builder: implement Nordic DFU file writer in Go

This avoids a dependency on nrfutil. I have verified that it creates
equivalent zip files to a wasp-os DFU zip file I downloaded here:
https://github.com/wasp-os/wasp-os/releases/
I have also tested that it produces valid DFU files that can be uploaded
using the dfu.py program here to my PineTime:
https://github.com/wasp-os/ota-dfu-python/tree/3d6fd30d33c2b20bc86ff6b9269fddf4a1d4c7c6

There are some minor differences in the generated file that should not
matter in practice (JSON whitespace, firmware file name, zip
compression).

2 files changed, 103 insertions, 18 deletions

diff --git a/builder/build.go b/builder/build.go
index fcd7a2f07..d95519b33 100644
--- a/builder/build.go
+++ b/builder/build.go
@@ -909,13 +909,8 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		}
 	case "nrf-dfu":
 		// special format for nrfutil for Nordic chips
-		tmphexpath := filepath.Join(tmpdir, "main.hex")
-		err := objcopy(result.Executable, tmphexpath, "hex")
-		if err != nil {
-			return result, err
-		}
 		result.Binary = filepath.Join(tmpdir, "main"+outext)
-		err = makeDFUFirmwareImage(config.Options, tmphexpath, result.Binary)
+		err = makeDFUFirmwareImage(config.Options, result.Executable, result.Binary)
 		if err != nil {
 			return result, err
 		}
diff --git a/builder/nrfutil.go b/builder/nrfutil.go
index 73b04925c..6f33eb7c6 100644
--- a/builder/nrfutil.go
+++ b/builder/nrfutil.go
@@ -1,27 +1,117 @@
 package builder
 
 import (
-	"fmt"
-	"io"
-	"os/exec"
+	"archive/zip"
+	"bytes"
+	"encoding/binary"
+	"encoding/json"
+	"os"
 
+	"github.com/sigurn/crc16"
 	"github.com/tinygo-org/tinygo/compileopts"
 )
 
-// https://infocenter.nordicsemi.com/index.jsp?topic=%2Fug_nrfutil%2FUG%2Fnrfutil%2Fnrfutil_intro.html
+// Structure of the manifest.json file.
+type jsonManifest struct {
+	Manifest struct {
+		Application struct {
+			BinaryFile     string        `json:"bin_file"`
+			DataFile       string        `json:"dat_file"`
+			InitPacketData nrfInitPacket `json:"init_packet_data"`
+		} `json:"application"`
+		DFUVersion float64 `json:"dfu_version"` // yes, this is a JSON number, not a string
+	} `json:"manifest"`
+}
+
+// Structure of the init packet.
+// Source:
+// https://github.com/adafruit/Adafruit_nRF52_Bootloader/blob/master/lib/sdk11/components/libraries/bootloader_dfu/dfu_init.h#L47-L57
+type nrfInitPacket struct {
+	ApplicationVersion uint32   `json:"application_version"`
+	DeviceRevision     uint16   `json:"device_revision"`
+	DeviceType         uint16   `json:"device_type"`
+	FirmwareCRC16      uint16   `json:"firmware_crc16"`
+	SoftDeviceRequired []uint16 `json:"softdevice_req"` // this is actually a variable length array
+}
 
+// Create the init packet (the contents of application.dat).
+func (p nrfInitPacket) createInitPacket() []byte {
+	buf := &bytes.Buffer{}
+	binary.Write(buf, binary.LittleEndian, p.DeviceType)                      // uint16_t device_type;
+	binary.Write(buf, binary.LittleEndian, p.DeviceRevision)                  // uint16_t device_rev;
+	binary.Write(buf, binary.LittleEndian, p.ApplicationVersion)              // uint32_t app_version;
+	binary.Write(buf, binary.LittleEndian, uint16(len(p.SoftDeviceRequired))) // uint16_t softdevice_len;
+	binary.Write(buf, binary.LittleEndian, p.SoftDeviceRequired)              // uint16_t softdevice[1];
+	binary.Write(buf, binary.LittleEndian, p.FirmwareCRC16)
+	return buf.Bytes()
+}
+
+// Make a Nordic DFU firmware image from an ELF file.
 func makeDFUFirmwareImage(options *compileopts.Options, infile, outfile string) error {
-	cmdLine := []string{"nrfutil", "pkg", "generate", "--hw-version", "52", "--sd-req", "0x0", "--debug-mode", "--application", infile, outfile}
+	// Read ELF file as input and convert it to a binary image file.
+	_, data, err := extractROM(infile)
+	if err != nil {
+		return err
+	}
+
+	// Create the zip file in memory.
+	// It won't be very large anyway.
+	buf := &bytes.Buffer{}
+	w := zip.NewWriter(buf)
+
+	// Write the application binary to the zip file.
+	binw, err := w.Create("application.bin")
+	if err != nil {
+		return err
+	}
+	_, err = binw.Write(data)
+	if err != nil {
+		return err
+	}
 
-	if options.PrintCommands != nil {
-		options.PrintCommands(cmdLine[0], cmdLine[1:]...)
+	// Create the init packet.
+	initPacket := nrfInitPacket{
+		ApplicationVersion: 0xffff_ffff, // appears to be unused by the Adafruit bootloader
+		DeviceRevision:     0xffff,      // DFU_DEVICE_REVISION_EMPTY
+		DeviceType:         0x0052,      // ADAFRUIT_DEVICE_TYPE
+		FirmwareCRC16:      crc16.Checksum(data, crc16.MakeTable(crc16.CRC16_CCITT_FALSE)),
+		SoftDeviceRequired: []uint16{0xfffe}, // DFU_SOFTDEVICE_ANY
 	}
 
-	cmd := exec.Command(cmdLine[0], cmdLine[1:]...)
-	cmd.Stdout = io.Discard
-	err := cmd.Run()
+	// Write the init packet to the zip file.
+	datw, err := w.Create("application.dat")
 	if err != nil {
-		return fmt.Errorf("could not run nrfutil pkg generate: %w", err)
+		return err
 	}
-	return nil
+	_, err = datw.Write(initPacket.createInitPacket())
+	if err != nil {
+		return err
+	}
+
+	// Create the JSON manifest.
+	manifest := &jsonManifest{}
+	manifest.Manifest.Application.BinaryFile = "application.bin"
+	manifest.Manifest.Application.DataFile = "application.dat"
+	manifest.Manifest.Application.InitPacketData = initPacket
+	manifest.Manifest.DFUVersion = 0.5
+
+	// Write the JSON manifest to the file.
+	jsonw, err := w.Create("manifest.json")
+	if err != nil {
+		return err
+	}
+	enc := json.NewEncoder(jsonw)
+	enc.SetIndent("", "    ")
+	err = enc.Encode(manifest)
+	if err != nil {
+		return err
+	}
+
+	// Finish the zip file.
+	err = w.Close()
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(outfile, buf.Bytes(), 0o666)
 }