// SPDX-License-Identifier: GPL-2.0+
/*
 * trans_usbg.c - USB peripheral usb9pfs configuration driver and transport.
 *
 * Copyright (C) 2024 Michael Grzeschik <m.grzeschik@pengutronix.de>
 */

/* Gadget usb9pfs only needs two bulk endpoints, and will use the usb9pfs
 * transport to mount host exported filesystem via usb gadget.
 */

/*     +--------------------------+    |    +--------------------------+
 *     |  9PFS mounting client    |    |    |  9PFS exporting server   |
 *  SW |                          |    |    |                          |
 *     |   (this:trans_usbg)      |    |    |(e.g. diod or nfs-ganesha)|
 *     +-------------^------------+    |    +-------------^------------+
 *                   |                 |                  |
 * ------------------|------------------------------------|-------------
 *                   |                 |                  |
 *     +-------------v------------+    |    +-------------v------------+
 *     |                          |    |    |                          |
 *  HW |   USB Device Controller  <--------->   USB Host Controller    |
 *     |                          |    |    |                          |
 *     +--------------------------+    |    +--------------------------+
 */

#include <linux/cleanup.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs_context.h>
#include <linux/usb/composite.h>
#include <linux/usb/func_utils.h>

#include <net/9p/9p.h>
#include <net/9p/client.h>
#include <net/9p/transport.h>

#define DEFAULT_BUFLEN        16384

struct f_usb9pfs {
	struct p9_client *client;

	/* 9p request lock for en/dequeue */
	spinlock_t lock;

	struct usb_request *in_req;
	struct usb_request *out_req;

	struct usb_ep *in_ep;
	struct usb_ep *out_ep;

	struct completion send;
	struct completion received;

	unsigned int buflen;

	struct usb_function function;
};

static inline struct f_usb9pfs *func_to_usb9pfs(struct usb_function *f)
{
	return container_of(f, struct f_usb9pfs, function);
}

struct f_usb9pfs_opts {
	struct usb_function_instance func_inst;
	unsigned int buflen;

	struct f_usb9pfs_dev *dev;

	/* Read/write access to configfs attributes is handled by configfs.
	 *
	 * This is to protect the data from concurrent access by read/write
	 * and create symlink/remove symlink.
	 */
	struct mutex lock;
	int refcnt;
};

struct f_usb9pfs_dev {
	struct f_usb9pfs *usb9pfs;
	struct f_usb9pfs_opts *opts;
	char tag[41];
	bool inuse;

	struct list_head usb9pfs_instance;
};

static DEFINE_MUTEX(usb9pfs_lock);
static struct list_head usbg_instance_list;

static int usb9pfs_queue_tx(struct f_usb9pfs *usb9pfs, struct p9_req_t *p9_tx_req,
			    gfp_t gfp_flags)
{
	struct usb_composite_dev *cdev = usb9pfs->function.config->cdev;
	struct usb_request *req = usb9pfs->in_req;
	int ret;

	if (!(p9_tx_req->tc.size % usb9pfs->in_ep->maxpacket))
		req->zero = 1;

	req->buf = p9_tx_req->tc.sdata;
	req->length = p9_tx_req->tc.size;
	req->context = p9_tx_req;

	dev_dbg(&cdev->gadget->dev, "%s usb9pfs send --> %d/%d, zero: %d\n",
		usb9pfs->in_ep->name, req->actual, req->length, req->zero);

	ret = usb_ep_queue(usb9pfs->in_ep, req, gfp_flags);
	if (ret)
		req->context = NULL;

	dev_dbg(&cdev->gadget->dev, "tx submit --> %d\n", ret);

	return ret;
}

static int usb9pfs_queue_rx(struct f_usb9pfs *usb9pfs, struct usb_request *req,
			    gfp_t gfp_flags)
{
	struct usb_composite_dev *cdev = usb9pfs->function.config->cdev;
	int ret;

	ret = usb_ep_queue(usb9pfs->out_ep, req, gfp_flags);

	dev_dbg(&cdev->gadget->dev, "rx submit --> %d\n", ret);

