Message-ID: <915890fa-e763-470a-a82b-eda97c47c0c8@collabora.com>
Date: Wed, 6 Mar 2024 16:18:54 +0100
From: Andrzej Pietrasiewicz <andrzej.p@...labora.com>
To: Michael Grzeschik <m.grzeschik@...gutronix.de>,
 Eric Van Hensbergen <ericvh@...nel.org>, Latchesar Ionkov
 <lucho@...kov.net>, Dominique Martinet <asmadeus@...ewreck.org>,
 Christian Schoenebeck <linux_oss@...debyte.com>,
 Jonathan Corbet <corbet@....net>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: v9fs@...ts.linux.dev, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
 kernel@...gutronix.de
Subject: Re: [PATCH v3 2/3] net/9p/usbg: Add new usb gadget function transport

Hi Michael,

W dniu 26.02.2024 o 14:56, Michael Grzeschik pisze:
> Add the new gadget function for 9pfs transport. This function is
> defining an simple 9pfs transport interface that consists of one in and
> one out endpoint. The endpoints transmit and receive the 9pfs protocol
> payload when mounting a 9p filesystem over usb.

Reading the earlier discussions on v1 and v2 I was somehow under the impression
that what you submitted here does not contain configfs support.

But once I started reading the code I realized it does.

It worked for me, both as a legacy gadget and when composed with configfs.

I noticed that when you stop the forwarder at the host side when 9pfs remains
mounted at the gadget side, umount hangs at the gadget side until the forwarder
is restarted. I know that once the host-side software dies, not much can be
done, however, unmounting a dead filesystem seems a reasonable thing to do
and the way usb9pfs worked for me it is not possible to unmount in this
situation. Any ideas on improving this?

Please find my review below.

> 
> Signed-off-by: Michael Grzeschik <m.grzeschik@...gutronix.de>
> 
> ---
> v2 -> v3: -
> v1 -> v2:
>    - move the driver to net/9p/trans_usbg.c
>    - fixed the commit title
>    - fix %zu usage in p9_debug for size_t type
>    - define functions static that are only used localy
>    - return 0 in alloc_requests
>    - use id_alloc_max instead of ida_simple_get
>    - call ida_free in usb9pfs_free_func
>    - call kfree for usb9pfs->tag and usb9pfs in usb9pfs_free_func
>    - add MODULE_ALIAS_9P to load this module automatic when mounting
>    - improved the documentation
>    - added depends on USB_GADGET in Kconfig
> ---
>   Documentation/filesystems/9p.rst |  15 +
>   net/9p/Kconfig                   |   6 +
>   net/9p/Makefile                  |   4 +
>   net/9p/trans_usbg.c              | 871 +++++++++++++++++++++++++++++++++++++++
>   4 files changed, 896 insertions(+)
> 
> diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst
> index 1e0e0bb6fdf91..64439068a8fc5 100644
> --- a/Documentation/filesystems/9p.rst
> +++ b/Documentation/filesystems/9p.rst
> @@ -53,6 +53,20 @@ mount points. Each 9P export is seen by the client as a virtio device with an
>   associated "mount_tag" property. Available mount tags can be
>   seen by reading /sys/bus/virtio/drivers/9pnet_virtio/virtio<n>/mount_tag files.
>   
> +USBG Usage
> +==========
> +
> +To mount a 9p FS on a USB Host accessible via the gadget at runtime::
> +
> +	mount -t 9p -o trans=usbg,aname=/path/to/fs <mount_tag> /mnt/9
> +
> +To mount a 9p FS on a USB Host accessible via the gadget as root filesystem::
> +
> +	root=<mount_tag> rootfstype=9p rootflags=trans=usbg,cache=loose,uname=root,access=0,dfltuid=0,dfltgid=0,aname=/path/to/rootfs
> +
> +where mount_tag is the tag associated by the usb gadget transport. The
> +pattern is usb9pfs0, usb9pfs1, ...

I'd reword to use "generated" instead of "associated" which would emphasize
that it is the usb gadget transport part that comes up with the name.
The word "associated" suggests that maybe the name comes from elsewhere.

I'm wondering if it would make sense to treat instance name, e.g.

"my_9pfs" from

$ mkdir /sys/kernel/config/usb_gadget/g9pfs/functions/usb9pfs.my_9pfs

as device name known above as "<mount_tag>?"

