stm32/usb: Add flow control option for USB VCP data received from host.

It's off by default and can be enabled at run-time with:

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

ports/stm32/usb.c

@@ -410,6 +410,27 @@ STATIC mp_obj_t pyb_usb_vcp_make_new(const mp_obj_type_t *type, size_t n_args, s
     }
 }
 
+// init(*, flow=-1)
+STATIC mp_obj_t pyb_usb_vcp_init(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
+    enum { ARG_flow };
+    static const mp_arg_t allowed_args[] = {
+        { MP_QSTR_flow, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = -1} },
+    };
+
+    // parse args
+    mp_arg_val_t args[MP_ARRAY_SIZE(allowed_args)];
+    pyb_usb_vcp_obj_t *self = MP_OBJ_TO_PTR(pos_args[0]);
+    mp_arg_parse_all(n_args - 1, pos_args + 1, kw_args, MP_ARRAY_SIZE(allowed_args), allowed_args, args);
+
+    // flow control
+    if (args[ARG_flow].u_int != -1) {
+        self->cdc_itf->flow = args[ARG_flow].u_int;
+    }
+
+    return mp_const_none;
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_KW(pyb_usb_vcp_init_obj, 1, pyb_usb_vcp_init);
+
 STATIC mp_obj_t pyb_usb_vcp_setinterrupt(mp_obj_t self_in, mp_obj_t int_chr_in) {
     mp_hal_set_interrupt_char(mp_obj_get_int(int_chr_in));
     return mp_const_none;
@@ -510,6 +531,7 @@ mp_obj_t pyb_usb_vcp___exit__(size_t n_args, const mp_obj_t *args) {
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(pyb_usb_vcp___exit___obj, 4, 4, pyb_usb_vcp___exit__);
 
 STATIC const mp_rom_map_elem_t pyb_usb_vcp_locals_dict_table[] = {
+    { MP_ROM_QSTR(MP_QSTR_init), MP_ROM_PTR(&pyb_usb_vcp_init_obj) },
     { MP_ROM_QSTR(MP_QSTR_setinterrupt), MP_ROM_PTR(&pyb_usb_vcp_setinterrupt_obj) },
     { MP_ROM_QSTR(MP_QSTR_isconnected), MP_ROM_PTR(&pyb_usb_vcp_isconnected_obj) },
     { MP_ROM_QSTR(MP_QSTR_any), MP_ROM_PTR(&pyb_usb_vcp_any_obj) },
@@ -524,6 +546,9 @@ STATIC const mp_rom_map_elem_t pyb_usb_vcp_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR___del__), MP_ROM_PTR(&mp_identity_obj) },
     { MP_ROM_QSTR(MP_QSTR___enter__), MP_ROM_PTR(&mp_identity_obj) },
     { MP_ROM_QSTR(MP_QSTR___exit__), MP_ROM_PTR(&pyb_usb_vcp___exit___obj) },
+
+    // class constants
+    { MP_ROM_QSTR(MP_QSTR_RTS), MP_ROM_INT(USBD_CDC_FLOWCONTROL_RTS) },
 };
 
 STATIC MP_DEFINE_CONST_DICT(pyb_usb_vcp_locals_dict, pyb_usb_vcp_locals_dict_table);

ports/stm32/usbd_cdc_interface.c

@@ -69,6 +69,7 @@ uint8_t *usbd_cdc_init(usbd_cdc_state_t *cdc_in) {
     // be filled (by usbd_cdc_tx_always) before the USB device is connected.
     cdc->rx_buf_put = 0;
     cdc->rx_buf_get = 0;
+    cdc->rx_buf_full = false;
     cdc->tx_buf_ptr_out = 0;
     cdc->tx_buf_ptr_out_shadow = 0;
     cdc->tx_need_empty_packet = 0;
@@ -236,6 +237,25 @@ void HAL_PCD_SOFCallback(PCD_HandleTypeDef *hpcd) {
     }
 }
 
+bool usbd_cdc_rx_buffer_full(usbd_cdc_itf_t *cdc) {
+    int get = cdc->rx_buf_get, put = cdc->rx_buf_put;
+    int remaining = (get - put) + (-((int) (get <= put)) & USBD_CDC_RX_DATA_SIZE);
+    return remaining < CDC_DATA_MAX_PACKET_SIZE + 1;
+}
+
+void usbd_cdc_rx_check_resume(usbd_cdc_itf_t *cdc) {
+    uint32_t irq_state = disable_irq();
+    if (cdc->rx_buf_full) {
+        if (!usbd_cdc_rx_buffer_full(cdc)) {
+            cdc->rx_buf_full = false;
+            enable_irq(irq_state);
+            USBD_CDC_ReceivePacket(&cdc->base, cdc->rx_packet_buf);
+            return;
+        }
+    }
+    enable_irq(irq_state);
+}
+
 // Data received over USB OUT endpoint is processed here.
 // len: number of bytes received into the buffer we passed to USBD_CDC_ReceivePacket
 // Returns USBD_OK if all operations are OK else USBD_FAIL
@@ -257,10 +277,14 @@ int8_t usbd_cdc_receive(usbd_cdc_state_t *cdc_in, size_t len) {
         }
     }
 
