rtds2gpu: update register type to work for more complex payloads

include/villas/fpga/ips/rtds2gpu/register_types.hpp

@@ -3,6 +3,7 @@
 
 #include <stdint.h>
 #include <cstddef>
+#include <cstdint>
 
 union axilite_reg_status_t {
 	uint32_t value;
@@ -26,16 +27,31 @@ union reg_doorbell_t {
 		count			: 6,
 		is_valid		: 1;
 	};
+
+	constexpr reg_doorbell_t() : value(0) {}
 };
 
 template<size_t N, typename T = uint32_t>
 struct Rtds2GpuMemoryBuffer {
-	static constexpr size_t valueCount = N;
-	static constexpr size_t dataOffset = offsetof(Rtds2GpuMemoryBuffer, data);
-	static constexpr size_t doorbellOffset = offsetof(Rtds2GpuMemoryBuffer, doorbell);
+	// this type is only for memory interpretation, it makes no sense to create
+	// an instance so it's forbidden
+	Rtds2GpuMemoryBuffer() = delete;
+
+	// T can be a more complex type that wraps multiple values
+	static constexpr size_t rawValueCount = N * (sizeof(T) / 4);
+
+	// As of C++14, offsetof() is not working for non-standard layout types (i.e.
+	// composed of non-POD members). This might work in C++17 though.
+	// More info: https://gist.github.com/graphitemaster/494f21190bb2c63c5516
+	//static constexpr size_t doorbellOffset = offsetof(Rtds2GpuMemoryBuffer, doorbell);
+	//static constexpr size_t dataOffset = offsetof(Rtds2GpuMemoryBuffer, data);
+
+	// HACK: This might break horribly, let's just hope C++17 will be there soon
+	static constexpr size_t dataOffset = 0;
+	static constexpr size_t doorbellOffset = N * sizeof(Rtds2GpuMemoryBuffer::data);
 
 	T data[N];
 	reg_doorbell_t doorbell;
-} __attribute__((packed));
+};
 
 #endif // REGISTER_TYPES_H