FunctionFS uses this strategy and IMO it makes sense to follow this pattern.

BTW, isn't this being called "mount_tag" an implementation detail of 9pfs?
When doing "mount" users usually expect a "device name" (even if not a "real"
device).

> +
>   Options
>   =======
>   
> @@ -68,6 +82,7 @@ Options
>   			virtio	  connect to the next virtio channel available
>   				  (from QEMU with trans_virtio module)
>   			rdma	  connect to a specified RDMA channel
> +			usbg	  connect to a specified usb gadget channel
>   			========  ============================================
>   
>     uname=name	user name to attempt mount as on the remote server.  The
> diff --git a/net/9p/Kconfig b/net/9p/Kconfig
> index 00ebce9e5a657..c3d357eb8bb37 100644
> --- a/net/9p/Kconfig
> +++ b/net/9p/Kconfig
> @@ -39,6 +39,12 @@ config NET_9P_XEN
>   	  This builds support for a transport for 9pfs between
>   	  two Xen domains.
>   
> +config NET_9P_USBG
> +	bool "9P USB Gadget Transport"
> +	depends on USB_GADGET
> +	help
> +	  This builds support for a transport for 9pfs over
> +	  usb gadget.
>   
>   config NET_9P_RDMA
>   	depends on INET && INFINIBAND && INFINIBAND_ADDR_TRANS
> diff --git a/net/9p/Makefile b/net/9p/Makefile
> index 1df9b344c30bd..22794a451c3f7 100644
> --- a/net/9p/Makefile
> +++ b/net/9p/Makefile
> @@ -4,6 +4,7 @@ obj-$(CONFIG_NET_9P_FD) += 9pnet_fd.o
>   obj-$(CONFIG_NET_9P_XEN) += 9pnet_xen.o
>   obj-$(CONFIG_NET_9P_VIRTIO) += 9pnet_virtio.o
>   obj-$(CONFIG_NET_9P_RDMA) += 9pnet_rdma.o
> +obj-$(CONFIG_NET_9P_USBG) += 9pnet_usbg.o
>   
>   9pnet-objs := \
>   	mod.o \
> @@ -23,3 +24,6 @@ obj-$(CONFIG_NET_9P_RDMA) += 9pnet_rdma.o
>   
>   9pnet_rdma-objs := \
>   	trans_rdma.o \
> +
> +9pnet_usbg-objs := \
> +	trans_usbg.o \
> diff --git a/net/9p/trans_usbg.c b/net/9p/trans_usbg.c
> new file mode 100644
> index 0000000000000..93d9606a806d8
> --- /dev/null
> +++ b/net/9p/trans_usbg.c
> @@ -0,0 +1,871 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * trans_usbg.c - USB peripheral usb9pfs configuration driver and transport.
> + *
> + * Copyright (C) 2024 Michael Grzeschik <m.grzeschik@...gutronix.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/slab.h>
> +#include <linux/kernel.h>
> +#include <linux/device.h>
> +#include <linux/module.h>
> +#include <linux/err.h>
> +#include <linux/usb/composite.h>
> +#include <linux/usb/u_f.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;
> +
> +	struct p9_req_t *p9_tx_req;
> +
> +	struct list_head tx_req_list;
> +
> +	/* 9p request lock for en/dequeue */
> +	spinlock_t lock;
> +	/* usb request lock for en/dequeue */
> +	spinlock_t req_lock;
> +
> +	struct usb_request *in_req;
> +	struct usb_request *out_req;
> +
> +	struct usb_ep *in_ep;
> +	struct usb_ep *out_ep;
> +
> +	unsigned int buflen;
> +
> +	bool inuse;
> +	char *tag;
> +
> +	int index;
> +
> +	struct usb_function function;
> +
> +	struct list_head function_list;

usb9pfs_instance ?

> +};
> +
> +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;
> +
> +	/* 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;
> +};
> +
> +static DEFINE_IDA(usb9pfs_ida);
> +static DEFINE_MUTEX(usb9pfs_ida_lock); /* protects access do usb9pfs_ida */
> +
> +static DEFINE_MUTEX(usb9pfs_lock);
> +static struct list_head usbg_function_list;

This name seems too generic to me, it creates an impression that the list
contains all functions of USB gadgets. It really doesn't and what it
contains is a number of 9pfs instances. What about usb9pfs_instance_list?