-    // initiate next USB packet transfer
-    USBD_CDC_ReceivePacket(&cdc->base, cdc->rx_packet_buf);
-
-    return USBD_OK;
+    if ((cdc->flow & USBD_CDC_FLOWCONTROL_RTS) && (usbd_cdc_rx_buffer_full(cdc))) {
+        cdc->rx_buf_full = true;
+        return USBD_BUSY;
+    } else {
+        // initiate next USB packet transfer
+        cdc->rx_buf_full = false;
+        return USBD_CDC_ReceivePacket(&cdc->base, cdc->rx_packet_buf);
+    }
 }
 
 int usbd_cdc_tx_half_empty(usbd_cdc_itf_t *cdc) {
@@ -339,6 +363,7 @@ int usbd_cdc_rx_num(usbd_cdc_itf_t *cdc) {
     if (rx_waiting < 0) {
         rx_waiting += USBD_CDC_RX_DATA_SIZE;
     }
+    usbd_cdc_rx_check_resume(cdc);
     return rx_waiting;
 }
 
@@ -359,6 +384,7 @@ int usbd_cdc_rx(usbd_cdc_itf_t *cdc, uint8_t *buf, uint32_t len, uint32_t timeou
                 // IRQs disabled so buffer will never be filled; return immediately
                 return i;
             }
+            usbd_cdc_rx_check_resume(cdc);
             __WFI(); // enter sleep mode, waiting for interrupt
         }
 
@@ -366,6 +392,7 @@ int usbd_cdc_rx(usbd_cdc_itf_t *cdc, uint8_t *buf, uint32_t len, uint32_t timeou
         buf[i] = cdc->rx_user_buf[cdc->rx_buf_get];
         cdc->rx_buf_get = (cdc->rx_buf_get + 1) & (USBD_CDC_RX_DATA_SIZE - 1);
     }
+    usbd_cdc_rx_check_resume(cdc);
 
     // Success, return number of bytes read
     return len;

ports/stm32/usbd_cdc_interface.h

@@ -39,6 +39,10 @@
 #define USBD_CDC_CONNECT_STATE_CONNECTING (1)
 #define USBD_CDC_CONNECT_STATE_CONNECTED (2)
 
+// Flow control settings
+#define USBD_CDC_FLOWCONTROL_NONE (0)
+#define USBD_CDC_FLOWCONTROL_RTS (1)
+
 typedef struct _usbd_cdc_itf_t {
     usbd_cdc_state_t base; // state for the base CDC layer
 
@@ -46,6 +50,7 @@ typedef struct _usbd_cdc_itf_t {
     uint8_t rx_user_buf[USBD_CDC_RX_DATA_SIZE]; // received data is buffered here until the user reads it
     volatile uint16_t rx_buf_put; // circular buffer index
     uint16_t rx_buf_get; // circular buffer index
+    uint8_t rx_buf_full; // rx from host will be blocked while this is true
 
     uint8_t tx_buf[USBD_CDC_TX_DATA_SIZE]; // data for USB IN endpoind is stored in this buffer
     uint16_t tx_buf_ptr_in; // increment this pointer modulo USBD_CDC_TX_DATA_SIZE when new data is available
@@ -55,6 +60,7 @@ typedef struct _usbd_cdc_itf_t {
 
     volatile uint8_t connect_state; // indicates if we are connected
     uint8_t attached_to_repl; // indicates if interface is connected to REPL
+    uint8_t flow; // USBD_CDC_FLOWCONTROL_* setting flags
 } usbd_cdc_itf_t;
 
 // This is implemented in usb.c

ports/stm32/usbdev/class/src/usbd_cdc_msc_hid.c

@@ -1128,14 +1128,13 @@ static uint8_t USBD_CDC_MSC_HID_DataOut(USBD_HandleTypeDef *pdev, uint8_t epnum)
 
         /* USB data will be immediately processed, this allow next USB traffic being
         NAKed till the end of the application Xfer */
-        usbd_cdc_receive(usbd->cdc, len);
+        return usbd_cdc_receive(usbd->cdc, len);
 
-        return USBD_OK;
     #if MICROPY_HW_USB_ENABLE_CDC2
     } else if ((usbd->usbd_mode & USBD_MODE_CDC2) && epnum == (CDC2_OUT_EP & 0x7f)) {
         size_t len = USBD_LL_GetRxDataSize(pdev, epnum);
-        usbd_cdc_receive(usbd->cdc2, len);
-        return USBD_OK;
+        return usbd_cdc_receive(usbd->cdc2, len);
+        
     #endif
     } else if ((usbd->usbd_mode & USBD_MODE_MSC) && epnum == (MSC_OUT_EP & 0x7f)) {
         MSC_BOT_DataOut(pdev, epnum);