usb: gadget: f_midi: allow customizing the USB MIDI interface string through configfs

When using f_midi from configfs the USB MIDI interface string is hardcoded
to 'MIDI function'.

This USB string descriptor is used by some third-party OS or software to
display the name of the MIDI device

Since we add an additional string option a new macro block was created to
factorize declarations

Signed-off-by: Victor Krawiec <victor.krawiec@arturia.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20251209164006.143219-1-victor.krawiec@arturia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/configfs-usb-gadget-midi

@@ -4,11 +4,12 @@ KernelVersion:	3.19
 Description:
 		The attributes:
 
-		==========	====================================
-		index		index value for the USB MIDI adapter
-		id		ID string for the USB MIDI adapter
-		buflen		MIDI buffer length
-		qlen		USB read request queue length
-		in_ports	number of MIDI input ports
-		out_ports	number of MIDI output ports
-		==========	====================================
+		================	====================================
+		index			index value for the USB MIDI adapter
+		id			ID string for the USB MIDI adapter
+		buflen			MIDI buffer length
+		qlen			USB read request queue length
+		in_ports		number of MIDI input ports
+		out_ports		number of MIDI output ports
+		interface_string	USB AudioControl interface string
+		================	====================================

Documentation/usb/gadget-testing.rst

@@ -368,14 +368,15 @@ Function-specific configfs interface
 The function name to use when creating the function directory is "midi".
 The MIDI function provides these attributes in its function directory:
 
-	=============== ====================================
-	buflen		MIDI buffer length
-	id		ID string for the USB MIDI adapter
-	in_ports	number of MIDI input ports
-	index		index value for the USB MIDI adapter
-	out_ports	number of MIDI output ports
-	qlen		USB read request queue length
-	=============== ====================================
+	================ ====================================
+	buflen		 MIDI buffer length
+	id		 ID string for the USB MIDI adapter
+	in_ports	 number of MIDI input ports
+	index		 index value for the USB MIDI adapter
+	out_ports	 number of MIDI output ports
+	qlen		 USB read request queue length
+	interface_string USB AudioControl interface string
+	================ ====================================
 
 Testing the MIDI function
 -------------------------

drivers/usb/gadget/function/f_midi.c

@@ -875,6 +875,7 @@ static int f_midi_bind(struct usb_configuration *c, struct usb_function *f)
 	struct usb_composite_dev *cdev = c->cdev;
 	struct f_midi *midi = func_to_midi(f);
 	struct usb_string *us;
+	struct f_midi_opts *opts;
 	int status, n, jack = 1, i = 0, endpoint_descriptor_index = 0;
 
 	midi->gadget = cdev->gadget;
@@ -883,6 +884,10 @@ static int f_midi_bind(struct usb_configuration *c, struct usb_function *f)
 	if (status < 0)
 		goto fail_register;
 
+	opts = container_of(f->fi, struct f_midi_opts, func_inst);
+	if (opts->interface_string)
+		midi_string_defs[STRING_FUNC_IDX].s = opts->interface_string;
+
 	/* maybe allocate device-global string ID */
 	us = usb_gstrings_attach(c->cdev, midi_strings,
 				 ARRAY_SIZE(midi_string_defs));
@@ -1178,59 +1183,60 @@ end:									\
 									\
 CONFIGFS_ATTR(f_midi_opts_, name);
 