> +
> +static int usb9pfs_tx_fill(struct f_usb9pfs *usb9pfs, struct usb_request *req,

given what this function effectively does...

> +			   gfp_t gfp_flags)
> +{
> +	struct usb_composite_dev *cdev = usb9pfs->function.config->cdev;
> +	int retval = -ENOMEM;
> +
> +	if (!(usb9pfs->p9_tx_req->tc.size % usb9pfs->in_ep->maxpacket))
> +		req->zero = 1;
> +
> +	req->buf = usb9pfs->p9_tx_req->tc.sdata;
> +	req->length = usb9pfs->p9_tx_req->tc.size;
> +
> +	dev_dbg(&cdev->gadget->dev, "%s usb9pfs send --> %d/%d, zero: %d\n",
> +		usb9pfs->in_ep->name, req->actual, req->length, req->zero);
> +
> +	retval = usb_ep_queue(usb9pfs->in_ep, req, gfp_flags);

.. (which is usb_ep_queue), I'd rather name it e.g. usb9pfs_tx_queue.
Or maybe usb9pfs_queue_tx to not create an impression that the function
is a queue of some sort (because rather than being a queue it does queueing).

> +
> +	dev_dbg(&cdev->gadget->dev, "tx submit --> %d\n", retval);
> +
> +	return retval;
> +}

The return value of this function is never used.

> +
> +static int usb9pfs_rx_fill(struct f_usb9pfs *usb9pfs, struct usb_request *req,

usb9pfs_queue_rx() ?

> +			   gfp_t gfp_flags)
> +{
> +	struct usb_composite_dev *cdev = usb9pfs->function.config->cdev;
> +	int retval = -ENOMEM;
> +
> +	retval = usb_ep_queue(usb9pfs->out_ep, req, gfp_flags);
> +
> +	dev_dbg(&cdev->gadget->dev, "rx submit --> %d\n", retval);
> +
> +	return retval;
> +}

The return value of this function is never used.

> +
> +static int usb9pfs_transmit(struct f_usb9pfs *usb9pfs)

Might be worth documenting that this needs to be called with req_lock held.

> +{
> +	struct p9_req_t *p9_req = NULL;
> +	unsigned long flags;
> +	int ret = 0;
> +
> +	spin_lock_irqsave(&usb9pfs->lock, flags);
> +
> +	if (usb9pfs->p9_tx_req) {
> +		spin_unlock_irqrestore(&usb9pfs->lock, flags);
> +		return -EBUSY;
> +	}
> +
> +	p9_req = list_first_entry_or_null(&usb9pfs->tx_req_list, struct p9_req_t, req_list);
> +	if (!p9_req) {
> +		spin_unlock_irqrestore(&usb9pfs->lock, flags);
> +		return -ENOENT;
> +	}
> +
> +	list_del(&p9_req->req_list);
> +
> +	usb9pfs->p9_tx_req = p9_req;
> +
> +	p9_req_get(usb9pfs->p9_tx_req);
> +
> +	usb9pfs_tx_fill(usb9pfs, usb9pfs->in_req, GFP_ATOMIC);
> +
> +	spin_unlock_irqrestore(&usb9pfs->lock, flags);
> +
> +	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;
> +
> +	/* reset zero packages */
> +	req->zero = 0;
> +
> +	dev_dbg(&cdev->gadget->dev, "%s usb9pfs complete --> %d, %d/%d\n",
> +		ep->name, req->status, req->actual, req->length);
> +
> +	switch (req->status) {

Are more cases expected here? I'm asking because for now it seems
that "if - else" would do the job.

> +	case 0:				/* normal completion? */
> +		spin_lock(&usb9pfs->req_lock);
> +
> +		WRITE_ONCE(usb9pfs->p9_tx_req->status, REQ_STATUS_SENT);
> +
> +		p9_req_put(usb9pfs->client, usb9pfs->p9_tx_req);
> +
> +		usb9pfs_rx_fill(usb9pfs, usb9pfs->out_req, GFP_ATOMIC);
> +
> +		spin_unlock(&usb9pfs->req_lock);
> +
> +		break;
> +
> +	default:
> +		dev_err(&cdev->gadget->dev, "%s usb9pfs complete --> %d, %d/%d\n",
> +			ep->name, req->status, req->actual, req->length);
> +		usb_ep_free_request(ep == usb9pfs->in_ep ?
> +				    usb9pfs->out_ep : usb9pfs->in_ep,
> +				    req->context);
> +		free_ep_req(ep, req);
> +		return;

Is this return necessary? The "default" branch does contain some statements,
so there's no need to silence the compiler.

