Make ITS reco start layer configurable

Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackingConfigParam.h

@@ -62,6 +62,7 @@ struct TrackerParamConfig : public o2::conf::ConfigurableParamHelper<TrackerPara
   bool useFastMaterial = false; // use faster material approximation for material budget accounting in the fits.
   int deltaRof = 0;             // configure the width of the window in ROFs to be considered for the tracking.
   int minTrackLgtIter[MaxIter] = {};                                        // minimum track length at each iteration, used only if >0, otherwise use code defaults
+  uint8_t startLayerMask[MaxIter] = {};                                     // mask of start layer for this iteration (if >0)
   float minPtIterLgt[MaxIter * (MaxTrackLength - MinTrackLength + 1)] = {}; // min.pT for given track length at this iteration, used only if >0, otherwise use code defaults
   float sysErrY2[7] = {0};      // systematic error^2 in Y per layer
   float sysErrZ2[7] = {0};      // systematic error^2 in Z per layer

Detectors/ITSMFT/ITS/tracking/src/TrackingInterface.cxx

@@ -78,6 +78,9 @@ void ITSTrackingInterface::initialise()
       param.TrackletsPerClusterLimit = 1.e3f;
       // check if something was overridden via configurable params
       if (ip < trackConf.MaxIter) {
+        if (trackConf.startLayerMask[ip] > 0) {
+          trackParams[2].StartLayerMask = trackConf.startLayerMask[ip];
+        }
         if (trackConf.minTrackLgtIter[ip] > 0) {
           param.MinTrackLength = trackConf.minTrackLgtIter[ip];
         }