tools: rewrite gen-device-svd in Go

This should make it more maintainable. Another big advantage that
generation time (including gofmt) is now 3 times faster. No real attempt
at refactoring has been made, that will need to be done at a later time.

1 files changed, 971 insertions, 0 deletions

diff --git a/tools/gen-device-svd/gen-device-svd.go b/tools/gen-device-svd/gen-device-svd.go
new file mode 100755
index 000000000..2a3d03910
--- /dev/null
+++ b/tools/gen-device-svd/gen-device-svd.go
@@ -0,0 +1,971 @@
+package main
+
+import (
+	"bufio"
+	"encoding/xml"
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+	"text/template"
+	"unicode"
+)
+
+var validName = regexp.MustCompile("^[a-zA-Z0-9_]+$")
+
+type SVDFile struct {
+	XMLName     xml.Name `xml:"device"`
+	Name        string   `xml:"name"`
+	Description string   `xml:"description"`
+	LicenseText string   `xml:"licenseText"`
+	Peripherals []struct {
+		Name        string `xml:"name"`
+		Description string `xml:"description"`
+		BaseAddress string `xml:"baseAddress"`
+		GroupName   string `xml:"groupName"`
+		DerivedFrom string `xml:"derivedFrom,attr"`
+		Interrupts  []struct {
+			Name  string `xml:"name"`
+			Index int    `xml:"value"`
+		} `xml:"interrupt"`
+		Registers []*SVDRegister `xml:"registers>register"`
+		Clusters  []*SVDCluster  `xml:"registers>cluster"`
+	} `xml:"peripherals>peripheral"`
+}
+
+type SVDRegister struct {
+	Name          string      `xml:"name"`
+	Description   string      `xml:"description"`
+	Dim           *string     `xml:"dim"`
+	DimIncrement  string      `xml:"dimIncrement"`
+	Size          *string     `xml:"size"`
+	Fields        []*SVDField `xml:"fields>field"`
+	Offset        *string     `xml:"offset"`
+	AddressOffset *string     `xml:"addressOffset"`
+}
+
+type SVDField struct {
+	Name             string  `xml:"name"`
+	Description      string  `xml:"description"`
+	Lsb              *uint32 `xml:"lsb"`
+	Msb              *uint32 `xml:"msb"`
+	BitOffset        *uint32 `xml:"bitOffset"`
+	BitWidth         *uint32 `xml:"bitWidth"`
+	BitRange         *string `xml:"bitRange"`
+	EnumeratedValues []struct {
+		Name        string `xml:"name"`
+		Description string `xml:"description"`
+		Value       string `xml:"value"`
+	} `xml:"enumeratedValues>enumeratedValue"`
+}
+
+type SVDCluster struct {
+	Dim           *int           `xml:"dim"`
+	DimIncrement  string         `xml:"dimIncrement"`
+	DimIndex      *string        `xml:"dimIndex"`
+	Name          string         `xml:"name"`
+	Description   string         `xml:"description"`
+	Registers     []*SVDRegister `xml:"register"`
+	Clusters      []*SVDCluster  `xml:"cluster"`
+	AddressOffset string         `xml:"addressOffset"`
+}
+
+type Device struct {
+	metadata    map[string]string
+	interrupts  []*interrupt
+	peripherals []*peripheral
+}
+
+type interrupt struct {
+	Name            string
+	peripheralIndex int
+	Value           int // interrupt number
+	Description     string
+}
+
+type peripheral struct {
+	Name        string
+	GroupName   string
+	BaseAddress uint64
+	Description string
+	ClusterName string
+	registers   []*PeripheralField
+	subtypes    []*peripheral
+}
+
+// A PeripheralField is a single field in a peripheral type. It may be a full
+// peripheral or a cluster within a peripheral.
+type PeripheralField struct {
+	name        string
+	address     uint64
+	description string
+	registers   []*PeripheralField // contains fields if this is a cluster
+	array       int
+	elementSize int
+	bitfields   []Bitfield
+}
+
+type Bitfield struct {
+	name        string
+	description string
+	value       uint32
+}
+
+func formatText(text string) string {
+	text = regexp.MustCompile(`[ \t\n]+`).ReplaceAllString(text, " ") // Collapse whitespace (like in HTML)
+	text = strings.Replace(text, "\\n ", "\n", -1)
+	text = strings.TrimSpace(text)
+	return text
+}
+
+// Replace characters that are not allowed in a symbol name with a '_'. This is
+// useful to be able to process SVD files with errors.