+#define F_MIDI_OPT_STRING(name)						\
+static ssize_t f_midi_opts_##name##_show(struct config_item *item, char *page) \
+{									\
+	struct f_midi_opts *opts = to_f_midi_opts(item);		\
+	ssize_t result;							\
+									\
+	mutex_lock(&opts->lock);					\
+	if (opts->name) {						\
+		result = strscpy(page, opts->name, PAGE_SIZE);		\
+	} else {							\
+		page[0] = 0;						\
+		result = 0;						\
+	}								\
+									\
+	mutex_unlock(&opts->lock);					\
+									\
+	return result;							\
+}									\
+									\
+static ssize_t f_midi_opts_##name##_store(struct config_item *item,	\
+					 const char *page, size_t len)	\
+{									\
+	struct f_midi_opts *opts = to_f_midi_opts(item);		\
+	int ret;							\
+	char *c;							\
+									\
+	mutex_lock(&opts->lock);					\
+	if (opts->refcnt > 1) {						\
+		ret = -EBUSY;						\
+		goto end;						\
+	}								\
+									\
+	c = kstrndup(page, len, GFP_KERNEL);				\
+	if (!c) {							\
+		ret = -ENOMEM;						\
+		goto end;						\
+	}								\
+	kfree(opts->name);						\
+	opts->name = c;							\
+	ret = len;							\
+end:									\
+	mutex_unlock(&opts->lock);					\
+	return ret;							\
+}									\
+									\
+CONFIGFS_ATTR(f_midi_opts_, name)
+
 F_MIDI_OPT_SIGNED(index, true, SNDRV_CARDS);
 F_MIDI_OPT(buflen, false, 0);
 F_MIDI_OPT(qlen, false, 0);
 F_MIDI_OPT(in_ports, true, MAX_PORTS);
 F_MIDI_OPT(out_ports, true, MAX_PORTS);
-
-static ssize_t f_midi_opts_id_show(struct config_item *item, char *page)
-{
-	struct f_midi_opts *opts = to_f_midi_opts(item);
-	ssize_t result;
-
-	mutex_lock(&opts->lock);
-	if (opts->id) {
-		result = strscpy(page, opts->id, PAGE_SIZE);
-	} else {
-		page[0] = 0;
-		result = 0;
-	}
-
-	mutex_unlock(&opts->lock);
-
-	return result;
-}
-
-static ssize_t f_midi_opts_id_store(struct config_item *item,
-				    const char *page, size_t len)
-{
-	struct f_midi_opts *opts = to_f_midi_opts(item);
-	int ret;
-	char *c;
-
-	mutex_lock(&opts->lock);
-	if (opts->refcnt > 1) {
-		ret = -EBUSY;
-		goto end;
-	}
-
-	c = kstrndup(page, len, GFP_KERNEL);
-	if (!c) {
-		ret = -ENOMEM;
-		goto end;
-	}
-	if (opts->id_allocated)
-		kfree(opts->id);
-	opts->id = c;
-	opts->id_allocated = true;
-	ret = len;
-end:
-	mutex_unlock(&opts->lock);
-	return ret;
-}
-
-CONFIGFS_ATTR(f_midi_opts_, id);
+F_MIDI_OPT_STRING(id);
+F_MIDI_OPT_STRING(interface_string);
 
 static struct configfs_attribute *midi_attrs[] = {
 	&f_midi_opts_attr_index,
@@ -1239,6 +1245,7 @@ static struct configfs_attribute *midi_attrs[] = {
 	&f_midi_opts_attr_in_ports,
 	&f_midi_opts_attr_out_ports,
 	&f_midi_opts_attr_id,
+	&f_midi_opts_attr_interface_string,
 	NULL,
 };
 
@@ -1262,8 +1269,8 @@ static void f_midi_free_inst(struct usb_function_instance *f)
 	mutex_unlock(&opts->lock);
 
 	if (free) {
-		if (opts->id_allocated)
-			kfree(opts->id);
+		kfree(opts->id);
+		kfree(opts->interface_string);
 		kfree(opts);
 	}
 }
@@ -1279,7 +1286,8 @@ static struct usb_function_instance *f_midi_alloc_inst(void)
 	mutex_init(&opts->lock);
 	opts->func_inst.free_func_inst = f_midi_free_inst;
 	opts->index = SNDRV_DEFAULT_IDX1;
-	opts->id = SNDRV_DEFAULT_STR1;
+	opts->id = NULL;
+	opts->interface_string = NULL;
 	opts->buflen = 512;
 	opts->qlen = 32;
 	opts->in_ports = 1;

drivers/usb/gadget/function/u_midi.h

@@ -19,7 +19,7 @@ struct f_midi_opts {
 	struct usb_function_instance	func_inst;
 	int				index;
 	char				*id;
-	bool				id_allocated;
+	char				*interface_string;
 	unsigned int			in_ports;
 	unsigned int			out_ports;
 	unsigned int			buflen;