pbio/drv/usb/usb_ev3.c: Implement status keepalive timer

Without this timer, if the hub state doesn't change and the user program
doesn't output anything, the hub won't be able to detect that the
host software has closed (because it never sends anything which could time out).

Keeping this state reasonably up-to-date is important for the hub
auto-power-off functionality.

Author: ArcaneNibble

lib/pbio/drv/usb/usb_ev3.c

@@ -883,6 +883,7 @@ static pbio_os_process_t pbdrv_usb_ev3_process;
 static pbio_error_t pbdrv_usb_ev3_process_thread(pbio_os_state_t *state, void *context) {
     static pbio_os_timer_t delay_timer;
     static pbio_os_timer_t tx_timeout_timer;
+    static pbio_os_timer_t keepalive_timer;
     static bool was_transmitting = false;
     static bool is_transmitting = false;
     static uint32_t prev_status_flags = ~0;
@@ -910,6 +911,7 @@ static pbio_error_t pbdrv_usb_ev3_process_thread(pbio_os_state_t *state, void *c
             if (usb_rx_sz && !usb_tx_response_is_not_ready) {
                 switch (ep1_rx_buf[0]) {
                     case PBIO_PYBRICKS_OUT_EP_MSG_SUBSCRIBE:
+                        pbio_os_timer_set(&keepalive_timer, 1000);
                         pbdrv_usb_is_events_subscribed = ep1_rx_buf[1];
                         ep1_tx_response_buf[0] = PBIO_PYBRICKS_IN_EP_MSG_RESPONSE;
                         pbio_set_uint32_le(&ep1_tx_response_buf[1], PBIO_PYBRICKS_ERROR_OK);
@@ -936,14 +938,24 @@ static pbio_error_t pbdrv_usb_ev3_process_thread(pbio_os_state_t *state, void *c
 
         // Send status flags if they've changed (and we can)
         new_status_flags = pbsys_status_get_flags();
-        if (pbdrv_usb_is_events_subscribed && !usb_tx_status_is_not_ready && new_status_flags != prev_status_flags) {
+        if (pbdrv_usb_is_events_subscribed && !usb_tx_status_is_not_ready &&
+            (new_status_flags != prev_status_flags || pbio_os_timer_is_expired(&keepalive_timer))) {
             ep1_tx_status_buf[0] = PBIO_PYBRICKS_IN_EP_MSG_EVENT;
             uint32_t usb_status_sz = PBIO_PYBRICKS_USB_MESSAGE_SIZE(pbsys_status_get_status_report(&ep1_tx_status_buf[1]));
 
             usb_tx_status_is_not_ready = true;
             usb_setup_tx_dma_desc(CPPI_DESC_TX_STATUS, ep1_tx_status_buf, usb_status_sz);
 
             prev_status_flags = new_status_flags;
+
+            if (new_status_flags != prev_status_flags) {
+                // If we are sending a status because the flags have changed,
+                // we can bump out the keepalive timer.
+                pbio_os_timer_set(&keepalive_timer, 1000);
+            } else {
+                // Otherwise, we want to send keepalives at a particular rate.
+                pbio_os_timer_extend(&keepalive_timer);
+            }
         }
 
         // Handle timeouts