+func cleanName(text string) string {
+	if !validName.MatchString(text) {
+		result := make([]rune, 0, len(text))
+		for _, c := range text {
+			if validName.MatchString(string(c)) {
+				result = append(result, c)
+			} else {
+				result = append(result, '_')
+			}
+		}
+		text = string(result)
+	}
+	return text
+}
+
+// Read ARM SVD files.
+func readSVD(path, sourceURL string) (*Device, error) {
+	// Open the XML file.
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	decoder := xml.NewDecoder(f)
+	device := &SVDFile{}
+	err = decoder.Decode(device)
+	if err != nil {
+		return nil, err
+	}
+
+	peripheralDict := map[string]*peripheral{}
+	groups := map[string]*peripheral{}
+
+	interrupts := make(map[string]*interrupt)
+	var peripheralsList []*peripheral
+
+	for _, periphEl := range device.Peripherals {
+		description := formatText(periphEl.Description)
+		baseAddress, err := strconv.ParseUint(periphEl.BaseAddress, 0, 32)
+		if err != nil {
+			return nil, fmt.Errorf("invalid base address: %w", err)
+		}
+		// Some group names (for example the STM32H7A3x) have an invalid
+		// group name. Replace invalid characters with "_".
+		groupName := cleanName(periphEl.GroupName)
+
+		for _, interrupt := range periphEl.Interrupts {
+			addInterrupt(interrupts, interrupt.Name, interrupt.Index, description)
+			// As a convenience, also use the peripheral name as the interrupt
+			// name. Only do that for the nrf for now, as the stm32 .svd files
+			// don't always put interrupts in the correct peripheral...
+			if len(periphEl.Interrupts) == 1 && strings.HasPrefix(device.Name, "nrf") {
+				addInterrupt(interrupts, periphEl.Name, interrupt.Index, description)
+			}
+		}
+
+		if _, ok := groups[groupName]; ok || periphEl.DerivedFrom != "" {
+			var derivedFrom *peripheral
+			if periphEl.DerivedFrom != "" {
+				derivedFrom = peripheralDict[periphEl.DerivedFrom]
+			} else {
+				derivedFrom = groups[groupName]
+			}
+			p := &peripheral{
+				Name:        periphEl.Name,
+				GroupName:   derivedFrom.GroupName,
+				Description: description,
+				BaseAddress: baseAddress,
+			}
+			if p.Description == "" {
+				p.Description = derivedFrom.Description
+			}
+			peripheralsList = append(peripheralsList, p)
+			peripheralDict[p.Name] = p
+			for _, subtype := range derivedFrom.subtypes {
+				peripheralsList = append(peripheralsList, &peripheral{
+					Name:        periphEl.Name + "_" + subtype.ClusterName,
+					GroupName:   subtype.GroupName,
+					Description: subtype.Description,
+					BaseAddress: baseAddress,
+				})
+			}
+			continue
+		}
+
+		p := &peripheral{
+			Name:        periphEl.Name,
+			GroupName:   groupName,
+			Description: description,
+			BaseAddress: baseAddress,
+			registers:   []*PeripheralField{},
+		}
+		if p.GroupName == "" {
+			p.GroupName = periphEl.Name
+		}
+		peripheralsList = append(peripheralsList, p)
+		peripheralDict[periphEl.Name] = p
+
+		if _, ok := groups[groupName]; !ok && groupName != "" {
+			groups[groupName] = p
+		}
+
+		for _, register := range periphEl.Registers {
+			regName := groupName // preferably use the group name
+			if regName == "" {
+				regName = periphEl.Name // fall back to peripheral name
+			}
+			p.registers = append(p.registers, parseRegister(regName, register, baseAddress, "")...)
+		}
+		for _, cluster := range periphEl.Clusters {
+			clusterName := strings.Replace(cluster.Name, "[%s]", "", -1)
+			if cluster.DimIndex != nil {
+				clusterName = strings.Replace(clusterName, "%s", "", -1)
+			}
+			clusterPrefix := clusterName + "_"
+			clusterOffset, err := strconv.ParseUint(cluster.AddressOffset, 0, 32)
+			if err != nil {
+				panic(err)
+			}
+			var dim, dimIncrement int
+			if cluster.Dim == nil {
+				if clusterOffset == 0 {
+					// make this a separate peripheral
+					cpRegisters := []*PeripheralField{}
+					for _, regEl := range cluster.Registers {
+						cpRegisters = append(cpRegisters, parseRegister(groupName, regEl, baseAddress, clusterName+"_")...)
