test(port_arm_m): keep the USB device alive after completing each test

The USB device is used to deliver the test result and receive commands,
so it must be kept alive even after the test completion.

All tests are now passing on `rp_pico`.

Author: yvt

src/r3_port_arm_m_test_driver/src/board_rp2040.rs

@@ -18,8 +18,15 @@ fn panic(info: &PanicInfo) -> ! {
 
     r3_support_rp2040::sprintln!("{}{}", mux::BEGIN_MAIN, info);
 
-    // TODO: keep polling
-    loop {}
+    enter_poll_loop();
+}
+
+/// Start polling USB so that we can deliver the test result and reset the
+/// device when requested.
+pub fn enter_poll_loop() -> ! {
+    loop {
+        usbstdio::poll::<Options>();
+    }
 }
 
 struct Logger;

src/r3_port_arm_m_test_driver/src/main.rs

@@ -49,6 +49,9 @@ macro_rules! instantiate_test {
                 crate::board_rp2040::mux::BEGIN_MAIN,
             );
 
+            #[cfg(feature = "board-rp_pico")]
+            board_rp2040::enter_poll_loop();
+
             loop {}
         }
 

src/r3_support_rp2040/src/usbstdio.rs

@@ -221,24 +221,6 @@ impl UsbStdioGlobal {
     }
 }
 
-// TODO:
-//
-//   - The USB controller needs to be periodically polled to work correctly.
-//     The panic handler should poll USB instead of doing nothing.
-//
-//   - We also need to handle incoming data. The test driver will need this to
-//     hold off the test execution until requested and to prepare the target for
-//     a subsequent test run.
-//
-//   - If there's incoming data, the interrupt will not be deassserted until
-//     `SerialPort::read` is called. And `SerialPort::read` does not consume
-//     the incoming data if its internal buffer is full.
-//
-//     The only way to do flow control seems to be disabling or ignoring USB
-//     interrupts. Of course, this can only be done for a few milliseconds, the
-//     upper bound defined by the USB specification.
-//
-
 struct Deque<T, const LEN: usize> {
     buf: [T; LEN],
     start: usize,