usb: gadget: f_fs: expose ready state in configfs

When a USB gadget is configured through configfs with 1 or more f_fs
functions, then the logic setting up the gadget configuration has to wait
until the user space code (typically separate applications) responsible for
those functions have written their descriptors before the gadget can be
activated.

The f_fs instance already knows if this has been done, so expose it through
a "ready" attribute in configfs for easier synchronization.

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

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

···
 Description:	The purpose of this directory is to create and remove it.
 
 		A corresponding USB function instance is created/removed.
-		There are no attributes here.
 
-		All parameters are set through FunctionFS.
+		All attributes are read only:
+
+		=============	============================================
+		ready		1 if the function is ready to be used, E.G.
+				if userspace has written descriptors and
+				strings to ep0, so the gadget can be
+				enabled - 0 otherwise.
+		=============	============================================
+
+		All other parameters are set through FunctionFS.

Documentation/usb/gadget-testing.rst

···
 process which implements the function proper). The gadget should be enabled
 by writing a suitable string to usb_gadget/<gadget>/UDC.
 
+The FFS function provides just one attribute in its function directory:
+
+	ready
+
+The attribute is read-only and signals if the function is ready (1) to be
+used, E.G. if userspace has written descriptors and strings to ep0, so
+the gadget can be enabled.
+
 Testing the FFS function
 ------------------------
 

drivers/usb/gadget/function/f_fs.c

···
 			    func_inst.group);
 }
 
+static ssize_t f_fs_opts_ready_show(struct config_item *item, char *page)
+{
+	struct f_fs_opts *opts = to_ffs_opts(item);
+	int ready;
+
+	ffs_dev_lock();
+	ready = opts->dev->desc_ready;
+	ffs_dev_unlock();
+
+	return sprintf(page, "%d\n", ready);
+}
+
+CONFIGFS_ATTR_RO(f_fs_opts_, ready);
+
+static struct configfs_attribute *ffs_attrs[] = {
+	&f_fs_opts_attr_ready,
+	NULL,
+};
+
 static void ffs_attr_release(struct config_item *item)
 {
 	struct f_fs_opts *opts = to_ffs_opts(item);
···
 
 static const struct config_item_type ffs_func_type = {
 	.ct_item_ops	= &ffs_item_ops,
+	.ct_attrs	= ffs_attrs,
 	.ct_owner	= THIS_MODULE,
 };