+					}
+					// handle sub-clusters of registers
+					for _, subClusterEl := range cluster.Clusters {
+						subclusterName := strings.Replace(subClusterEl.Name, "[%s]", "", -1)
+						subclusterPrefix := subclusterName + "_"
+						subclusterOffset, err := strconv.ParseUint(subClusterEl.AddressOffset, 0, 32)
+						if err != nil {
+							panic(err)
+						}
+						subdim := *subClusterEl.Dim
+						subdimIncrement, err := strconv.ParseInt(subClusterEl.DimIncrement, 0, 32)
+						if err != nil {
+							panic(err)
+						}
+
+						if subdim > 1 {
+							subcpRegisters := []*PeripheralField{}
+							subregSize := 0
+							for _, regEl := range subClusterEl.Registers {
+								size, err := strconv.ParseInt(*regEl.Size, 0, 32)
+								if err != nil {
+									panic(err)
+								}
+								subregSize += int(size)
+								subcpRegisters = append(subcpRegisters, parseRegister(groupName, regEl, baseAddress+subclusterOffset, subclusterPrefix)...)
+							}
+							cpRegisters = append(cpRegisters, &PeripheralField{
+								name:        subclusterName,
+								address:     baseAddress + subclusterOffset,
+								description: subClusterEl.Description,
+								registers:   subcpRegisters,
+								array:       subdim,
+								elementSize: int(subdimIncrement),
+							})
+						} else {
+							for _, regEl := range subClusterEl.Registers {
+								cpRegisters = append(cpRegisters, parseRegister(regEl.Name, regEl, baseAddress+subclusterOffset, subclusterPrefix)...)
+							}
+						}
+					}
+
+					sort.SliceStable(cpRegisters, func(i, j int) bool {
+						return cpRegisters[i].address < cpRegisters[j].address
+					})
+					clusterPeripheral := &peripheral{
+						Name:        periphEl.Name + "_" + clusterName,
+						GroupName:   groupName + "_" + clusterName,
+						Description: description + " - " + clusterName,
+						ClusterName: clusterName,
+						BaseAddress: baseAddress,
+						registers:   cpRegisters,
+					}
+					peripheralsList = append(peripheralsList, clusterPeripheral)
+					peripheralDict[clusterPeripheral.Name] = clusterPeripheral
+					p.subtypes = append(p.subtypes, clusterPeripheral)
+					continue
+				}
+				dim = -1
+				dimIncrement = -1
+			} else {
+				dim = *cluster.Dim
+				if dim == 1 {
+					dimIncrement = -1
+				} else {
+					inc, err := strconv.ParseUint(cluster.DimIncrement, 0, 32)
+					if err != nil {
+						panic(err)
+					}
+					dimIncrement = int(inc)
+				}
+			}
+			clusterRegisters := []*PeripheralField{}
+			for _, regEl := range cluster.Registers {
+				regName := groupName
+				if regName == "" {
+					regName = periphEl.Name
+				}
+				clusterRegisters = append(clusterRegisters, parseRegister(regName, regEl, baseAddress+clusterOffset, clusterPrefix)...)
+			}
+			sort.SliceStable(clusterRegisters, func(i, j int) bool {
+				return clusterRegisters[i].address < clusterRegisters[j].address
+			})
+			if dimIncrement == -1 {
+				lastReg := clusterRegisters[len(clusterRegisters)-1]
+				lastAddress := lastReg.address
+				if lastReg.array != -1 {
+					lastAddress = lastReg.address + uint64(lastReg.array*lastReg.elementSize)
+				}
+				firstAddress := clusterRegisters[0].address
+				dimIncrement = int(lastAddress - firstAddress)
+			}
+			p.registers = append(p.registers, &PeripheralField{
+				name:        clusterName,
+				address:     baseAddress + clusterOffset,
+				description: cluster.Description,
+				registers:   clusterRegisters,
+				array:       dim,
+				elementSize: dimIncrement,
+			})
+		}
+		sort.SliceStable(p.registers, func(i, j int) bool {
+			return p.registers[i].address < p.registers[j].address
+		})
+	}
+
+	// Make a sorted list of interrupts.
