Require 64 bytes extra CDC RX buffer

Once the buffer has less that an endpoint's worth, TinyUSB won't
request more from the host. When that happens, ctrl-c will no
longer be sent to the device and TinyUSB won't find the wanted
character.

Fixes #4444

Author: tannewt

ports/broadcom/Makefile

@@ -107,7 +107,7 @@ CFLAGS += $(OPTIMIZATION_FLAGS)
 
 # TinyUSB defines
 CFLAGS += -DCFG_TUSB_MCU=OPT_MCU_BCM2711 -DCFG_TUD_MIDI_RX_BUFSIZE=512 \
-          -DCFG_TUD_CDC_RX_BUFSIZE=512 -DCFG_TUD_MIDI_TX_BUFSIZE=512 \
+          -DCFG_TUD_CDC_RX_BUFSIZE=640 -DCFG_TUD_MIDI_TX_BUFSIZE=512 \
           -DCFG_TUD_CDC_TX_BUFSIZE=512 -DCFG_TUD_MSC_BUFSIZE=1024
 
 #Debugging/Optimization

ports/mimxrt10xx/Makefile

@@ -48,7 +48,7 @@ INC += \
 CFLAGS += -ftree-vrp -DNDEBUG
 
 # TinyUSB defines
-CFLAGS += -DCFG_TUSB_MCU=OPT_MCU_MIMXRT10XX -DCFG_TUD_CDC_RX_BUFSIZE=512 -DCFG_TUD_CDC_TX_BUFSIZE=512
+CFLAGS += -DCFG_TUSB_MCU=OPT_MCU_MIMXRT10XX -DCFG_TUD_CDC_RX_BUFSIZE=640 -DCFG_TUD_CDC_TX_BUFSIZE=512
 ifeq ($(CHIP_FAMILY),$(filter $(CHIP_FAMILY),MIMXRT1011 MIMXRT1015))
 CFLAGS += -DCFG_TUD_MIDI_RX_BUFSIZE=512 -DCFG_TUD_MIDI_TX_BUFSIZE=64 -DCFG_TUD_MSC_BUFSIZE=512
 else

supervisor/shared/usb/usb.c

@@ -317,13 +317,20 @@ bool tud_vendor_control_xfer_cb(uint8_t rhport, uint8_t stage, tusb_control_requ
 
 #if MICROPY_KBD_EXCEPTION && CIRCUITPY_USB_CDC
 
+// The CDC RX buffer impacts monitoring for ctrl-c. TinyUSB will only ask for
+// more from CDC if the free space in the buffer is greater than the endpoint
+// size. Setting CFG_TUD_CDC_RX_BUFSIZE to the endpoint size and then sending
+// any character will prevent ctrl-c from working. Require at least a 64
+// character buffer.
+#if CFG_TUD_CDC_RX_BUFSIZE < CFG_TUD_CDC_EP_BUFSIZE + 64
+#error "CFG_TUD_CDC_RX_BUFSIZE must be 64 bytes bigger than endpoint size."
+#endif
+
 /**
  * Callback invoked when received an "wanted" char.
  * @param itf           Interface index (for multiple cdc interfaces)
  * @param wanted_char   The wanted char (set previously)
  */
-
-// Only called when console is enabled.
 void tud_cdc_rx_wanted_cb(uint8_t itf, char wanted_char) {
     // Workaround for using shared/runtime/interrupt_char.c
     // Compare mp_interrupt_char with wanted_char and ignore if not matched