	return ret;
}

static int usb9pfs_transmit(struct f_usb9pfs *usb9pfs, struct p9_req_t *p9_req)
{
	int ret = 0;

	guard(spinlock_irqsave)(&usb9pfs->lock);

	ret = usb9pfs_queue_tx(usb9pfs, p9_req, GFP_ATOMIC);
	if (ret)
		return ret;

	list_del(&p9_req->req_list);

	p9_req_get(p9_req);

	return ret;
}

static void usb9pfs_tx_complete(struct usb_ep *ep, struct usb_request *req)
{
	struct f_usb9pfs *usb9pfs = ep->driver_data;
	struct usb_composite_dev *cdev = usb9pfs->function.config->cdev;
	struct p9_req_t *p9_tx_req = req->context;
	unsigned long flags;

	/* reset zero packages */
	req->zero = 0;

	if (req->status) {
		dev_err(&cdev->gadget->dev, "%s usb9pfs complete --> %d, %d/%d\n",
			ep->name, req->status, req->actual, req->length);
		return;
	}

	dev_dbg(&cdev->gadget->dev, "%s usb9pfs complete --> %d, %d/%d\n",
		ep->name, req->status, req->actual, req->length);

	spin_lock_irqsave(&usb9pfs->lock, flags);
	WRITE_ONCE(p9_tx_req->status, REQ_STATUS_SENT);

	p9_req_put(usb9pfs->client, p9_tx_req);

	req->context = NULL;

	spin_unlock_irqrestore(&usb9pfs->lock, flags);

	complete(&usb9pfs->send);
}

static struct p9_req_t *usb9pfs_rx_header(struct f_usb9pfs *usb9pfs, void *buf)
{
	struct p9_req_t *p9_rx_req;
	struct p9_fcall	rc;
	int ret;

	/* start by reading header */
	rc.sdata = buf;
	rc.offset = 0;
	rc.capacity = P9_HDRSZ;
	rc.size = P9_HDRSZ;

	p9_debug(P9_DEBUG_TRANS, "mux %p got %zu bytes\n", usb9pfs,
		 rc.capacity - rc.offset);

	ret = p9_parse_header(&rc, &rc.size, NULL, NULL, 0);
	if (ret) {
		p9_debug(P9_DEBUG_ERROR,
			 "error parsing header: %d\n", ret);
		return NULL;
	}

	p9_debug(P9_DEBUG_TRANS,
		 "mux %p pkt: size: %d bytes tag: %d\n",
		 usb9pfs, rc.size, rc.tag);

	p9_rx_req = p9_tag_lookup(usb9pfs->client, rc.tag);
	if (!p9_rx_req || p9_rx_req->status != REQ_STATUS_SENT) {
		p9_debug(P9_DEBUG_ERROR, "Unexpected packet tag %d\n", rc.tag);
		return NULL;
	}

	if (rc.size > p9_rx_req->rc.capacity) {
		p9_debug(P9_DEBUG_ERROR,
			 "requested packet size too big: %d for tag %d with capacity %zd\n",
			 rc.size, rc.tag, p9_rx_req->rc.capacity);
		p9_req_put(usb9pfs->client, p9_rx_req);
		return NULL;
	}

	if (!p9_rx_req->rc.sdata) {
		p9_debug(P9_DEBUG_ERROR,
			 "No recv fcall for tag %d (req %p), disconnecting!\n",
			 rc.tag, p9_rx_req);
		p9_req_put(usb9pfs->client, p9_rx_req);
		return NULL;
	}

	return p9_rx_req;
}

static void usb9pfs_rx_complete(struct usb_ep *ep, struct usb_request *req)
{
	struct f_usb9pfs *usb9pfs = ep->driver_data;
	struct usb_composite_dev *cdev = usb9pfs->function.config->cdev;
	struct p9_req_t *p9_rx_req;
	unsigned int req_size = req->actual;
	int status = REQ_STATUS_RCVD;

	if (req->status) {
		dev_err(&cdev->gadget->dev, "%s usb9pfs complete --> %d, %d/%d\n",
			ep->name, req->status, req->actual, req->length);
		return;
	}

