Jolt.js/nRF52840: USB flow control

[gfwilliams]

Right now we don't do USB flow control on nRF52840 - this should be as easy as just delaying app_usbd_cdc_acm_read until we have space for more data, but it doesn't appear to work.

In fact increasing the rx buffer from 1 byte to 64 bytes (which would be sensible) doesn't work either, so I think something strange is going on here.

diff --git a/targets/nrf5x/jshardware.c b/targets/nrf5x/jshardware.c
index b63478fea..7f1eac97b 100644
--- a/targets/nrf5x/jshardware.c
+++ b/targets/nrf5x/jshardware.c
@@ -177,6 +177,10 @@ static char m_tx_buffer[NRF_DRV_USBD_EPSIZE];
 static bool m_usb_connected = false;
 static bool m_usb_open = false;
 static bool m_usb_transmitting = false;
+/** Set to true in cdc_acm_user_ev_handler if we need to call
+ cdc_acm_user_setup_rx when there's sufficient space in our buffer
+ so we ack the transfer and can get more data */
+static volatile bool m_usb_wait_rx_ack = false;
 
 void on_usb_disconnected() {
   m_usb_open = false;
@@ -187,6 +191,15 @@ void on_usb_disconnected() {
   jshTransmitClearDevice(EV_USBSERIAL); // clear the transmit queue
 }
 
+static ret_code_t cdc_acm_user_setup_rx() {
+  m_usb_wait_rx_ack = false;
+  return app_usbd_cdc_acm_read(&m_app_cdc_acm,
+                               m_rx_buffer,
+                               sizeof(m_rx_buffer));
+  // we expect ret=NRF_ERROR_IO_PENDING here
+  // jsiConsolePrintf("app_usbd_cdc_acm_read -> %d\n", ret);
+}
+
 /**
  * @brief User event handler @ref app_usbd_cdc_acm_user_ev_handler_t (headphones)
  * */
@@ -204,12 +217,7 @@ static void cdc_acm_user_ev_handler(app_usbd_class_inst_t const * p_inst,
         m_usb_open = true;
         m_usb_transmitting = false;
         /*Setup first transfer*/
-        ret_code_t ret = app_usbd_cdc_acm_read(&m_app_cdc_acm,
-                                                m_rx_buffer,
-                                                sizeof(m_rx_buffer));
-        UNUSED_VARIABLE(ret);
-        // we expect ret=NRF_ERROR_IO_PENDING here
-        // jsiConsolePrintf("app_usbd_cdc_acm_read -> %d\n", ret);
+        cdc_acm_user_setup_rx();
 
         // USB connected - so move console device over to it
         if (jsiGetConsoleDevice()!=EV_LIMBO) {
@@ -236,11 +244,12 @@ static void cdc_acm_user_ev_handler(app_usbd_class_inst_t const * p_inst,
           size_t size = app_usbd_cdc_acm_rx_size(p_cdc_acm);
           jshPushIOCharEvents(EV_USBSERIAL,  m_rx_buffer, size);
           jshHadEvent();
-
-          /*Setup next transfer*/
-          ret = app_usbd_cdc_acm_read(&m_app_cdc_acm,
-                                                m_rx_buffer,
-                                                sizeof(m_rx_buffer));
+          /* If we pass the XOFF threshold, don't request more data and in the idle loop we'll check if we have space and if so will setup the next transfer */
+          if (jshGetEventsUsed() > IOBUFFER_XOFF) {
+            m_usb_wait_rx_ack = true;
+            return;
+          }
+          ret = cdc_acm_user_setup_rx();
         } while (ret == NRF_SUCCESS);
         break;
     }
@@ -653,6 +662,10 @@ void SysTick_Handler(void)  {
     jsiOneSecondAfterStartup();
   }
 
+  // Kick USB if it's needed after RX fifo overflowed
+  if (m_usb_wait_rx_ack && (jshGetEventsUsed() < IOBUFFER_XON))
+    cdc_acm_user_setup_rx();
+
   JsSysTime currTime = jshGetSystemTime();
   JsSysTime t = (currTime - lastSysTickTime);
   if (t>0xFFFFFFFFU) t = 0xFFFFFFFFU;

When sending with the IDE the reads end up seemingly delayed by several characters.

If we can't fix this we could always turn on XON/XOFF flow control for USB on nRF52840?