DataFlash: parameterise dataflash-over-mavlink cachesize

libraries/DataFlash/DataFlash.cpp

@@ -50,6 +50,13 @@ const AP_Param::GroupInfo DataFlash_Class::var_info[] = {
     // @User: Standard
     AP_GROUPINFO("_FILE_DSRMROT",  4, DataFlash_Class, _params.file_disarm_rot,       0),
 
+    // @Param: _MAV_BUFSIZE
+    // @DisplayName: Maximum DataFlash MAVLink Backend buffer size
+    // @Description: Maximum amount of memory to allocate to DataFlash-over-mavlink
+    // @User: Advanced
+    // @Units: kB
+    AP_GROUPINFO("_MAV_BUFSIZE",  5, DataFlash_Class, _params.mav_bufsize,       8),
+
     AP_GROUPEND
 };
 

libraries/DataFlash/DataFlash.h

@@ -198,6 +198,7 @@ public:
         AP_Int8 file_disarm_rot;
         AP_Int8 log_disarmed;
         AP_Int8 log_replay;
+        AP_Int8 mav_bufsize; // in kilobytes
     } _params;
 
     const struct LogStructure *structure(uint16_t num) const;

libraries/DataFlash/DataFlash_MAVLink.cpp

@@ -35,7 +35,7 @@ void DataFlash_MAVLink::Init()
 
     _blocks = nullptr;
     while (_blockcount >= 8) { // 8 is a *magic* number
-        _blocks = (struct dm_block *) calloc(_blockcount, sizeof(_blocks[0]));
+        _blocks = (struct dm_block *) calloc(_blockcount, sizeof(struct dm_block));
         if (_blocks != nullptr) {
             break;
         }

libraries/DataFlash/DataFlash_MAVLink.h

@@ -23,10 +23,12 @@ public:
     // constructor
     DataFlash_MAVLink(DataFlash_Class &front, DFMessageWriter_DFLogStart *writer) :
         DataFlash_Backend(front, writer),
-        _max_blocks_per_send_blocks(8),
-        _blockcount(32) // this may get reduced in Init if allocation fails
+        _max_blocks_per_send_blocks(8)
         ,_perf_packing(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "DM_packing"))
-        { }
+        {
+            _blockcount = 1024*((uint8_t)_front._params.mav_bufsize) / sizeof(struct dm_block);
+            // ::fprintf(stderr, "DM: Using %u blocks\n", _blockcount);
+        }
 
     // initialisation
     void Init() override;