	p9_rx_req = usb9pfs_rx_header(usb9pfs, req->buf);
	if (!p9_rx_req)
		return;

	if (req_size > p9_rx_req->rc.capacity) {
		dev_err(&cdev->gadget->dev,
			"%s received data size %u exceeds buffer capacity %zu\n",
			ep->name, req_size, p9_rx_req->rc.capacity);
		req_size = 0;
		status = REQ_STATUS_ERROR;
	}

	memcpy(p9_rx_req->rc.sdata, req->buf, req_size);

	p9_rx_req->rc.size = req_size;

	p9_client_cb(usb9pfs->client, p9_rx_req, status);
	p9_req_put(usb9pfs->client, p9_rx_req);

	complete(&usb9pfs->received);
}

static void disable_ep(struct usb_composite_dev *cdev, struct usb_ep *ep)
{
	int value;

	value = usb_ep_disable(ep);
	if (value < 0)
		dev_info(&cdev->gadget->dev,
			 "disable %s --> %d\n", ep->name, value);
}

static void disable_usb9pfs(struct f_usb9pfs *usb9pfs)
{
	struct usb_composite_dev *cdev =
		usb9pfs->function.config->cdev;

	if (usb9pfs->in_req) {
		usb_ep_free_request(usb9pfs->in_ep, usb9pfs->in_req);
		usb9pfs->in_req = NULL;
	}

	if (usb9pfs->out_req) {
		usb_ep_free_request(usb9pfs->out_ep, usb9pfs->out_req);
		usb9pfs->out_req = NULL;
	}

	disable_ep(cdev, usb9pfs->in_ep);
	disable_ep(cdev, usb9pfs->out_ep);
	dev_dbg(&cdev->gadget->dev, "%s disabled\n",
		usb9pfs->function.name);
}

static int alloc_requests(struct usb_composite_dev *cdev,
			  struct f_usb9pfs *usb9pfs)
{
	int ret;

	usb9pfs->in_req = usb_ep_alloc_request(usb9pfs->in_ep, GFP_ATOMIC);
	if (!usb9pfs->in_req) {
		ret = -ENOENT;
		goto fail;
	}

	usb9pfs->out_req = alloc_ep_req(usb9pfs->out_ep, usb9pfs->buflen);
	if (!usb9pfs->out_req) {
		ret = -ENOENT;
		goto fail_in;
	}

	usb9pfs->in_req->complete = usb9pfs_tx_complete;
	usb9pfs->out_req->complete = usb9pfs_rx_complete;

	/* length will be set in complete routine */
	usb9pfs->in_req->context = usb9pfs;
	usb9pfs->out_req->context = usb9pfs;

	return 0;

fail_in:
	usb_ep_free_request(usb9pfs->in_ep, usb9pfs->in_req);
fail:
	return ret;
}

static int enable_endpoint(struct usb_composite_dev *cdev,
			   struct f_usb9pfs *usb9pfs, struct usb_ep *ep)
{
	int ret;

	ret = config_ep_by_speed(cdev->gadget, &usb9pfs->function, ep);
	if (ret)
		return ret;

	ret = usb_ep_enable(ep);
	if (ret < 0)
		return ret;

	ep->driver_data = usb9pfs;

	return 0;
}

static int
enable_usb9pfs(struct usb_composite_dev *cdev, struct f_usb9pfs *usb9pfs)
{
	struct p9_client *client;
	int ret = 0;

	ret = enable_endpoint(cdev, usb9pfs, usb9pfs->in_ep);
	if (ret)
		goto out;

	ret = enable_endpoint(cdev, usb9pfs, usb9pfs->out_ep);
	if (ret)
		goto disable_in;

	ret = alloc_requests(cdev, usb9pfs);
	if (ret)
		goto disable_out;

	client = usb9pfs->client;
	if (client)
		client->status = Connected;

	dev_dbg(&cdev->gadget->dev, "%s enabled\n", usb9pfs->function.name);
	return 0;

