rp2/mphalport: Run TinyUSB stack while waiting for CDC input/output.

The recent change in bcbdee2357 means that
TinyUSB can no longer be run from within a soft (or hard) IRQ handler, ie
when the scheduler is locked.  That means that Python code that calls
`print(...)` from within a scheduled function may block indefinitely if the
USB CDC buffers are full.

This commit fixes that problem by explicitly running the TinyUSB stack when
waiting within stdio tx/rx functions.

Signed-off-by: Damien George <damien@micropython.org>

ports/rp2/mphalport.c

@@ -30,6 +30,7 @@
 #include "extmod/misc.h"
 #include "shared/runtime/interrupt_char.h"
 #include "shared/timeutils/timeutils.h"
+#include "shared/tinyusb/mp_usbd.h"
 #include "tusb.h"
 #include "uart.h"
 #include "hardware/rtc.h"
@@ -54,6 +55,19 @@ ringbuf_t stdin_ringbuf = { stdin_ringbuf_array, sizeof(stdin_ringbuf_array) };
 
 #endif
 
+#if MICROPY_HW_USB_CDC
+// Explicitly run the USB stack in case the scheduler is locked (eg we are in an
+// interrupt handler) and there is in/out data pending on the USB CDC interface.
+#define MICROPY_EVENT_POLL_HOOK_WITH_USB \
+    do { \
+        MICROPY_EVENT_POLL_HOOK; \
+        mp_usbd_task(); \
+    } while (0)
+
+#else
+#define MICROPY_EVENT_POLL_HOOK_WITH_USB MICROPY_EVENT_POLL_HOOK
+#endif
+
 #if MICROPY_HW_USB_CDC
 
 uint8_t cdc_itf_pending; // keep track of cdc interfaces which need attention to poll
@@ -135,7 +149,7 @@ int mp_hal_stdin_rx_chr(void) {
             return dupterm_c;
         }
         #endif
-        MICROPY_EVENT_POLL_HOOK
+        MICROPY_EVENT_POLL_HOOK_WITH_USB;
     }
 }
 
@@ -155,7 +169,7 @@ void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
             int timeout = 0;
             // Wait with a max of USC_CDC_TIMEOUT ms
             while (n > tud_cdc_write_available() && timeout++ < MICROPY_HW_USB_CDC_TX_TIMEOUT) {
-                MICROPY_EVENT_POLL_HOOK
+                MICROPY_EVENT_POLL_HOOK_WITH_USB;
             }
             if (timeout >= MICROPY_HW_USB_CDC_TX_TIMEOUT) {
                 break;