HAL_ChibiOS: prevent long timeouts in DShot

this prevents bad calculated timeouts in DShot. The timeout would
sometimes come out as 0xFFFFFFFF, which led to an assert and could
block the thread

This fix is meant to be minimilistic to allow it to be merged easily
into 4.2. A better fix would fix all the uint32_t wrap handling in
DShot

libraries/AP_HAL_ChibiOS/RCOutput.cpp

@@ -237,14 +237,23 @@ void RCOutput::dshot_collect_dma_locks(uint32_t time_out_us)
             // if we have time left wait for the event
             eventmask_t mask = 0;
             const uint32_t pulse_elapsed_us = now - group.last_dmar_send_us;
+            uint32_t wait_us = 0;
             if (now < time_out_us) {
-                mask = chEvtWaitOneTimeout(group.dshot_event_mask,
-                    chTimeUS2I(MAX(time_out_us - now, group.dshot_pulse_send_time_us - pulse_elapsed_us)));
+                wait_us = MAX(time_out_us - now, group.dshot_pulse_send_time_us - pulse_elapsed_us);
             } else if (pulse_elapsed_us < group.dshot_pulse_send_time_us) {
                 // better to let the burst write in progress complete rather than cancelling mid way through
-                mask = chEvtWaitOneTimeout(group.dshot_event_mask,
-                    chTimeUS2I(group.dshot_pulse_send_time_us - pulse_elapsed_us));
+                wait_us = group.dshot_pulse_send_time_us - pulse_elapsed_us;
             }
+
+            // waiting for a very short period of time can cause a
+            // timer wrap with ChibiOS timers. Use CH_CFG_ST_TIMEDELTA
+            // as minimum. Don't allow for a very long delay (over 1200us)
+            // to prevent bugs in handling timer wrap
+            const uint32_t max_delay_us = 1200;
+            const uint32_t min_delay_us = 10; // matches our CH_CFG_ST_TIMEDELTA
+            wait_us = constrain_uint32(wait_us, min_delay_us, max_delay_us);
+            mask = chEvtWaitOneTimeout(group.dshot_event_mask, chTimeUS2I(wait_us));
+
             // no time left cancel and restart
             if (!mask) {
                 dma_cancel(group);