disable_out:
	usb_ep_disable(usb9pfs->out_ep);
disable_in:
	usb_ep_disable(usb9pfs->in_ep);
out:
	return ret;
}

static int p9_usbg_create(struct p9_client *client, struct fs_context *fc)
{
	const char *devname = fc->source;
	struct f_usb9pfs_dev *dev;
	struct f_usb9pfs *usb9pfs;
	int ret = -ENOENT;
	int found = 0;

	if (!devname)
		return -EINVAL;

	guard(mutex)(&usb9pfs_lock);

	list_for_each_entry(dev, &usbg_instance_list, usb9pfs_instance) {
		if (!strncmp(devname, dev->tag, strlen(devname))) {
			if (!dev->inuse) {
				dev->inuse = true;
				found = 1;
				break;
			}
			ret = -EBUSY;
			break;
		}
	}

	if (!found) {
		pr_err("no channels available for device %s\n", devname);
		return ret;
	}

	usb9pfs = dev->usb9pfs;
	if (!usb9pfs)
		return -EINVAL;

	client->trans = (void *)usb9pfs;
	if (!usb9pfs->in_req)
		client->status = Disconnected;
	else
		client->status = Connected;
	usb9pfs->client = client;

	client->trans_mod->maxsize = usb9pfs->buflen;

	complete(&usb9pfs->received);

	return 0;
}

static void usb9pfs_clear_tx(struct f_usb9pfs *usb9pfs)
{
	struct p9_req_t *req;

	guard(spinlock_irqsave)(&usb9pfs->lock);

	req = usb9pfs->in_req->context;
	if (!req)
		return;

	if (!req->t_err)
		req->t_err = -ECONNRESET;

	p9_client_cb(usb9pfs->client, req, REQ_STATUS_ERROR);
}

static void p9_usbg_close(struct p9_client *client)
{
	struct f_usb9pfs *usb9pfs;
	struct f_usb9pfs_dev *dev;
	struct f_usb9pfs_opts *opts;

	if (!client)
		return;

	usb9pfs = client->trans;
	if (!usb9pfs)
		return;

	client->status = Disconnected;

	usb9pfs_clear_tx(usb9pfs);

	opts = container_of(usb9pfs->function.fi,
			    struct f_usb9pfs_opts, func_inst);

	dev = opts->dev;

	mutex_lock(&usb9pfs_lock);
	dev->inuse = false;
	mutex_unlock(&usb9pfs_lock);
}

static int p9_usbg_request(struct p9_client *client, struct p9_req_t *p9_req)
{
	struct f_usb9pfs *usb9pfs = client->trans;
	int ret;

	if (client->status != Connected)
		return -EBUSY;

	ret = wait_for_completion_killable(&usb9pfs->received);
	if (ret)
		return ret;

	ret = usb9pfs_transmit(usb9pfs, p9_req);
	if (ret)
		return ret;

	ret = wait_for_completion_killable(&usb9pfs->send);
	if (ret)
		return ret;

	return usb9pfs_queue_rx(usb9pfs, usb9pfs->out_req, GFP_ATOMIC);
}

static int p9_usbg_cancel(struct p9_client *client, struct p9_req_t *req)
{
	struct f_usb9pfs *usb9pfs = client->trans;
	int ret = 1;

	p9_debug(P9_DEBUG_TRANS, "client %p req %p\n", client, req);

	guard(spinlock_irqsave)(&usb9pfs->lock);

	if (req->status == REQ_STATUS_UNSENT) {
		list_del(&req->req_list);
		WRITE_ONCE(req->status, REQ_STATUS_FLSHD);
		p9_req_put(client, req);
		ret = 0;
	}

	return ret;
}

static struct p9_trans_module p9_usbg_trans = {
	.name = "usbg",
	.create = p9_usbg_create,
	.close = p9_usbg_close,
	.request = p9_usbg_request,
	.cancel = p9_usbg_cancel,
	.supports_vmalloc = false,
	.owner = THIS_MODULE,
};

