usb: gadget: add RNDIS configfs options for class/subclass/protocol

This adds 3 new options to the RNDIS gadget function configs. It allows
overriding the default USB interface class/subclass/protocol.

The motivation for this is that if you set the values to "ef" (Misc),
"04" (RNDIS), "01" (Ethernet) respectively, then the device will be
recognized by the rndiscmp.inf file in Windows Vista and newer and will
cause Windows to load the correct RNDIS driver without the need for a
custom (signed) .inf file.

Signed-off-by: David Lechner <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

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

···
 				Ethernet over USB link
 		dev_addr	- MAC address of device's end of this
 				Ethernet over USB link
+		class		- USB interface class, default is 02 (hex)
+		subclass	- USB interface subclass, default is 06 (hex)
+		protocol	- USB interface protocol, default is 00 (hex)

drivers/usb/gadget/function/f_rndis.c

···
 		f->os_desc_table[0].os_desc = &rndis_opts->rndis_os_desc;
 	}
 
+	rndis_iad_descriptor.bFunctionClass = rndis_opts->class;
+	rndis_iad_descriptor.bFunctionSubClass = rndis_opts->subclass;
+	rndis_iad_descriptor.bFunctionProtocol = rndis_opts->protocol;
+
 	/*
 	 * in drivers/usb/gadget/configfs.c:configfs_composite_bind()
 	 * configurations are bound in sequence with list_for_each_entry,
···
 /* f_rndis_opts_ifname */
 USB_ETHERNET_CONFIGFS_ITEM_ATTR_IFNAME(rndis);
 
+/* f_rndis_opts_class */
+USB_ETHER_CONFIGFS_ITEM_ATTR_U8_RW(rndis, class);
+
+/* f_rndis_opts_subclass */
+USB_ETHER_CONFIGFS_ITEM_ATTR_U8_RW(rndis, subclass);
+
+/* f_rndis_opts_protocol */
+USB_ETHER_CONFIGFS_ITEM_ATTR_U8_RW(rndis, protocol);
+
 static struct configfs_attribute *rndis_attrs[] = {
 	&rndis_opts_attr_dev_addr,
 	&rndis_opts_attr_host_addr,
 	&rndis_opts_attr_qmult,
 	&rndis_opts_attr_ifname,
+	&rndis_opts_attr_class,
+	&rndis_opts_attr_subclass,
+	&rndis_opts_attr_protocol,
 	NULL,
 };
 
···
 		return ERR_CAST(net);
 	}
 	INIT_LIST_HEAD(&opts->rndis_os_desc.ext_prop);
+
+	opts->class = rndis_iad_descriptor.bFunctionClass;
+	opts->subclass = rndis_iad_descriptor.bFunctionSubClass;
+	opts->protocol = rndis_iad_descriptor.bFunctionProtocol;
 
 	descs[0] = &opts->rndis_os_desc;
 	names[0] = "rndis";

drivers/usb/gadget/function/u_ether_configfs.h

···
 									\
 	CONFIGFS_ATTR_RO(_f_##_opts_, ifname)
 
+#define USB_ETHER_CONFIGFS_ITEM_ATTR_U8_RW(_f_, _n_)			\
+	static ssize_t _f_##_opts_##_n_##_show(struct config_item *item,\
+					       char *page)		\
+	{								\
+		struct f_##_f_##_opts *opts = to_f_##_f_##_opts(item);	\
+		int ret;						\
+									\
+		mutex_lock(&opts->lock);				\
+		ret = sprintf(page, "%02x\n", opts->_n_);		\
+		mutex_unlock(&opts->lock);				\
+									\
+		return ret;						\
+	}								\
+									\
+	static ssize_t _f_##_opts_##_n_##_store(struct config_item *item,\
+						const char *page,	\
+						size_t len)		\
+	{								\
+		struct f_##_f_##_opts *opts = to_f_##_f_##_opts(item);	\
+		int ret;						\
+		u8 val;							\
+									\
+		mutex_lock(&opts->lock);				\
+		ret = sscanf(page, "%02hhx", &val);			\
+		if (ret > 0) {						\
+			opts->_n_ = val;				\
+			ret = len;					\
+		}							\
+		mutex_unlock(&opts->lock);				\
+									\
+		return ret;						\
+	}								\
+									\
+	CONFIGFS_ATTR(_f_##_opts_, _n_)
+
 #endif /* __U_ETHER_CONFIGFS_H */

drivers/usb/gadget/function/u_rndis.h

···
 	struct usb_os_desc		rndis_os_desc;
 	char				rndis_ext_compat_id[16];
 
+	u8				class;
+	u8				subclass;
+	u8				protocol;
+
 	/*
 	 * Read/write access to configfs attributes is handled by configfs.
 	 *