usb: gadget: f_uac1: allow changing interface name via configfs

This adds "function_name" configfs entry to change string value
of the iInterface field. This field will be shown in Windows' audio
settings panel, so being able to change it is useful. It will default
to "AC Interface" just as before if unchanged.

Signed-off-by: Yunhao Tian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/ABI/testing/configfs-usb-gadget-uac1

···
 					(in 1/256 dB)
 		req_number		the number of pre-allocated requests
 					for both capture and playback
+		function_name		name of the interface
 		=====================	=======================================

Documentation/usb/gadget-testing.rst

···
 	p_volume_res     playback volume control resolution (in 1/256 dB)
 	req_number       the number of pre-allocated request for both capture
 	                 and playback
+	function_name    name of the interface
 	================ ====================================================
 
 The attributes have sane default values.

drivers/usb/gadget/function/f_uac1.c

···
 };
 
 static struct usb_string strings_uac1[] = {
-	[STR_AC_IF].s = "AC Interface",
+	/* [STR_AC_IF].s = DYNAMIC, */
 	[STR_USB_OUT_IT].s = "Playback Input terminal",
 	[STR_USB_OUT_IT_CH_NAMES].s = "Playback Channels",
 	[STR_IO_OUT_OT].s = "Playback Output terminal",
···
 
 	audio_opts = container_of(f->fi, struct f_uac1_opts, func_inst);
 
+	strings_uac1[STR_AC_IF].s = audio_opts->function_name;
+
 	us = usb_gstrings_attach(cdev, uac1_strings, ARRAY_SIZE(strings_uac1));
 	if (IS_ERR(us))
 		return PTR_ERR(us);
···
 									\
 CONFIGFS_ATTR(f_uac1_opts_, name)
 
+#define UAC1_ATTRIBUTE_STRING(name)					\
+static ssize_t f_uac1_opts_##name##_show(struct config_item *item,	\
+					 char *page)			\
+{									\
+	struct f_uac1_opts *opts = to_f_uac1_opts(item);		\
+	int result;							\
+									\
+	mutex_lock(&opts->lock);					\
+	result = snprintf(page, sizeof(opts->name), "%s", opts->name);	\
+	mutex_unlock(&opts->lock);					\
+									\
+	return result;							\
+}									\
+									\
+static ssize_t f_uac1_opts_##name##_store(struct config_item *item,	\
+					  const char *page, size_t len)	\
+{									\
+	struct f_uac1_opts *opts = to_f_uac1_opts(item);		\
+	int ret = 0;							\
+									\
+	mutex_lock(&opts->lock);					\
+	if (opts->refcnt) {						\
+		ret = -EBUSY;						\
+		goto end;						\
+	}								\
+									\
+	ret = snprintf(opts->name, min(sizeof(opts->name), len),	\
+			"%s", page);					\
+									\
+end:									\
+	mutex_unlock(&opts->lock);					\
+	return ret;							\
+}									\
+									\
+CONFIGFS_ATTR(f_uac1_opts_, name)
+
 UAC1_ATTRIBUTE(u32, c_chmask);
 UAC1_RATE_ATTRIBUTE(c_srate);
 UAC1_ATTRIBUTE(u32, c_ssize);
···
 UAC1_ATTRIBUTE(s16, c_volume_min);
 UAC1_ATTRIBUTE(s16, c_volume_max);
 UAC1_ATTRIBUTE(s16, c_volume_res);
+UAC1_ATTRIBUTE_STRING(function_name);
 
 static struct configfs_attribute *f_uac1_attrs[] = {
 	&f_uac1_opts_attr_c_chmask,
···
 	&f_uac1_opts_attr_c_volume_min,
 	&f_uac1_opts_attr_c_volume_max,
 	&f_uac1_opts_attr_c_volume_res,
+
+	&f_uac1_opts_attr_function_name,
 
 	NULL,
 };
···
 	opts->c_volume_res = UAC1_DEF_RES_DB;
 
 	opts->req_number = UAC1_DEF_REQ_NUM;
+
+	snprintf(opts->function_name, sizeof(opts->function_name), "AC Interface");
+
 	return &opts->func_inst;
 }
 

drivers/usb/gadget/function/u_uac1.h

···
 	int				req_number;
 	unsigned			bound:1;
 
+	char			function_name[32];
+
 	struct mutex			lock;
 	int				refcnt;
 };