/*-------------------------------------------------------------------------*/

#define USB_PROTOCOL_9PFS	0x09

static struct usb_interface_descriptor usb9pfs_intf = {
	.bLength =		sizeof(usb9pfs_intf),
	.bDescriptorType =	USB_DT_INTERFACE,

	.bNumEndpoints =	2,
	.bInterfaceClass =	USB_CLASS_VENDOR_SPEC,
	.bInterfaceSubClass =	USB_SUBCLASS_VENDOR_SPEC,
	.bInterfaceProtocol =   USB_PROTOCOL_9PFS,

	/* .iInterface = DYNAMIC */
};

/* full speed support: */

static struct usb_endpoint_descriptor fs_usb9pfs_source_desc = {
	.bLength =		USB_DT_ENDPOINT_SIZE,
	.bDescriptorType =	USB_DT_ENDPOINT,

	.bEndpointAddress =	USB_DIR_IN,
	.bmAttributes =		USB_ENDPOINT_XFER_BULK,
};

static struct usb_endpoint_descriptor fs_usb9pfs_sink_desc = {
	.bLength =		USB_DT_ENDPOINT_SIZE,
	.bDescriptorType =	USB_DT_ENDPOINT,

	.bEndpointAddress =	USB_DIR_OUT,
	.bmAttributes =		USB_ENDPOINT_XFER_BULK,
};

static struct usb_descriptor_header *fs_usb9pfs_descs[] = {
	(struct usb_descriptor_header *)&usb9pfs_intf,
	(struct usb_descriptor_header *)&fs_usb9pfs_sink_desc,
	(struct usb_descriptor_header *)&fs_usb9pfs_source_desc,
	NULL,
};

/* high speed support: */

static struct usb_endpoint_descriptor hs_usb9pfs_source_desc = {
	.bLength =		USB_DT_ENDPOINT_SIZE,
	.bDescriptorType =	USB_DT_ENDPOINT,

	.bmAttributes =		USB_ENDPOINT_XFER_BULK,
	.wMaxPacketSize =	cpu_to_le16(512),
};

static struct usb_endpoint_descriptor hs_usb9pfs_sink_desc = {
	.bLength =		USB_DT_ENDPOINT_SIZE,
	.bDescriptorType =	USB_DT_ENDPOINT,

	.bmAttributes =		USB_ENDPOINT_XFER_BULK,
	.wMaxPacketSize =	cpu_to_le16(512),
};

static struct usb_descriptor_header *hs_usb9pfs_descs[] = {
	(struct usb_descriptor_header *)&usb9pfs_intf,
	(struct usb_descriptor_header *)&hs_usb9pfs_source_desc,
	(struct usb_descriptor_header *)&hs_usb9pfs_sink_desc,
	NULL,
};

/* super speed support: */

static struct usb_endpoint_descriptor ss_usb9pfs_source_desc = {
	.bLength =		USB_DT_ENDPOINT_SIZE,
	.bDescriptorType =	USB_DT_ENDPOINT,

	.bmAttributes =		USB_ENDPOINT_XFER_BULK,
	.wMaxPacketSize =	cpu_to_le16(1024),
};

static struct usb_ss_ep_comp_descriptor ss_usb9pfs_source_comp_desc = {
	.bLength =		USB_DT_SS_EP_COMP_SIZE,
	.bDescriptorType =	USB_DT_SS_ENDPOINT_COMP,
	.bMaxBurst =		0,
	.bmAttributes =		0,
	.wBytesPerInterval =	0,
};

static struct usb_endpoint_descriptor ss_usb9pfs_sink_desc = {
	.bLength =		USB_DT_ENDPOINT_SIZE,
	.bDescriptorType =	USB_DT_ENDPOINT,

