STM32CubeWL: Fix RX timing issue due to relative timestamps

For the RX window timing, a timer is started after TxDone. However,
since this is done in the mainloop rather than the TxDone ISR, the exact
timing of the RX window depends on how soon after the interrupt
LoRaMacProcess() is called (but it will always be at least a little too
late).

The TxDone interrupt already captures a timestamp, so this commit
uses that to correctly position the RX window. There is still a small
race condition where interrupts could still cause a delay, but the
influence of that should be small and rare enough in practice, so this
is left unfixed here (a proper fix would require changes to the timer
subsystem, which are a bit too invasive to make here).

This fixes Lora-net/LoRaMac-node#1349

Author: matthijskooijman

src/STM32CubeWL/LoRaWAN/Mac/LoRaMac.c

@@ -959,19 +959,30 @@ static void ProcessRadioTxDone( void )
     {
         Radio.Sleep( );
     }
+    // XXX: There is a race condition here: If an interrupt (or any
+    // other delay) happens between calculating the offset and actually
+    // setting the timer, the timer will be set too late. In practice,
+    // assuming that an IRQ is rare an also does not take more than
+    // a few ms, this is probably still within the allowed margins. This
+    // could be fixed by disabling interrupts, but that requires
+    // disabling them for quite some time, which probably has more
+    // impact. A proper fix would be to allow setting an absolute
+    // timestamp into a timer instead of relative, but that is a change
+    // best made upstream in Semtech's codebase.
+    uint32_t offset = TimerGetCurrentTime() - TxDoneParams.CurTime;
 #if ( !defined(DISABLE_LORAWAN_RX_WINDOW) || (DISABLE_LORAWAN_RX_WINDOW == 0) )
     // Setup timers
-    TimerSetValue( &MacCtx.RxWindowTimer1, MacCtx.RxWindow1Delay );
+    TimerSetValue( &MacCtx.RxWindowTimer1, MacCtx.RxWindow1Delay - offset );
     TimerStart( &MacCtx.RxWindowTimer1 );
-    TimerSetValue( &MacCtx.RxWindowTimer2, MacCtx.RxWindow2Delay );
+    TimerSetValue( &MacCtx.RxWindowTimer2, MacCtx.RxWindow2Delay - offset );
     TimerStart( &MacCtx.RxWindowTimer2 );
 #else
     if (Nvm.MacGroup2.NetworkActivation == ACTIVATION_TYPE_NONE)
     {
         // Setup timers
-        TimerSetValue( &MacCtx.RxWindowTimer1, MacCtx.RxWindow1Delay );
+        TimerSetValue( &MacCtx.RxWindowTimer1, MacCtx.RxWindow1Delay - offset );
         TimerStart( &MacCtx.RxWindowTimer1 );
-        TimerSetValue( &MacCtx.RxWindowTimer2, MacCtx.RxWindow2Delay );
+        TimerSetValue( &MacCtx.RxWindowTimer2, MacCtx.RxWindow2Delay - offset );
         TimerStart( &MacCtx.RxWindowTimer2 );
     }
     else