feat(cdc): add a 1200 bps touch hook

The Arduino convention for asking a board to reboot into its bootloader is
for the host to set the CDC line coding to 1200 bps and then close the port.
The core tracks both halves already, in linecoding.bitrate and dtrState, but
nothing acts on the combination, so a sketch has to poll Serial.baud() and
Serial.dtr() from loop() and misses the event whenever loop() is busy.

Add cdc_1200bps_touchHook(), a weak no-op alongside yield() and
dtr_togglingHook(), called from CDC_SET_CONTROL_LINE_STATE when the port is
closed at 1200 bps. dtr_togglingHook() cannot serve here because it is
invoked from USBD_CDC_Receive and so requires the host to send data, which a
1200 bps touch does not.

The hook only reports the event. It adds no series specific code, no backup
register use and no policy about which bootloader to enter, so boards and
sketches remain free to decide that.

Refs #706, #710

Author: mikeysklar

cores/arduino/hooks.c

@@ -49,4 +49,26 @@ static void __empty_dtr_toggling(uint8_t *buf, uint32_t *len)
   (void)len;
 }
 void dtr_togglingHook(uint8_t *buf, uint32_t *len) __attribute__((weak, alias("__empty_dtr_toggling")));
-#endif
+#endif
+
+#if defined(USBCON) && defined(USBD_USE_CDC)
+/**
+ * Empty cdc_1200bps_touch() hook.
+ *
+ * Called when the host closes the CDC port while the line coding is set to
+ * 1200 bps, which is the Arduino convention for asking the board to reboot
+ * into its bootloader.
+ *
+ * Its defined as a weak symbol and it can be redefined to implement the
+ * bootloader entry a given board needs.
+ *
+ * It is called from the USB control transfer callback, so it runs in handler
+ * mode. Branching to a bootloader from here leaves the core inside an
+ * exception that never returns; schedule the work and perform it from thread
+ * mode or after a reset instead.
+ */
+static void __empty_1200bps_touch(void)
+{
+}
+void cdc_1200bps_touchHook(void) __attribute__((weak, alias("__empty_1200bps_touch")));
+#endif /* USBCON && USBD_USE_CDC */

libraries/USBDevice/src/cdc/usbd_cdc_if.c

@@ -60,6 +60,8 @@ __IO bool rtsState = false;
 __IO bool receivePended = true;
 static uint32_t transmitStart = 0;
 
+extern void cdc_1200bps_touchHook(void);
+
 #ifdef DTR_TOGGLING_SEQ
   /* DTR toggling sequence management */
   extern void dtr_togglingHook(uint8_t *buf, uint32_t *len);
@@ -195,6 +197,11 @@ static int8_t USBD_CDC_Control(uint8_t cmd, uint8_t *pbuf, uint16_t length)
         transmitStart = 0;
       }
       rtsState = (((USBD_SetupReqTypedef *)pbuf)->wValue & CLS_RTS);
+      /* Host closing the port at 1200 bps is the Arduino convention for
+         requesting a reboot into the bootloader. */
+      if (!dtrState && linecoding.bitrate == 1200) {
+        cdc_1200bps_touchHook();
+      }
 #ifdef DTR_TOGGLING_SEQ
       dtr_toggling++; /* Count DTR toggling */
 #endif