+	interruptList := make([]*interrupt, 0, len(interrupts))
+	for _, intr := range interrupts {
+		interruptList = append(interruptList, intr)
+	}
+	sort.SliceStable(interruptList, func(i, j int) bool {
+		if interruptList[i].Value != interruptList[j].Value {
+			return interruptList[i].Value < interruptList[j].Value
+		}
+		return interruptList[i].peripheralIndex < interruptList[j].peripheralIndex
+	})
+
+	// Properly format the license block, with comments.
+	licenseBlock := ""
+	if text := formatText(device.LicenseText); text != "" {
+		licenseBlock = "//     " + strings.Replace(text, "\n", "\n//     ", -1)
+		licenseBlock = regexp.MustCompile(`\s+\n`).ReplaceAllString(licenseBlock, "\n")
+	}
+
+	return &Device{
+		metadata: map[string]string{
+			"file":             filepath.Base(path),
+			"descriptorSource": sourceURL,
+			"name":             device.Name,
+			"nameLower":        strings.ToLower(device.Name),
+			"description":      strings.TrimSpace(device.Description),
+			"licenseBlock":     licenseBlock,
+		},
+		interrupts:  interruptList,
+		peripherals: peripheralsList,
+	}, nil
+}
+
+func addInterrupt(interrupts map[string]*interrupt, name string, index int, description string) {
+	if _, ok := interrupts[name]; ok {
+		if interrupts[name].Value != index {
+			// Note: some SVD files like the one for STM32H7x7 contain mistakes.
+			// Instead of throwing an error, simply log it.
+			fmt.Fprintf(os.Stderr, "interrupt with the same name has different indexes: %s (%d vs %d)",
+				name, interrupts[name].Value, index)
+		}
+		parts := strings.Split(interrupts[name].Description, " // ")
+		hasDescription := false
+		for _, part := range parts {
+			if part == description {
+				hasDescription = true
+			}
+		}
+		if !hasDescription {
+			interrupts[name].Description += " // " + description
+		}
+	} else {
+		interrupts[name] = &interrupt{
+			Name:            name,
+			peripheralIndex: len(interrupts),
+			Value:           index,
+			Description:     description,
+		}
+	}
+}
+
+func parseBitfields(groupName, regName string, fieldEls []*SVDField, bitfieldPrefix string) []Bitfield {
+	var fields []Bitfield
+	for _, fieldEl := range fieldEls {
+		// Some bitfields (like the STM32H7x7) contain invalid bitfield
+		// names like "CNT[31]". Replace invalid characters with "_" when
+		// needed.
+		fieldName := cleanName(fieldEl.Name)
+		if !unicode.IsUpper(rune(fieldName[0])) && !unicode.IsDigit(rune(fieldName[0])) {
+			fieldName = strings.ToUpper(fieldName)
+		}
+
+		// Find the lsb/msb that is encoded in various ways.
+		// Standards are great, that's why there are so many to choose from!
+		var lsb, msb uint32
+		if fieldEl.Lsb != nil && fieldEl.Msb != nil {
+			// try to use lsb/msb tags
+			lsb = *fieldEl.Lsb
+			msb = *fieldEl.Msb
+		} else if fieldEl.BitOffset != nil && fieldEl.BitWidth != nil {
+			// try to use bitOffset/bitWidth tags
+			lsb = *fieldEl.BitOffset
+			msb = *fieldEl.BitWidth + lsb - 1
+		} else if fieldEl.BitRange != nil {
+			// try use bitRange
+			// example string: "[20:16]"
+			parts := strings.Split(strings.Trim(*fieldEl.BitRange, "[]"), ":")
+			l, err := strconv.ParseUint(parts[1], 0, 32)
+			if err != nil {
+				panic(err)
+			}
+			lsb = uint32(l)
+			m, err := strconv.ParseUint(parts[0], 0, 32)
+			if err != nil {
+				panic(err)
+			}
+			msb = uint32(m)
+		} else {
+			// this is an error. what to do?
