stm32/usbd_cdc_interface: Add CTS flow control option for USB VCP.

Enabled by default, but disabled when REPL is connected to the VCP (this is
the existing behaviour).  Can be configured at run-time with, eg:

    pyb.USB_VCP().init(flow=pyb.USB_VCP.RTS | pyb.USB_VCP.CTS)

ports/stm32/usb.c

@@ -634,6 +634,12 @@ STATIC void pyb_usb_vcp_print(const mp_print_t *print, mp_obj_t self_in, mp_prin
 
 void usb_vcp_attach_to_repl(const pyb_usb_vcp_obj_t *self, bool attached) {
     self->cdc_itf->attached_to_repl = attached;
+    if (attached) {
+        // Default behavior is non-blocking when attached to repl
+        self->cdc_itf->flow &= ~USBD_CDC_FLOWCONTROL_CTS;
+    } else {
+        self->cdc_itf->flow |= USBD_CDC_FLOWCONTROL_CTS;
+    }
 }
 
 /// \classmethod \constructor()
@@ -791,6 +797,7 @@ STATIC const mp_rom_map_elem_t pyb_usb_vcp_locals_dict_table[] = {
 
     // class constants
     { MP_ROM_QSTR(MP_QSTR_RTS), MP_ROM_INT(USBD_CDC_FLOWCONTROL_RTS) },
+    { MP_ROM_QSTR(MP_QSTR_CTS), MP_ROM_INT(USBD_CDC_FLOWCONTROL_CTS) },
 };
 
 STATIC MP_DEFINE_CONST_DICT(pyb_usb_vcp_locals_dict, pyb_usb_vcp_locals_dict_table);
@@ -808,18 +815,13 @@ STATIC mp_uint_t pyb_usb_vcp_read(mp_obj_t self_in, void *buf, mp_uint_t size, i
 
 STATIC mp_uint_t pyb_usb_vcp_write(mp_obj_t self_in, const void *buf, mp_uint_t size, int *errcode) {
     pyb_usb_vcp_obj_t *self = MP_OBJ_TO_PTR(self_in);
-    if (self->cdc_itf->attached_to_repl) {
+    int ret = usbd_cdc_tx_flow(self->cdc_itf, (const byte*)buf, size);
-        usbd_cdc_tx_always(self->cdc_itf, (const byte*)buf, size);
-        return size;
-    } else {
-        int ret = usbd_cdc_tx(self->cdc_itf, (const byte*)buf, size, 0);
     if (ret == 0) {
         // return EAGAIN error to indicate non-blocking
         *errcode = MP_EAGAIN;
         return MP_STREAM_ERROR;
     }
     return ret;
-    }
 }
 
 STATIC mp_uint_t pyb_usb_vcp_ioctl(mp_obj_t self_in, mp_uint_t request, uintptr_t arg, int *errcode) {

ports/stm32/usbd_cdc_interface.c

@@ -74,6 +74,12 @@ uint8_t *usbd_cdc_init(usbd_cdc_state_t *cdc_in) {
     cdc->rx_buf_full = false;
     cdc->tx_need_empty_packet = 0;
     cdc->connect_state = USBD_CDC_CONNECT_STATE_DISCONNECTED;
+    if (cdc->attached_to_repl) {
+        // Default behavior is non-blocking when attached to repl
+        cdc->flow &= ~USBD_CDC_FLOWCONTROL_CTS;
+    } else {
+        cdc->flow |= USBD_CDC_FLOWCONTROL_CTS;
+    }
 
     // Return the buffer to place the first USB OUT packet
     return cdc->rx_packet_buf;
@@ -292,6 +298,19 @@ int usbd_cdc_tx_half_empty(usbd_cdc_itf_t *cdc) {
     return tx_waiting <= USBD_CDC_TX_DATA_SIZE / 2;
 }
 
+// Writes only the data that fits if flow & CTS, else writes all data
+// Returns number of bytes actually written to the device
+int usbd_cdc_tx_flow(usbd_cdc_itf_t *cdc, const uint8_t *buf, uint32_t len) {
+    if (cdc->flow & USBD_CDC_FLOWCONTROL_CTS) {
+        // Only write as much as can fit in tx buffer
+        return usbd_cdc_tx(cdc, buf, len, 0);
+    } else {
+        // Never block, keep most recent data in rolling buffer
+        usbd_cdc_tx_always(cdc, buf, len);
+        return len;
+    }
+}
+
 // timout in milliseconds.
 // Returns number of bytes written to the device.
 int usbd_cdc_tx(usbd_cdc_itf_t *cdc, const uint8_t *buf, uint32_t len, uint32_t timeout) {

ports/stm32/usbd_cdc_interface.h

@@ -46,6 +46,7 @@
 // Flow control settings
 #define USBD_CDC_FLOWCONTROL_NONE (0)
 #define USBD_CDC_FLOWCONTROL_RTS (1)
+#define USBD_CDC_FLOWCONTROL_CTS (2)
 
 typedef struct _usbd_cdc_itf_t {
     usbd_cdc_state_t base; // state for the base CDC layer
@@ -75,6 +76,7 @@ static inline int usbd_cdc_is_connected(usbd_cdc_itf_t *cdc) {
 }
 
 int usbd_cdc_tx_half_empty(usbd_cdc_itf_t *cdc);
+int usbd_cdc_tx_flow(usbd_cdc_itf_t *cdc, const uint8_t *buf, uint32_t len);
 int usbd_cdc_tx(usbd_cdc_itf_t *cdc, const uint8_t *buf, uint32_t len, uint32_t timeout);
 void usbd_cdc_tx_always(usbd_cdc_itf_t *cdc, const uint8_t *buf, uint32_t len);