runtime: print trace level log messages to UART during startup.

There's no way to retrieve them otherwise if the startup kernel
hangs.

This commit was mistakenly removed in 88ad054.

artiq/firmware/runtime/logger.rs

@@ -1,11 +1,12 @@
 use core::{mem, ptr};
-use core::cell::RefCell;
+use core::cell::{Cell, RefCell};
 use log::{self, Log, LogLevel, LogMetadata, LogRecord, LogLevelFilter};
 use log_buffer::LogBuffer;
 use clock;
 
 pub struct BufferLogger {
-    buffer: RefCell<LogBuffer<&'static mut [u8]>>
+    buffer: RefCell<LogBuffer<&'static mut [u8]>>,
+    trace_to_uart: Cell<bool>
 }
 
 unsafe impl Sync for BufferLogger {}
@@ -15,7 +16,8 @@ static mut LOGGER: *const BufferLogger = ptr::null();
 impl BufferLogger {
     pub fn new(buffer: &'static mut [u8]) -> BufferLogger {
         BufferLogger {
-            buffer: RefCell::new(LogBuffer::new(buffer))
+            buffer: RefCell::new(LogBuffer::new(buffer)),
+            trace_to_uart: Cell::new(true)
         }
     }
 
@@ -48,6 +50,14 @@ impl BufferLogger {
     pub fn extract<R, F: FnOnce(&str) -> R>(&self, f: F) -> R {
         f(self.buffer.borrow_mut().extract())
     }
+
+    pub fn disable_trace_to_uart(&self) {
+        if self.trace_to_uart.get() {
+            trace!("disabling tracing to UART; all further trace messages \
+                    are sent to core log only");
+            self.trace_to_uart.set(false)
+        }
+    }
 }
 
 impl Log for BufferLogger {
@@ -61,7 +71,10 @@ impl Log for BufferLogger {
             writeln!(self.buffer.borrow_mut(),
                      "[{:12}us] {:>5}({}): {}",
                      clock::get_us(), record.level(), record.target(), record.args()).unwrap();
-            if record.level() <= LogLevel::Info {
+
+            // Printing to UART is really slow, so avoid doing that when we have an alternative
+            // route to retrieve the debug messages.
+            if self.trace_to_uart.get() || record.level() <= LogLevel::Info {
                 println!("[{:12}us] {:>5}({}): {}",
                          clock::get_us(), record.level(), record.target(), record.args());
             }

artiq/firmware/runtime/session.rs

@@ -590,6 +590,8 @@ pub fn thread(waiter: Waiter, spawner: Spawner) {
         }
     }
 
+    BufferLogger::with_instance(|logger| logger.disable_trace_to_uart());
+
     let addr = SocketAddr::new(IP_ANY, 1381);
     let listener = TcpListener::bind(waiter, addr).expect("cannot bind socket");
     listener.set_keepalive(true);