+			fmt.Fprintln(os.Stderr, "unable to find lsb/msb in field:", fieldName)
+			continue
+		}
+
+		fields = append(fields, Bitfield{
+			name:        fmt.Sprintf("%s_%s%s_%s_Pos", groupName, bitfieldPrefix, regName, fieldName),
+			description: fmt.Sprintf("Position of %s field.", fieldName),
+			value:       lsb,
+		})
+		fields = append(fields, Bitfield{
+			name:        fmt.Sprintf("%s_%s%s_%s_Msk", groupName, bitfieldPrefix, regName, fieldName),
+			description: fmt.Sprintf("Bit mask of %s field.", fieldName),
+			value:       (0xffffffff >> (31 - (msb - lsb))) << lsb,
+		})
+		if lsb == msb { // single bit
+			fields = append(fields, Bitfield{
+				name:        fmt.Sprintf("%s_%s%s_%s", groupName, bitfieldPrefix, regName, fieldName),
+				description: fmt.Sprintf("Bit %s.", fieldName),
+				value:       1 << lsb,
+			})
+		}
+		for _, enumEl := range fieldEl.EnumeratedValues {
+			enumName := enumEl.Name
+			if !unicode.IsUpper(rune(enumName[0])) && !unicode.IsDigit(rune(enumName[0])) {
+				enumName = strings.ToUpper(enumName)
+			}
+			enumDescription := strings.Replace(enumEl.Description, "\n", " ", -1)
+			enumValue, err := strconv.ParseUint(enumEl.Value, 0, 32)
+			if err != nil {
+				panic(err)
+			}
+			fields = append(fields, Bitfield{
+				name:        fmt.Sprintf("%s_%s%s_%s_%s", groupName, bitfieldPrefix, regName, fieldName, enumName),
+				description: enumDescription,
+				value:       uint32(enumValue),
+			})
+		}
+	}
+	return fields
+}
+
+type Register struct {
+	element     *SVDRegister
+	baseAddress uint64
+}
+
+func NewRegister(element *SVDRegister, baseAddress uint64) *Register {
+	return &Register{
+		element:     element,
+		baseAddress: baseAddress,
+	}
+}
+
+func (r *Register) name() string {
+	return strings.Replace(r.element.Name, "[%s]", "", -1)
+}
+
+func (r *Register) description() string {
+	return strings.Replace(r.element.Description, "\n", " ", -1)
+}
+
+func (r *Register) address() uint64 {
+	offsetString := r.element.Offset
+	if offsetString == nil {
+		offsetString = r.element.AddressOffset
+	}
+	addr, err := strconv.ParseUint(*offsetString, 0, 32)
+	if err != nil {
+		panic(err)
+	}
+	return r.baseAddress + addr
+}
+
+func (r *Register) dim() int {
+	if r.element.Dim == nil {
+		return -1 // no dim elements
+	}
+	dim, err := strconv.ParseInt(*r.element.Dim, 0, 32)
+	if err != nil {
+		panic(err)
+	}
+	return int(dim)
+}
+
+func (r *Register) size() int {
+	if r.element.Size != nil {
+		size, err := strconv.ParseInt(*r.element.Size, 0, 32)
+		if err != nil {
+			panic(err)
+		}
+		return int(size) / 8
+	}
+	return 4
+}
+
+func parseRegister(groupName string, regEl *SVDRegister, baseAddress uint64, bitfieldPrefix string) []*PeripheralField {
+	reg := NewRegister(regEl, baseAddress)
+
+	if reg.dim() != -1 {
+		dimIncrement, err := strconv.ParseUint(regEl.DimIncrement, 0, 32)
+		if err != nil {
+			panic(err)
+		}
+		if strings.Contains(reg.name(), "%s") {
+			// a "spaced array" of registers, special processing required
+			// we need to generate a separate register for each "element"
+			var results []*PeripheralField
+			for i := uint64(0); i < uint64(reg.dim()); i++ {
+				regAddress := reg.address() + (i * dimIncrement)
+				results = append(results, &PeripheralField{
+					name:        strings.Replace(reg.name(), "%s", strconv.FormatUint(i, 10), -1),
+					address:     regAddress,
+					description: reg.description(),
+					array:       -1,
+					elementSize: reg.size(),
+				})
+			}
+			// set first result bitfield
+			shortName := strings.ToUpper(strings.Replace(strings.Replace(reg.name(), "_%s", "", -1), "%s", "", -1))
+			results[0].bitfields = parseBitfields(groupName, shortName, regEl.Fields, bitfieldPrefix)
+			return results
+		}
+	}
+	regName := reg.name()
+	if !unicode.IsUpper(rune(regName[0])) && !unicode.IsDigit(rune(regName[0])) {
+		regName = strings.ToUpper(regName)
+	}
+
+	return []*PeripheralField{&PeripheralField{
+		name:        regName,
+		address:     reg.address(),
+		description: reg.description(),
+		bitfields:   parseBitfields(groupName, regName, regEl.Fields, bitfieldPrefix),
+		array:       reg.dim(),
+		elementSize: reg.size(),
+	}}
+}
+
+// The Go module for this device.
