fix: make wakeup_on_write optional for backward compatibility

Move wakeup_on_write from attrs to optionalAttrs so it is written
with IgnoreErrors=true. On kernels without rv1106-system#57, the
configfs attribute does not exist and writing to it would fail,
breaking the entire USB gadget initialization.

With this change, wakeup_on_write is silently skipped on unpatched
kernels while still being set on patched ones.

Author: jlian

internal/usbgadget/config.go

@@ -9,8 +9,9 @@ type gadgetConfigItem struct {
 	order       uint
 	device      string
 	path        []string
-	attrs       gadgetAttributes
-	configAttrs gadgetAttributes
+	attrs         gadgetAttributes
+	optionalAttrs gadgetAttributes // written with IgnoreErrors; for kernel features that may not exist
+	configAttrs   gadgetAttributes
 	configPath  []string
 	reportDesc  []byte
 }

internal/usbgadget/config_tx.go

@@ -221,6 +221,16 @@ func (tx *UsbGadgetTransaction) writeGadgetItemConfig(item gadgetConfigItem, dep
 		)...)
 	}
 
+	if len(item.optionalAttrs) > 0 {
+		// write optional attributes (IgnoreErrors=true for backward compat with older kernels)
+		files = append(files, tx.writeGadgetAttrsOptional(
+			gadgetItemPath,
+			item.optionalAttrs,
+			component,
+			beforeChange,
+		)...)
+	}
+
 	// write report descriptor if available
 	reportDescPath := path.Join(gadgetItemPath, "report_desc")
 	if item.reportDesc != nil {
@@ -295,6 +305,24 @@ func (tx *UsbGadgetTransaction) writeGadgetAttrs(basePath string, attrs gadgetAt
 	return files
 }
 
+func (tx *UsbGadgetTransaction) writeGadgetAttrsOptional(basePath string, attrs gadgetAttributes, component string, beforeChange []string) (files []string) {
+	files = make([]string, 0)
+	for key, val := range attrs {
+		filePath := filepath.Join(basePath, key)
+		tx.addFileChange(component, RequestedFileChange{
+			Path:            filePath,
+			ExpectedState:   FileStateFileContentMatch,
+			ExpectedContent: []byte(val),
+			Description:     "write optional gadget attribute",
+			DependsOn:       []string{basePath},
+			BeforeChange:    beforeChange,
+			IgnoreErrors:    true,
+		})
+		files = append(files, filePath)
+	}
+	return files
+}
+
 func (tx *UsbGadgetTransaction) addReorderSymlinkChange(path string, target string, deps []string) {
 	tx.log.Trace().Str("path", path).Str("target", target).Msg("add reorder symlink change")
 

internal/usbgadget/hid_keyboard.go

@@ -23,6 +23,8 @@ var keyboardConfig = gadgetConfigItem{
 		"subclass":        "1",
 		"report_length":   "8",
 		"no_out_endpoint": "0",
+	},
+	optionalAttrs: gadgetAttributes{
 		"wakeup_on_write": "1",
 	},
 	reportDesc: keyboardReportDesc,

internal/usbgadget/hid_mouse_absolute.go

@@ -15,6 +15,8 @@ var absoluteMouseConfig = gadgetConfigItem{
 		"subclass":        "0",
 		"report_length":   "6",
 		"no_out_endpoint": "1",
+	},
+	optionalAttrs: gadgetAttributes{
 		"wakeup_on_write": "1",
 	},
 	reportDesc: absoluteMouseCombinedReportDesc,

internal/usbgadget/hid_mouse_relative.go

@@ -15,6 +15,8 @@ var relativeMouseConfig = gadgetConfigItem{
 		"subclass":        "1",
 		"report_length":   "5",
 		"no_out_endpoint": "1",
+	},
+	optionalAttrs: gadgetAttributes{
 		"wakeup_on_write": "1",
 	},
 	reportDesc: relativeMouseCombinedReportDesc,