	.bmAttributes =		USB_ENDPOINT_XFER_BULK,
	.wMaxPacketSize =	cpu_to_le16(1024),
};

static struct usb_ss_ep_comp_descriptor ss_usb9pfs_sink_comp_desc = {
	.bLength =		USB_DT_SS_EP_COMP_SIZE,
	.bDescriptorType =	USB_DT_SS_ENDPOINT_COMP,
	.bMaxBurst =		0,
	.bmAttributes =		0,
	.wBytesPerInterval =	0,
};

static struct usb_descriptor_header *ss_usb9pfs_descs[] = {
	(struct usb_descriptor_header *)&usb9pfs_intf,
	(struct usb_descriptor_header *)&ss_usb9pfs_source_desc,
	(struct usb_descriptor_header *)&ss_usb9pfs_source_comp_desc,
	(struct usb_descriptor_header *)&ss_usb9pfs_sink_desc,
	(struct usb_descriptor_header *)&ss_usb9pfs_sink_comp_desc,
	NULL,
};

/* function-specific strings: */
static struct usb_string strings_usb9pfs[] = {
	[0].s = "usb9pfs input to output",
	{  }			/* end of list */
};

static struct usb_gadget_strings stringtab_usb9pfs = {
	.language	= 0x0409,	/* en-us */
	.strings	= strings_usb9pfs,
};

static struct usb_gadget_strings *usb9pfs_strings[] = {
	&stringtab_usb9pfs,
	NULL,
};

/*-------------------------------------------------------------------------*/

static int usb9pfs_func_bind(struct usb_configuration *c,
			     struct usb_function *f)
{
	struct f_usb9pfs *usb9pfs = func_to_usb9pfs(f);
	struct f_usb9pfs_opts *opts;
	struct usb_composite_dev *cdev = c->cdev;
	int ret;
	int id;

	/* allocate interface ID(s) */
	id = usb_interface_id(c, f);
	if (id < 0)
		return id;
	usb9pfs_intf.bInterfaceNumber = id;

	id = usb_string_id(cdev);
	if (id < 0)
		return id;
	strings_usb9pfs[0].id = id;
	usb9pfs_intf.iInterface = id;

	/* allocate endpoints */
	usb9pfs->in_ep = usb_ep_autoconfig(cdev->gadget,
					   &fs_usb9pfs_source_desc);
	if (!usb9pfs->in_ep)
		goto autoconf_fail;

	usb9pfs->out_ep = usb_ep_autoconfig(cdev->gadget,
					    &fs_usb9pfs_sink_desc);
	if (!usb9pfs->out_ep)
		goto autoconf_fail;

	/* support high speed hardware */
	hs_usb9pfs_source_desc.bEndpointAddress =
		fs_usb9pfs_source_desc.bEndpointAddress;
	hs_usb9pfs_sink_desc.bEndpointAddress =
		fs_usb9pfs_sink_desc.bEndpointAddress;

	/* support super speed hardware */
	ss_usb9pfs_source_desc.bEndpointAddress =
		fs_usb9pfs_source_desc.bEndpointAddress;
	ss_usb9pfs_sink_desc.bEndpointAddress =
		fs_usb9pfs_sink_desc.bEndpointAddress;

	ret = usb_assign_descriptors(f, fs_usb9pfs_descs, hs_usb9pfs_descs,
				     ss_usb9pfs_descs, ss_usb9pfs_descs);
	if (ret)
		return ret;

	opts = container_of(f->fi, struct f_usb9pfs_opts, func_inst);
	opts->dev->usb9pfs = usb9pfs;

	dev_dbg(&cdev->gadget->dev, "%s speed %s: IN/%s, OUT/%s\n",
		(gadget_is_superspeed(c->cdev->gadget) ? "super" :
		(gadget_is_dualspeed(c->cdev->gadget) ? "dual" : "full")),
			f->name, usb9pfs->in_ep->name, usb9pfs->out_ep->name);

	return 0;

autoconf_fail:
	ERROR(cdev, "%s: can't autoconfigure on %s\n",
	      f->name, cdev->gadget->name);
	return -ENODEV;
}

static void usb9pfs_func_unbind(struct usb_configuration *c,
				struct usb_function *f)
{
	struct f_usb9pfs *usb9pfs = func_to_usb9pfs(f);

	disable_usb9pfs(usb9pfs);
}

static void usb9pfs_free_func(struct usb_function *f)
{
	struct f_usb9pfs *usb9pfs = func_to_usb9pfs(f);
	struct f_usb9pfs_opts *opts;