> +	}
> +}
> +
> +static struct p9_req_t *usb9pfs_rx_header(struct f_usb9pfs *usb9pfs,
> +					  struct usb_request *req)

It seems that the logic of this function does not depend on anything USB.
It is all p9 stuff, so I'd rather accept a plain buffer instead of usb_request.

> +{
> +	struct p9_req_t *p9_rx_req;
> +	struct p9_fcall	rc;
> +	int ret;
> +
> +	/* start by reading header */
> +	rc.sdata = req->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;
> +	}
> +

I know nothing about p9 proper, but it seems that at this point
you have p9_rx_req...

> +	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);
> +		return NULL;

.. which you don't "put" here, but...

> +	}
> +
> +	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);

.. you do "put" it here. Not sure if this is ok or not ok, just asking,
because judging by the return value the caller of this function seems unable
to know which of the above "return" statements caused a NULL, hence it is
unable to tell whether it (the caller) should "put" or not.

> +		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 long flags;
> +
> +	switch (req->status) {
> +	case 0:				/* normal completion? */
> +		spin_lock_irqsave(&usb9pfs->req_lock, flags);
> +		p9_rx_req = usb9pfs_rx_header(usb9pfs, req);
> +		if (!p9_rx_req) {
> +			spin_unlock_irqrestore(&usb9pfs->req_lock, flags);
> +			goto free_req;
> +		}
> +
> +		memcpy(p9_rx_req->rc.sdata, req->buf, req->actual);
> +		p9_rx_req->rc.size = req->actual;
> +
> +		p9_client_cb(usb9pfs->client, p9_rx_req, REQ_STATUS_RCVD);
> +		p9_req_put(usb9pfs->client, p9_rx_req);
> +
> +		usb9pfs->p9_tx_req = NULL;
> +
> +		usb9pfs_transmit(usb9pfs);
> +
> +		spin_unlock_irqrestore(&usb9pfs->req_lock, flags);
> +
> +		return;
> +free_req:
> +	default:
> +		dev_err(&cdev->gadget->dev, "%s usb9pfs complete --> %d, %d/%d\n",
> +			ep->name, req->status, req->actual, req->length);
> +		usb_ep_free_request(ep == usb9pfs->in_ep ?
> +				    usb9pfs->out_ep : usb9pfs->in_ep,
> +				    req->context);
> +		free_ep_req(ep, req);
> +		return;

I assume this return means that p9_client_cb() should not be called if
either "free_req" or "default" cases happen?

But then anything after the end of this "switch" instruction seems unreachable
code.

> +	}
> +
> +	p9_client_cb(usb9pfs->client, p9_rx_req, REQ_STATUS_ERROR);
> +}
> +
> +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_endpoints(struct usb_composite_dev *cdev,
> +			      struct usb_ep *in, struct usb_ep *out,
> +			      struct usb_ep *iso_in, struct usb_ep *iso_out)

iso_in and iso_out are never used

> +{
> +	disable_ep(cdev, in);
> +	disable_ep(cdev, out);

I'm not sure how useful this whole function is...

> +}
> +
> +static void disable_usb9pfs(struct f_usb9pfs *usb9pfs)
> +{
> +	struct usb_composite_dev *cdev;
> +
> +	cdev = usb9pfs->function.config->cdev;
> +	disable_endpoints(cdev, usb9pfs->in_ep, usb9pfs->out_ep, NULL, NULL);

.. why not
	disable_ep(cdev, usb9pfs->in_ep);
	disable_ep(cdev, usb9pfs->out_ep);
?
> +	dev_dbg(&cdev->gadget->dev, "%s disabled\n", usb9pfs->function.name);
> +}
> +
> +static inline struct usb_request *usb9pfs_alloc_ep_req(struct usb_ep *ep, int len)
> +{
> +	return alloc_ep_req(ep, len);
> +}

What is the purpose of this wrapper?