+func writeGo(outdir string, device *Device) error {
+	outf, err := os.Create(filepath.Join(outdir, device.metadata["nameLower"]+".go"))
+	if err != nil {
+		return err
+	}
+	defer outf.Close()
+	w := bufio.NewWriter(outf)
+
+	maxInterruptValue := 0
+	for _, intr := range device.interrupts {
+		if intr.Value > maxInterruptValue {
+			maxInterruptValue = intr.Value
+		}
+	}
+
+	t := template.Must(template.New("go").Parse(`// Automatically generated file. DO NOT EDIT.
+// Generated by gen-device-svd.go from {{.metadata.file}}, see {{.metadata.descriptorSource}}
+
+// +build {{.pkgName}},{{.metadata.nameLower}}
+
+// {{.metadata.description}}
+//
+{{.metadata.licenseBlock}}
+package {{.pkgName}}
+
+import (
+	"runtime/volatile"
+	"unsafe"
+)
+
+// Some information about this device.
+const (
+	DEVICE	 = "{{.metadata.name}}"
+)
+
+// Interrupt numbers
+const ({{range .interrupts}}
+	IRQ_{{.Name}} = {{.Value}} // {{.Description}}{{end}}
+	IRQ_max = {{.interruptMax}} // Highest interrupt number on this device.
+)
+
+// Peripherals.
+var (
+{{range .peripherals}}	{{.Name}} = (*{{.GroupName}}_Type)(unsafe.Pointer(uintptr(0x{{printf "%x" .BaseAddress}}))) // {{.Description}}
+{{end}})
+`))
+	err = t.Execute(w, map[string]interface{}{
+		"metadata":     device.metadata,
+		"interrupts":   device.interrupts,
+		"peripherals":  device.peripherals,
+		"pkgName":      filepath.Base(strings.TrimRight(outdir, "/")),
+		"interruptMax": maxInterruptValue,
+	})
+	if err != nil {
+		return err
+	}
+
+	// Define peripheral struct types.
+	for _, peripheral := range device.peripherals {
+		if peripheral.registers == nil {
+			// This peripheral was derived from another peripheral. No new type
+			// needs to be defined for it.
+			continue
+		}
+		fmt.Fprintf(w, "\n// %s\ntype %s_Type struct {\n", peripheral.Description, peripheral.GroupName)
+		address := peripheral.BaseAddress
+		for _, register := range peripheral.registers {
+			if register.registers == nil && address > register.address {
+				// In Nordic SVD files, these registers are deprecated or
+				// duplicates, so can be ignored.
+				//fmt.Fprintf(os.Stderr, "skip: %s.%s 0x%x - 0x%x %d\n", peripheral.Name, register.name, address, register.address, register.elementSize)
+				continue
+			}
+
+			var regType string
+			eSize := register.elementSize
+			switch eSize {
+			case 4:
+				regType = "volatile.Register32"
+			case 2:
+				regType = "volatile.Register16"
+			case 1:
+				regType = "volatile.Register8"
+			default:
+				eSize = 4
+				regType = "volatile.Register32"
+			}
+
+			// insert padding, if needed
+			if address < register.address {
+				bytesNeeded := register.address - address
+				switch bytesNeeded {
+				case 1:
+					w.WriteString("\t_ volatile.Register8\n")
+				case 2:
+					w.WriteString("\t_ volatile.Register16\n")
+				case 3:
+					w.WriteString("\t_ [3]volatile.Register8\n")
+				default:
+					numSkip := (register.address - address) / uint64(eSize)
+					if numSkip == 1 {
+						fmt.Fprintf(w, "\t_ %s\n", regType)
+					} else {
+						fmt.Fprintf(w, "\t_ [%d]%s\n", numSkip, regType)
+					}
+				}
+				address = register.address
+			}
+
+			lastCluster := false
+			if register.registers != nil {
+				// This is a cluster, not a register. Create the cluster type.
