xc7patch: Generate Xilinx BIT header in output bitstream

Xilinx tools generate and expect an additional header to be added to the
bitstream.  This header is the only difference between .bit and .bin
files and is not required by the part in any way.  OpenOCD only supports
reading .bit files so the easiest path to a using FOSS tools from FASM
down is to generate a .bit header in xc7patch's output. To distinguish
xc7patch-generated files from Xilinx-generated files, the source file
field includes a Generator tag indicating that the bitstream was produced by
xc7patch.  Vivado uses the same technique to record the Vivado version
in a Version tag.

Signed-off-by: Rick Altherr <kc8apf@kc8apf.net>

tools/CMakeLists.txt

@@ -21,6 +21,7 @@ target_link_libraries(gen_part_base_yaml
 add_executable(xc7patch xc7patch.cc)
 target_link_libraries(xc7patch
 	absl::strings
+	absl::time
 	gflags
 	libprjxray
 )

tools/xc7patch.cc

@@ -5,7 +5,10 @@
 #include <string>
 #include <vector>
 
+#include <absl/strings/str_cat.h>
 #include <absl/strings/str_split.h>
+#include <absl/time/clock.h>
+#include <absl/time/time.h>
 #include <gflags/gflags.h>
 #include <prjxray/memory_mapped_file.h>
 #include <prjxray/xilinx/xc7series/bitstream_reader.h>
@@ -18,6 +21,7 @@
 #include <prjxray/xilinx/xc7series/nop_packet.h>
 #include <prjxray/xilinx/xc7series/part.h>
 
+DEFINE_string(part_name, "", "");
 DEFINE_string(part_file, "", "Definition file for target 7-series part");
 DEFINE_string(bitstream_file,
               "",
@@ -315,6 +319,61 @@ int main(int argc, char* argv[]) {
 		return 1;
 	}
 
+	// Xilinx BIT header.
+	// Sync header
+	std::vector<uint8_t> bit_header{0x0,  0x9,  0x0f, 0xf0, 0x0f,
+	                                0xf0, 0x0f, 0xf0, 0x0f, 0xf0,
+	                                0x00, 0x00, 0x01, 'a'};
+	auto build_source = absl::StrCat(FLAGS_frm_file, ";Generator=xc7patch");
+	bit_header.push_back(
+	    static_cast<uint8_t>((build_source.size() + 1) >> 8));
+	bit_header.push_back(static_cast<uint8_t>(build_source.size() + 1));
+	bit_header.insert(bit_header.end(), build_source.begin(),
+	                  build_source.end());
+	bit_header.push_back(0x0);
+
+	// Source file.
+	bit_header.push_back('b');
+	bit_header.push_back(
+	    static_cast<uint8_t>((FLAGS_part_name.size() + 1) >> 8));
+	bit_header.push_back(static_cast<uint8_t>(FLAGS_part_name.size() + 1));
+	bit_header.insert(bit_header.end(), FLAGS_part_name.begin(),
+	                  FLAGS_part_name.end());
+	bit_header.push_back(0x0);
+
+	// Build timestamp.
+	auto build_time = absl::Now();
+	auto build_date_string =
+	    absl::FormatTime("%E4Y/%m/%d", build_time, absl::UTCTimeZone());
+	auto build_time_string =
+	    absl::FormatTime("%H:%M:%S", build_time, absl::UTCTimeZone());
+
+	bit_header.push_back('c');
+	bit_header.push_back(
+	    static_cast<uint8_t>((build_date_string.size() + 1) >> 8));
+	bit_header.push_back(
+	    static_cast<uint8_t>(build_date_string.size() + 1));
+	bit_header.insert(bit_header.end(), build_date_string.begin(),
+	                  build_date_string.end());
+	bit_header.push_back(0x0);
+
+	bit_header.push_back('d');
+	bit_header.push_back(
+	    static_cast<uint8_t>((build_time_string.size() + 1) >> 8));
+	bit_header.push_back(
+	    static_cast<uint8_t>(build_time_string.size() + 1));
+	bit_header.insert(bit_header.end(), build_time_string.begin(),
+	                  build_time_string.end());
+	bit_header.push_back(0x0);
+
+	bit_header.insert(bit_header.end(), {'e', 0x0, 0x0, 0x0, 0x0});
+	out_file.write(reinterpret_cast<const char*>(bit_header.data()),
+	               bit_header.size());
+
+	auto end_of_header_pos = out_file.tellp();
+	auto header_data_length_pos =
+	    end_of_header_pos - static_cast<std::ofstream::off_type>(4);
+
 	for (uint32_t word : out_bitstream_writer) {
 		out_file.put((word >> 24) & 0xFF);
 		out_file.put((word >> 16) & 0xFF);
@@ -322,5 +381,13 @@ int main(int argc, char* argv[]) {
 		out_file.put((word)&0xFF);
 	}
 
+	uint32_t length_of_data = out_file.tellp() - end_of_header_pos;
+
+	out_file.seekp(header_data_length_pos);
+	out_file.put((length_of_data >> 24) & 0xFF);
+	out_file.put((length_of_data >> 16) & 0xFF);
+	out_file.put((length_of_data >> 8) & 0xFF);
+	out_file.put((length_of_data)&0xFF);
+
 	return 0;
 }