	kfree(usb9pfs);

	opts = container_of(f->fi, struct f_usb9pfs_opts, func_inst);

	mutex_lock(&opts->lock);
	opts->refcnt--;
	mutex_unlock(&opts->lock);

	usb_free_all_descriptors(f);
}

static int usb9pfs_set_alt(struct usb_function *f,
			   unsigned int intf, unsigned int alt)
{
	struct f_usb9pfs *usb9pfs = func_to_usb9pfs(f);
	struct usb_composite_dev *cdev = f->config->cdev;

	return enable_usb9pfs(cdev, usb9pfs);
}

static void usb9pfs_disable(struct usb_function *f)
{
	struct f_usb9pfs *usb9pfs = func_to_usb9pfs(f);

	usb9pfs_clear_tx(usb9pfs);
}

static struct usb_function *usb9pfs_alloc(struct usb_function_instance *fi)
{
	struct f_usb9pfs_opts *usb9pfs_opts;
	struct f_usb9pfs *usb9pfs;

	usb9pfs = kzalloc_obj(*usb9pfs);
	if (!usb9pfs)
		return ERR_PTR(-ENOMEM);

	spin_lock_init(&usb9pfs->lock);

	init_completion(&usb9pfs->send);
	init_completion(&usb9pfs->received);

	usb9pfs_opts = container_of(fi, struct f_usb9pfs_opts, func_inst);

	mutex_lock(&usb9pfs_opts->lock);
	usb9pfs_opts->refcnt++;
	mutex_unlock(&usb9pfs_opts->lock);

	usb9pfs->buflen = usb9pfs_opts->buflen;

	usb9pfs->function.name = "usb9pfs";
	usb9pfs->function.bind = usb9pfs_func_bind;
	usb9pfs->function.unbind = usb9pfs_func_unbind;
	usb9pfs->function.set_alt = usb9pfs_set_alt;
	usb9pfs->function.disable = usb9pfs_disable;
	usb9pfs->function.strings = usb9pfs_strings;

	usb9pfs->function.free_func = usb9pfs_free_func;

	return &usb9pfs->function;
}

static inline struct f_usb9pfs_opts *to_f_usb9pfs_opts(struct config_item *item)
{
	return container_of(to_config_group(item), struct f_usb9pfs_opts,
			    func_inst.group);
}

static inline struct f_usb9pfs_opts *fi_to_f_usb9pfs_opts(struct usb_function_instance *fi)
{
	return container_of(fi, struct f_usb9pfs_opts, func_inst);
}

static void usb9pfs_attr_release(struct config_item *item)
{
	struct f_usb9pfs_opts *usb9pfs_opts = to_f_usb9pfs_opts(item);

	usb_put_function_instance(&usb9pfs_opts->func_inst);
}

static struct configfs_item_operations usb9pfs_item_ops = {
	.release		= usb9pfs_attr_release,
};

static ssize_t f_usb9pfs_opts_buflen_show(struct config_item *item, char *page)
{
	struct f_usb9pfs_opts *opts = to_f_usb9pfs_opts(item);
	int ret;

	mutex_lock(&opts->lock);
	ret = sysfs_emit(page, "%d\n", opts->buflen);
	mutex_unlock(&opts->lock);

	return ret;
}

static ssize_t f_usb9pfs_opts_buflen_store(struct config_item *item,
					   const char *page, size_t len)
{
	struct f_usb9pfs_opts *opts = to_f_usb9pfs_opts(item);
	int ret;
	u32 num;

	guard(mutex)(&opts->lock);

	if (opts->refcnt)
		return -EBUSY;

	ret = kstrtou32(page, 0, &num);
	if (ret)
		return ret;

	opts->buflen = num;

	return len;
}

CONFIGFS_ATTR(f_usb9pfs_opts_, buflen);

static struct configfs_attribute *usb9pfs_attrs[] = {
	&f_usb9pfs_opts_attr_buflen,
	NULL,
};