> +
> +static int alloc_requests(struct usb_composite_dev *cdev,
> +			  struct f_usb9pfs *usb9pfs)
> +{
> +	usb9pfs->in_req = usb_ep_alloc_request(usb9pfs->in_ep, GFP_ATOMIC);
> +	if (!usb9pfs->in_req)
> +		goto fail;
> +
> +	usb9pfs->out_req = usb9pfs_alloc_ep_req(usb9pfs->out_ep, usb9pfs->buflen);
> +	if (!usb9pfs->out_req)
> +		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 0;
> +}

This funcion returns a zero no matter what. Such a return value carries
no information. I saw something in the changelog regarding returning 0,
was it about this function? But then why return always 0 instead of void?

> +
> +static int enable_endpoint(struct usb_composite_dev *cdev,
> +			   struct f_usb9pfs *usb9pfs, struct usb_ep *ep)
> +{
> +	int result;
> +
> +	result = config_ep_by_speed(cdev->gadget, &usb9pfs->function, ep);
> +	if (result)
> +		goto out;
> +
> +	result = usb_ep_enable(ep);
> +	if (result < 0)
> +		goto out;
> +
> +	ep->driver_data = usb9pfs;
> +
> +	result = 0;
> +
> +out:
> +	return result;

I assume this is "a single point of return". You'd save 3 lines of code
by returning "result" on the spot instead of gotos and explicitly
returning a zero. Who knows, maybe modern compilers are smart enough
to figure this out. Not a deal breaker for me, though.

> +}
> +
> +static int
> +enable_usb9pfs(struct usb_composite_dev *cdev, struct f_usb9pfs *usb9pfs)
> +{
> +	int result = 0;
> +
> +	result = enable_endpoint(cdev, usb9pfs, usb9pfs->in_ep);
> +	if (result)
> +		goto out;
> +
> +	result = enable_endpoint(cdev, usb9pfs, usb9pfs->out_ep);
> +	if (result)
> +		goto disable_in;
> +
> +	result = alloc_requests(cdev, usb9pfs);
> +	if (result)
> +		goto disable_out;
> +
> +	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 result;
> +}
> +
> +static int p9_usbg_create(struct p9_client *client, const char *devname, char *args)
> +{
> +	struct usb_composite_dev *cdev;
> +	struct f_usb9pfs *usb9pfs;
> +	struct f_usb9pfs *tmp;
> +	struct usb_function *f;
> +	int ret = -ENOENT;
> +	int found = 0;
> +
> +	if (!devname)
> +		return -EINVAL;
> +
> +	mutex_lock(&usb9pfs_lock);
> +	list_for_each_entry_safe(usb9pfs, tmp, &usbg_function_list, function_list) {
> +		if (!usb9pfs->tag)
> +			continue;
> +		if (!strncmp(devname, usb9pfs->tag, strlen(devname))) {
> +			if (!usb9pfs->inuse) {
> +				usb9pfs->inuse = true;
> +				found = 1;
> +				break;
> +			}
> +			ret = -EBUSY;
> +			break;
> +		}
> +	}
> +	mutex_unlock(&usb9pfs_lock);
> +
> +	if (!found) {
> +		pr_err("no channels available for device %s\n", devname);
> +		return ret;
> +	}
> +
> +	INIT_LIST_HEAD(&usb9pfs->tx_req_list);
> +
> +	spin_lock_init(&usb9pfs->lock);
> +	spin_lock_init(&usb9pfs->req_lock);
> +
> +	f = &usb9pfs->function;
> +	cdev = f->config->cdev;
> +
> +	client->trans = (void *)usb9pfs;
> +	client->status = Connected;
> +	usb9pfs->client = client;
> +
> +	client->trans_mod->maxsize = usb9pfs->buflen;
> +
> +	disable_usb9pfs(usb9pfs);
> +
> +	return enable_usb9pfs(cdev, usb9pfs);

This looks a bit weird. Clearly some side effect is intended here.
Can you document here, why and what is expected?