+				regType = "struct {\n"
+				subaddress := register.address
+				var subregSize uint64
+				var subregType string
+				for _, subregister := range register.registers {
+					switch subregister.elementSize {
+					case 4:
+						subregType = "volatile.Register32"
+					case 2:
+						subregType = "volatile.Register16"
+					case 1:
+						subregType = "volatile.Register8"
+					}
+					if subregType == "" {
+						panic("unknown element size")
+					}
+
+					if subregister.array != -1 {
+						subregType = fmt.Sprintf("[%d]%s", subregister.array, subregType)
+					}
+					if subaddress != subregister.address {
+						bytesNeeded := subregister.address - subaddress
+						if bytesNeeded == 1 {
+							regType += "\t\t_ volatile.Register8\n"
+						} else if bytesNeeded == 2 {
+							regType += "\t\t_ volatile.Register16\n"
+						} else {
+							numSkip := (subregister.address - subaddress)
+							if numSkip < 1 {
+								continue
+							} else if numSkip == 1 {
+								regType += "\t\t_ volatile.Register8\n"
+							} else {
+								regType += fmt.Sprintf("\t\t_ [%d]volatile.Register8\n", numSkip)
+							}
+						}
+						subaddress += bytesNeeded
+					}
+					if subregister.array != -1 {
+						subregSize = uint64(subregister.array * subregister.elementSize)
+					} else {
+						subregSize = uint64(subregister.elementSize)
+					}
+					subaddress += subregSize
+					regType += fmt.Sprintf("\t\t%s %s\n", subregister.name, subregType)
+				}
+				if register.array != -1 {
+					if subaddress != register.address+uint64(register.elementSize) {
+						numSkip := ((register.address + uint64(register.elementSize)) - subaddress) / subregSize
+						if numSkip <= 1 {
+							regType += fmt.Sprintf("\t\t_ %s\n", subregType)
+						} else {
+							regType += fmt.Sprintf("\t\t_ [%d]%s\n", numSkip, subregType)
+						}
+					}
+				} else {
+					lastCluster = true
+				}
+				regType += "\t}"
+				address = subaddress
+			}
+
+			if register.array != -1 {
+				regType = fmt.Sprintf("[%d]%s", register.array, regType)
+			}
+			fmt.Fprintf(w, "\t%s %s\n", register.name, regType)
+
+			// next address
+			if lastCluster {
+				lastCluster = false
+			} else if register.array != -1 {
+				address = register.address + uint64(register.elementSize*register.array)
+			} else {
+				address = register.address + uint64(register.elementSize)
+			}
+		}
+		w.WriteString("}\n")
+	}
+
+	// Define bitfields.
+	for _, peripheral := range device.peripherals {
+		if peripheral.registers == nil {
+			// This peripheral was derived from another peripheral. Bitfields are
+			// already defined.
+			continue
+		}
+		fmt.Fprintf(w, "\n// Bitfields for %s: %s\nconst(", peripheral.Name, peripheral.Description)
+		for _, register := range peripheral.registers {
+			if len(register.bitfields) != 0 {
+				writeGoRegisterBitfields(w, register, register.name)
+			}
+			if register.registers == nil {
+				continue
+			}
+			for _, subregister := range register.registers {
+				writeGoRegisterBitfields(w, subregister, register.name+"."+subregister.name)
+			}
+		}
+		w.WriteString(")\n")
+	}
+
+	return w.Flush()
+}
+
+func writeGoRegisterBitfields(w *bufio.Writer, register *PeripheralField, name string) {
+	w.WriteString("\n\t// " + name)
+	if register.description != "" {
+		w.WriteString(": " + register.description)
+	}
+	w.WriteByte('\n')
+	for _, bitfield := range register.bitfields {
+		fmt.Fprintf(w, "\t%s = 0x%x", bitfield.name, bitfield.value)
+		if bitfield.description != "" {
+			w.WriteString(" // " + bitfield.description)
+		}
+		w.WriteByte('\n')
+	}
+}
+
+// The interrupt vector, which is hard to write directly in Go.
+func writeAsm(outdir string, device *Device) error {
+	outf, err := os.Create(filepath.Join(outdir, device.metadata["nameLower"]+".s"))
+	if err != nil {
+		return err
+	}
+	defer outf.Close()
+	w := bufio.NewWriter(outf)
+
+	t := template.Must(template.New("go").Parse(`// Automatically generated file. DO NOT EDIT.
