FunctionFS: enable multiple functions

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/usb/functionfs.txt

@@ -0,0 +1,67 @@
+*How FunctionFS works*
+
+From kernel point of view it is just a composite function with some
+unique behaviour.  It may be added to an USB configuration only after
+the user space driver has registered by writing descriptors and
+strings (the user space program has to provide the same information
+that kernel level composite functions provide when they are added to
+the configuration).
+
+This in particular means that the composite initialisation functions
+may not be in init section (ie. may not use the __init tag).
+
+From user space point of view it is a file system which when
+mounted provides an "ep0" file.  User space driver need to
+write descriptors and strings to that file.  It does not need
+to worry about endpoints, interfaces or strings numbers but
+simply provide descriptors such as if the function was the
+only one (endpoints and strings numbers starting from one and
+interface numbers starting from zero).  The FunctionFS changes
+them as needed also handling situation when numbers differ in
+different configurations.
+
+When descriptors and strings are written "ep#" files appear
+(one for each declared endpoint) which handle communication on
+a single endpoint.  Again, FunctionFS takes care of the real
+numbers and changing of the configuration (which means that
+"ep1" file may be really mapped to (say) endpoint 3 (and when
+configuration changes to (say) endpoint 2)).  "ep0" is used
+for receiving events and handling setup requests.
+
+When all files are closed the function disables itself.
+
+What I also want to mention is that the FunctionFS is designed in such
+a way that it is possible to mount it several times so in the end
+a gadget could use several FunctionFS functions. The idea is that
+each FunctionFS instance is identified by the device name used
+when mounting.
+
+One can imagine a gadget that has an Ethernet, MTP and HID interfaces
+where the last two are implemented via FunctionFS.  On user space
+level it would look like this:
+
+$ insmod g_ffs.ko idVendor=<ID> iSerialNumber=<string> functions=mtp,hid
+$ mkdir /dev/ffs-mtp && mount -t functionfs mtp /dev/ffs-mtp
+$ ( cd /dev/ffs-mtp && mtp-daemon ) &
+$ mkdir /dev/ffs-hid && mount -t functionfs hid /dev/ffs-hid
+$ ( cd /dev/ffs-hid && hid-daemon ) &
+
+On kernel level the gadget checks ffs_data->dev_name to identify
+whether it's FunctionFS designed for MTP ("mtp") or HID ("hid").
+
+If no "functions" module parameters is supplied, the driver accepts
+just one function with any name.
+
+When "functions" module parameter is supplied, only functions
+with listed names are accepted. In particular, if the "functions"
+parameter's value is just a one-element list, then the behaviour
+is similar to when there is no "functions" at all; however,
+only a function with the specified name is accepted.
+
+The gadget is registered only after all the declared function
+filesystems have been mounted and USB descriptors of all functions
+have been written to their ep0's.
+
+Conversely, the gadget is unregistered after the first USB function
+closes its endpoints.
+

drivers/usb/gadget/f_fs.c

@@ -1031,6 +1031,12 @@ struct ffs_sb_fill_data {
 	struct ffs_file_perms perms;
 	umode_t root_mode;
 	const char *dev_name;
+	union {
+		/* set by ffs_fs_mount(), read by ffs_sb_fill() */
+		void *private_data;
+		/* set by ffs_sb_fill(), read by ffs_fs_mount */
+		struct ffs_data *ffs_data;
+	};
 };
 
 static int ffs_sb_fill(struct super_block *sb, void *_data, int silent)
@@ -1047,8 +1053,14 @@ static int ffs_sb_fill(struct super_block *sb, void *_data, int silent)
 		goto Enomem;
 
 	ffs->sb              = sb;
-	ffs->dev_name        = data->dev_name;
+	ffs->dev_name        = kstrdup(data->dev_name, GFP_KERNEL);
+	if (unlikely(!ffs->dev_name))
+		goto Enomem;
 	ffs->file_perms      = data->perms;
+	ffs->private_data    = data->private_data;
+
+	/* used by the caller of this function */
+	data->ffs_data       = ffs;
 
 	sb->s_fs_info        = ffs;
 	sb->s_blocksize      = PAGE_CACHE_SIZE;
@@ -1167,20 +1179,29 @@ ffs_fs_mount(struct file_system_type *t, int flags,
 		},
 		.root_mode = S_IFDIR | 0500,
 	};
+	struct dentry *rv;
 	int ret;
+	void *ffs_dev;
 
 	ENTER();
 
-	ret = functionfs_check_dev_callback(dev_name);
-	if (unlikely(ret < 0))
-		return ERR_PTR(ret);
-
 	ret = ffs_fs_parse_opts(&data, opts);
 	if (unlikely(ret < 0))
 		return ERR_PTR(ret);
 
+	ffs_dev = functionfs_acquire_dev_callback(dev_name);
+	if (IS_ERR(ffs_dev))
+		return ffs_dev;
+
 	data.dev_name = dev_name;
-	return mount_single(t, flags, &data, ffs_sb_fill);
+	data.private_data = ffs_dev;
+	rv = mount_nodev(t, flags, &data, ffs_sb_fill);
+
+	/* data.ffs_data is set by ffs_sb_fill */
+	if (IS_ERR(rv))
+		functionfs_release_dev_callback(data.ffs_data);
+
+	return rv;
 }
 
 static void
@@ -1189,8 +1210,10 @@ ffs_fs_kill_sb(struct super_block *sb)
 	ENTER();
 
 	kill_litter_super(sb);
-	if (sb->s_fs_info)
+	if (sb->s_fs_info) {
+		functionfs_release_dev_callback(sb->s_fs_info);
 		ffs_data_put(sb->s_fs_info);
+	}
 }
 
 static struct file_system_type ffs_fs_type = {
@@ -1256,6 +1279,7 @@ static void ffs_data_put(struct ffs_data *ffs)
 		ffs_data_clear(ffs);
 		BUG_ON(waitqueue_active(&ffs->ev.waitq) ||
 		       waitqueue_active(&ffs->ep0req_completion.wait));
+		kfree(ffs->dev_name);
 		kfree(ffs);
 	}
 }