_tracer) {