+// Generated by gen-device-svd.go from {{.file}}, see {{.descriptorSource}}
+
+// {{.description}}
+//
+{{.licenseBlock}}
+
+.syntax unified
+
+// This is the default handler for interrupts, if triggered but not defined.
+.section .text.Default_Handler
+.global  Default_Handler
+.type    Default_Handler, %function
+Default_Handler:
+    wfe
+    b    Default_Handler
+
+// Avoid the need for repeated .weak and .set instructions.
+.macro IRQ handler
+    .weak  \handler
+    .set   \handler, Default_Handler
+.endm
+
+// Must set the "a" flag on the section:
+// https://svnweb.freebsd.org/base/stable/11/sys/arm/arm/locore-v4.S?r1=321049&r2=321048&pathrev=321049
+// https://sourceware.org/binutils/docs/as/Section.html#ELF-Version
+.section .isr_vector, "a", %progbits
+.global  __isr_vector
+    // Interrupt vector as defined by Cortex-M, starting with the stack top.
+    // On reset, SP is initialized with *0x0 and PC is loaded with *0x4, loading
+    // _stack_top and Reset_Handler.
+    .long _stack_top
+    .long Reset_Handler
+    .long NMI_Handler
+    .long HardFault_Handler
+    .long MemoryManagement_Handler
+    .long BusFault_Handler
+    .long UsageFault_Handler
+    .long 0
+    .long 0
+    .long 0
+    .long 0
+    .long SVC_Handler
+    .long DebugMon_Handler
+    .long 0
+    .long PendSV_Handler
+    .long SysTick_Handler
+
+    // Extra interrupts for peripherals defined by the hardware vendor.
+`))
+	err = t.Execute(w, device.metadata)
+	if err != nil {
+		return err
+	}
+	num := 0
+	for _, intr := range device.interrupts {
+		if intr.Value == num-1 {
+			continue
+		}
+		if intr.Value < num {
+			panic("interrupt numbers are not sorted")
+		}
+		for intr.Value > num {
+			w.WriteString("    .long 0\n")
+			num++
+		}
+		num++
+		fmt.Fprintf(w, "    .long %s_IRQHandler\n", intr.Name)
+	}
+
+	w.WriteString(`
+    // Define default implementations for interrupts, redirecting to
+    // Default_Handler when not implemented.
+    IRQ NMI_Handler
+    IRQ HardFault_Handler
+    IRQ MemoryManagement_Handler
+    IRQ BusFault_Handler
+    IRQ UsageFault_Handler
+    IRQ SVC_Handler
+    IRQ DebugMon_Handler
+    IRQ PendSV_Handler
+    IRQ SysTick_Handler
+`)
+	for _, intr := range device.interrupts {
+		fmt.Fprintf(w, "    IRQ %s_IRQHandler\n", intr.Name)
+	}
+	return w.Flush()
+}
+
+func generate(indir, outdir, sourceURL string) error {
+	if _, err := os.Stat(indir); os.IsNotExist(err) {
+		fmt.Fprintln(os.Stderr, "cannot find input directory:", indir)
+		os.Exit(1)
+	}
+	os.MkdirAll(outdir, 0777)
+
+	infiles, err := filepath.Glob(filepath.Join(indir, "*.svd"))
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "could not read .svd files:", err)
+		os.Exit(1)
+	}
+	sort.Strings(infiles)
+	for _, infile := range infiles {
+		fmt.Println(infile)
+		device, err := readSVD(infile, sourceURL)
+		if err != nil {
+			return fmt.Errorf("failed to read: %w", err)
+		}
+		err = writeGo(outdir, device)
+		if err != nil {
+			return fmt.Errorf("failed to write Go file: %w", err)
+		}
+		err = writeAsm(outdir, device)
+		if err != nil {
+			return fmt.Errorf("failed to write assembly file: %w", err)
+		}
+	}
+	return nil
+}
+
+func main() {
+	sourceURL := flag.String("source", "<unknown>", "source SVD file")
+	flag.Parse()
+	if flag.NArg() != 2 {
+		fmt.Fprintln(os.Stderr, "provide exactly two arguments: input directory (with .svd files) and output directory for generated files")
+		flag.PrintDefaults()
+		return
+	}
+	indir := flag.Arg(0)
+	outdir := flag.Arg(1)
+	err := generate(indir, outdir, *sourceURL)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}