add buffer check before triggering new usb read

ports/atmel-samd/usb.c

@@ -25,6 +25,7 @@
  */
 
 #include "usb.h"
+#include <stdio.h>
 
 #include <stdint.h>
 
@@ -49,6 +50,7 @@
 #include "usb_mass_storage.h"
 
 #include "supervisor/shared/autoreload.h"
+#include "supervisor/serial.h"
 
 // Store received characters on our own so that we can filter control characters
 // and act immediately on CTRL-C for example.
@@ -126,7 +128,7 @@ static bool read_complete(const uint8_t ep, const enum usb_xfer_code rc, const u
         atomic_leave_critical(&flags);
         return true;
     }
-
+    
     for (uint16_t i = 0; i < count; i++) {
         uint8_t c = cdc_packet_buffer[i];
         if (c == mp_interrupt_char) {
@@ -150,15 +152,15 @@ static bool read_complete(const uint8_t ep, const enum usb_xfer_code rc, const u
         }
     }
     atomic_leave_critical(&flags);
-
+    
     // Trigger a follow up read if we have space.
-    if (usb_rx_count < USB_RX_BUF_SIZE) {
+    /*if (usb_rx_count < USB_RX_BUF_SIZE - 64) {
         int32_t result = start_read();
         if (result != ERR_NONE) {
             return true;
         }
-    }
-
+    }*/
+    
     /* No error. */
     return false;
 }
@@ -170,7 +172,9 @@ static bool write_complete(const uint8_t ep,
         return false; // No errors.
     }
     // This is called after writes are finished.
+
     usb_transmitting = false;
+
     /* No error. */
     return false;
 }
@@ -243,9 +247,16 @@ static bool cdc_enabled(void) {
 }
 
 bool usb_bytes_available(void) {
+    // Check if the buffer has data, but not enough
+    // space to hold another read.
+    if (usb_rx_count > 64) {
+        return usb_rx_count > 0;
+    }
+    // Buffer has enough room
     if (cdc_enabled() && !pending_read) {
         start_read();
     }
+    // Buffer is empty and/or no new data is available
     if (usb_rx_count == 0) {
         return false;
     }
@@ -263,16 +274,16 @@ int usb_read(void) {
     data = usb_rx_buf[usb_rx_buf_head];
     usb_rx_buf_head++;
     usb_rx_count--;
-    if ((USB_RX_BUF_SIZE) == usb_rx_buf_head) {
+    if (usb_rx_buf_head == USB_RX_BUF_SIZE) {
       usb_rx_buf_head = 0;
     }
     CRITICAL_SECTION_LEAVE();
 
     // Trigger a new read because we just cleared some space.
-    if (!pending_read && usb_rx_count == USB_RX_BUF_SIZE - 1) {
+    /*if (!pending_read && usb_rx_count == USB_RX_BUF_SIZE - 1) {
         start_read();
-    }
-
+    }*/
+     
     return data;
 }