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

Add a configfs entry, "function_name", to change the iInterface field
for VideoControl. This name is used on host devices for user selection,
useful when multiple cameras are present. The default will remain "UVC
Camera".

Signed-off-by: Dan Vacura <w36195@motorola.com>
Link: https://lore.kernel.org/r/20220401160447.5919-1-w36195@motorola.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -7,6 +7,7 @@ Description:	UVC function directory
 		streaming_maxburst	0..15 (ss only)
 		streaming_maxpacket	1..1023 (fs), 1..3072 (hs/ss)
 		streaming_interval	1..16
+		function_name		string [32]
 		===================	=============================
 
 What:		/config/usb-gadget/gadget/functions/uvc.name/control

Documentation/usb/gadget-testing.rst

@@ -787,6 +787,7 @@ The uvc function provides these attributes in its function directory:
 	streaming_maxpacket maximum packet size this endpoint is capable of
 			    sending or receiving when this configuration is
 			    selected
+	function_name       name of the interface
 	=================== ================================================
 
 There are also "control" and "streaming" subdirectories, each of which contain

drivers/usb/gadget/function/f_uvc.c

@@ -44,7 +44,7 @@ MODULE_PARM_DESC(trace, "Trace level bitmask");
 #define UVC_STRING_STREAMING_IDX		1
 
 static struct usb_string uvc_en_us_strings[] = {
-	[UVC_STRING_CONTROL_IDX].s = "UVC Camera",
+	/* [UVC_STRING_CONTROL_IDX].s = DYNAMIC, */
 	[UVC_STRING_STREAMING_IDX].s = "Video Streaming",
 	{  }
 };
@@ -676,6 +676,7 @@ uvc_function_bind(struct usb_configuration *c, struct usb_function *f)
 	uvc_hs_streaming_ep.bEndpointAddress = uvc->video.ep->address;
 	uvc_ss_streaming_ep.bEndpointAddress = uvc->video.ep->address;
 
+	uvc_en_us_strings[UVC_STRING_CONTROL_IDX].s = opts->function_name;
 	us = usb_gstrings_attach(cdev, uvc_function_strings,
 				 ARRAY_SIZE(uvc_en_us_strings));
 	if (IS_ERR(us)) {
@@ -866,6 +867,7 @@ static struct usb_function_instance *uvc_alloc_inst(void)
 
 	opts->streaming_interval = 1;
 	opts->streaming_maxpacket = 1024;
+	snprintf(opts->function_name, sizeof(opts->function_name), "UVC Camera");
 
 	ret = uvcg_attach_configfs(opts);
 	if (ret < 0) {

drivers/usb/gadget/function/u_uvc.h

@@ -27,6 +27,7 @@ struct f_uvc_opts {
 
 	unsigned int					control_interface;
 	unsigned int					streaming_interface;
+	char						function_name[32];
 
 	/*
 	 * Control descriptors array pointers for full-/high-speed and

drivers/usb/gadget/function/uvc_configfs.c

@@ -2425,10 +2425,51 @@ UVCG_OPTS_ATTR(streaming_maxburst, streaming_maxburst, 15);
 
 #undef UVCG_OPTS_ATTR
 
+#define UVCG_OPTS_STRING_ATTR(cname, aname)				\
+static ssize_t f_uvc_opts_string_##cname##_show(struct config_item *item,\
+					 char *page)			\
+{									\
+	struct f_uvc_opts *opts = to_f_uvc_opts(item);			\
+	int result;							\
+									\
+	mutex_lock(&opts->lock);					\
+	result = snprintf(page, sizeof(opts->aname), "%s", opts->aname);\
+	mutex_unlock(&opts->lock);					\
+									\
+	return result;							\
+}									\
+									\
+static ssize_t f_uvc_opts_string_##cname##_store(struct config_item *item,\
+					  const char *page, size_t len)	\
+{									\
+	struct f_uvc_opts *opts = to_f_uvc_opts(item);			\
+	int ret = 0;							\
+									\
+	mutex_lock(&opts->lock);					\
+	if (opts->refcnt) {						\
+		ret = -EBUSY;						\
+		goto end;						\
+	}								\
+									\
+	ret = snprintf(opts->aname, min(sizeof(opts->aname), len),	\
+			"%s", page);					\
+									\
+end:									\
+	mutex_unlock(&opts->lock);					\
+	return ret;							\
+}									\
+									\
+UVC_ATTR(f_uvc_opts_string_, cname, aname)
+
+UVCG_OPTS_STRING_ATTR(function_name, function_name);
+
+#undef UVCG_OPTS_STRING_ATTR
+
 static struct configfs_attribute *uvc_attrs[] = {
 	&f_uvc_opts_attr_streaming_interval,
 	&f_uvc_opts_attr_streaming_maxpacket,
 	&f_uvc_opts_attr_streaming_maxburst,
+	&f_uvc_opts_string_attr_function_name,
 	NULL,
 };