mimxrt: Fix USB CDC handling so it works reliably.

On i.MX the SysTick IRQ cannot wake the CPU from a WFI so the CPU was
blocked on WFI waiting for USB data in mp_hal_stdin_rx_chr() even though it
had already arrived (because it may arrive just after calling the check
tud_cdc_available()).  This commit fixes this problem by using SEV/WFE to
indicate that there has been a USB event.

The mp_hal_stdout_tx_strn() function is also fixed so that it doesn't
overflow the USB buffers.
Signed-off-by: Damien George <damien@micropython.org>

ports/mimxrt/board_init.c

@@ -92,9 +92,11 @@ void SysTick_Handler(void) {
 void USB_OTG1_IRQHandler(void) {
     tud_int_handler(0);
     tud_task();
+    __SEV();
 }
 
 void USB_OTG2_IRQHandler(void) {
     tud_int_handler(1);
     tud_task();
+    __SEV();
 }

ports/mimxrt/mpconfigport.h

@@ -94,7 +94,7 @@ extern const struct _mp_obj_module_t mp_module_utime;
     do { \
         extern void mp_handle_pending(bool); \
         mp_handle_pending(true); \
-        __WFI(); \
+        __WFE(); \
     } while (0);
 
 #define MICROPY_MAKE_POINTER_CALLABLE(p) ((void *)((mp_uint_t)(p) | 1))

ports/mimxrt/mphalport.c

@@ -75,7 +75,7 @@ int mp_hal_stdin_rx_chr(void) {
                 return buf[0];
             }
         }
-        __WFI();
+        MICROPY_EVENT_POLL_HOOK
     }
 }
 
@@ -83,17 +83,16 @@ void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
     if (tud_cdc_connected()) {
         for (size_t i = 0; i < len;) {
             uint32_t n = len - i;
-            uint32_t n2 = tud_cdc_write(str + i, n);
-            if (n2 < n) {
-                while (!tud_cdc_write_flush()) {
-                    __WFI();
-                }
+            if (n > CFG_TUD_CDC_EP_BUFSIZE) {
+                n = CFG_TUD_CDC_EP_BUFSIZE;
             }
+            while (n > tud_cdc_write_available()) {
+                __WFE();
+            }
+            uint32_t n2 = tud_cdc_write(str + i, n);
+            tud_cdc_write_flush();
             i += n2;
         }
-        while (!tud_cdc_write_flush()) {
-            __WFI();
-        }
     }
     // TODO
     // while (len--) {