static const struct config_item_type usb9pfs_func_type = {
	.ct_item_ops	= &usb9pfs_item_ops,
	.ct_attrs	= usb9pfs_attrs,
	.ct_owner	= THIS_MODULE,
};

static struct f_usb9pfs_dev *_usb9pfs_do_find_dev(const char *tag)
{
	struct f_usb9pfs_dev *usb9pfs_dev;

	if (!tag)
		return NULL;

	list_for_each_entry(usb9pfs_dev, &usbg_instance_list, usb9pfs_instance) {
		if (strcmp(usb9pfs_dev->tag, tag) == 0)
			return usb9pfs_dev;
	}

	return NULL;
}

static int usb9pfs_tag_instance(struct f_usb9pfs_dev *dev, const char *tag)
{
	struct f_usb9pfs_dev *existing;
	int ret = 0;

	guard(mutex)(&usb9pfs_lock);

	existing = _usb9pfs_do_find_dev(tag);
	if (!existing)
		strscpy(dev->tag, tag, ARRAY_SIZE(dev->tag));
	else if (existing != dev)
		ret = -EBUSY;

	return ret;
}

static int usb9pfs_set_inst_tag(struct usb_function_instance *fi, const char *tag)
{
	if (strlen(tag) >= sizeof_field(struct f_usb9pfs_dev, tag))
		return -ENAMETOOLONG;
	return usb9pfs_tag_instance(fi_to_f_usb9pfs_opts(fi)->dev, tag);
}

static void usb9pfs_free_instance(struct usb_function_instance *fi)
{
	struct f_usb9pfs_opts *usb9pfs_opts =
		container_of(fi, struct f_usb9pfs_opts, func_inst);
	struct f_usb9pfs_dev *dev = usb9pfs_opts->dev;

	mutex_lock(&usb9pfs_lock);
	list_del(&dev->usb9pfs_instance);
	mutex_unlock(&usb9pfs_lock);

	kfree(usb9pfs_opts);
}

static struct usb_function_instance *usb9pfs_alloc_instance(void)
{
	struct f_usb9pfs_opts *usb9pfs_opts;
	struct f_usb9pfs_dev *dev;

	usb9pfs_opts = kzalloc_obj(*usb9pfs_opts);
	if (!usb9pfs_opts)
		return ERR_PTR(-ENOMEM);

	mutex_init(&usb9pfs_opts->lock);

	usb9pfs_opts->func_inst.set_inst_name = usb9pfs_set_inst_tag;
	usb9pfs_opts->func_inst.free_func_inst = usb9pfs_free_instance;

	usb9pfs_opts->buflen = DEFAULT_BUFLEN;

	dev = kzalloc_obj(*dev);
	if (!dev) {
		kfree(usb9pfs_opts);
		return ERR_PTR(-ENOMEM);
	}

	usb9pfs_opts->dev = dev;
	dev->opts = usb9pfs_opts;

	config_group_init_type_name(&usb9pfs_opts->func_inst.group, "",
				    &usb9pfs_func_type);

	mutex_lock(&usb9pfs_lock);
	list_add_tail(&dev->usb9pfs_instance, &usbg_instance_list);
	mutex_unlock(&usb9pfs_lock);

	return &usb9pfs_opts->func_inst;
}
DECLARE_USB_FUNCTION(usb9pfs, usb9pfs_alloc_instance, usb9pfs_alloc);

static int __init usb9pfs_modinit(void)
{
	int ret;

	INIT_LIST_HEAD(&usbg_instance_list);

	ret = usb_function_register(&usb9pfsusb_func);
	if (!ret)
		v9fs_register_trans(&p9_usbg_trans);

	return ret;
}

static void __exit usb9pfs_modexit(void)
{
	usb_function_unregister(&usb9pfsusb_func);
	v9fs_unregister_trans(&p9_usbg_trans);
}

module_init(usb9pfs_modinit);
module_exit(usb9pfs_modexit);

MODULE_ALIAS_9P("usbg");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("USB gadget 9pfs transport");
MODULE_AUTHOR("Michael Grzeschik");