scsidevice: add get_more_data() method.

It is required for supporting large data transfers split
into multiple chunks.

devices/common/scsi/sc53c94.h

@@ -180,6 +180,7 @@ public:
     // ScsiDevice methods
     void notify(ScsiMsg msg_type, int param);
     bool prepare_data() { return false; };
+    bool get_more_data() { return false; };
     bool has_data() { return this->data_fifo_pos != 0; };
     int  send_data(uint8_t* dst_ptr, int count);
     void process_command() {};

devices/common/scsi/scsi.h

@@ -183,6 +183,7 @@ public:
     virtual int  rcv_data(const uint8_t* src_ptr, const int count);
 
     virtual bool prepare_data() = 0;
+    virtual bool get_more_data() = 0;
     virtual void process_command() = 0;
 
     void set_bus_object_ptr(ScsiBus *bus_obj_ptr) {

devices/common/scsi/scsidevice.cpp

@@ -113,7 +113,7 @@ void ScsiDevice::prepare_xfer(ScsiBus* bus_obj, int& bytes_in, int& bytes_out)
     switch (this->cur_phase) {
     case ScsiPhase::COMMAND:
         this->data_ptr = this->cmd_buf;
-        this->data_size = 0; //bytes_in;
+        this->data_size = 0;
         bytes_out = 0;
         break;
     case ScsiPhase::STATUS:
@@ -189,6 +189,18 @@ int ScsiDevice::send_data(uint8_t* dst_ptr, const int count)
     this->data_ptr  += actual_count;
     this->data_size -= actual_count;
 
+    // attempt to return the requested amount of data
+    // when data_size drops down to zero
+    if (!this->data_size) {
+        if (this->get_more_data() && count > actual_count) {
+            dst_ptr += actual_count;
+            actual_count = std::min(this->data_size, count - actual_count);
+            std::memcpy(dst_ptr, this->data_ptr, actual_count);
+            this->data_ptr  += actual_count;
+            this->data_size -= actual_count;
+        }
+    }
+
     return actual_count;
 }
 

devices/common/scsi/scsihd.h

@@ -43,6 +43,7 @@ public:
     void insert_image(std::string filename);
     void process_command();
     bool prepare_data();
+    bool get_more_data() { return false; };
 
 protected:
     int  test_unit_ready();