> +}
> +
> +static void p9_usbg_close(struct p9_client *client)
> +{
> +	struct f_usb9pfs *usb9pfs = client->trans;
> +
> +	disable_usb9pfs(usb9pfs);
> +}
> +
> +static int p9_usbg_request(struct p9_client *client, struct p9_req_t *p9_req)
> +{
> +	struct f_usb9pfs *usb9pfs = client->trans;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&usb9pfs->lock, flags);
> +	list_add_tail(&p9_req->req_list, &usb9pfs->tx_req_list);
> +	spin_unlock_irqrestore(&usb9pfs->lock, flags);
> +
> +	spin_lock_irqsave(&usb9pfs->req_lock, flags);
> +	usb9pfs_transmit(usb9pfs);
> +	spin_unlock_irqrestore(&usb9pfs->req_lock, flags);
> +
> +	return 0;
> +}
> +
> +/* We don't currently allow canceling of requests */

What would we gain by allowing those? In other words,
what are we losing?

Regards,

Andrzej

> +static int p9_usbg_cancel(struct p9_client *client, struct p9_req_t *req)
> +{
> +	return 1;
> +}
> +
> +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,
> +	.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 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;
> +
> +	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_free_func(struct usb_function *f)
> +{
> +	struct f_usb9pfs *usb9pfs = func_to_usb9pfs(f);
> +	struct f_usb9pfs_opts *opts;
> +
> +	ida_free(&usb9pfs_ida, usb9pfs->index);
> +	kfree(usb9pfs->tag);
> +	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)
> +{
> +	return 0;
> +}
> +
> +static void usb9pfs_disable(struct usb_function *f)
> +{
> +}
> +
> +static struct usb_function *usb9pfs_alloc(struct usb_function_instance *fi)
> +{
> +	struct f_usb9pfs_opts *usb9pfs_opts;
> +	struct f_usb9pfs *usb9pfs;
> +
> +	usb9pfs = kzalloc(sizeof(*usb9pfs), GFP_KERNEL);
> +	if (!usb9pfs)
> +		return ERR_PTR(-ENOMEM);
> +
> +	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.set_alt = usb9pfs_set_alt;
> +	usb9pfs->function.disable = usb9pfs_disable;
> +	usb9pfs->function.strings = usb9pfs_strings;
> +
> +	usb9pfs->function.free_func = usb9pfs_free_func;
> +
> +	mutex_lock(&usb9pfs_ida_lock);
> +
> +	usb9pfs->index = ida_alloc_max(&usb9pfs_ida, U8_MAX, GFP_KERNEL);
> +	if (usb9pfs->index < 0) {
> +		struct usb_function *ret = ERR_PTR(usb9pfs->index);
> +
> +		kfree(usb9pfs);
> +		mutex_unlock(&usb9pfs_ida_lock);
> +		return ret;
> +	}
> +
> +	mutex_unlock(&usb9pfs_ida_lock);
> +
> +	usb9pfs->tag = kasprintf(GFP_KERNEL, "%s%d", usb9pfs->function.name,
> +				 usb9pfs->index);
> +
> +	INIT_LIST_HEAD(&usb9pfs->function_list);
> +
> +	mutex_lock(&usb9pfs_lock);
> +	list_add_tail(&usb9pfs->function_list, &usbg_function_list);
> +	mutex_unlock(&usb9pfs_lock);
> +
> +	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 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 result;
> +
> +	mutex_lock(&opts->lock);
> +	result = sprintf(page, "%d\n", opts->buflen);
> +	mutex_unlock(&opts->lock);
> +
> +	return result;
> +}
> +
> +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;
> +
> +	mutex_lock(&opts->lock);
> +	if (opts->refcnt) {
> +		ret = -EBUSY;
> +		goto end;
> +	}
> +
> +	ret = kstrtou32(page, 0, &num);
> +	if (ret)
> +		goto end;
> +
> +	opts->buflen = num;
> +	ret = len;
> +end:
> +	mutex_unlock(&opts->lock);
> +	return ret;
> +}
> +
> +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 void usb9pfs_free_instance(struct usb_function_instance *fi)
> +{
> +	struct f_usb9pfs_opts *usb9pfs_opts;
> +
> +	usb9pfs_opts = container_of(fi, struct f_usb9pfs_opts, func_inst);
> +	kfree(usb9pfs_opts);
> +}
> +
> +static struct usb_function_instance *usb9pfs_alloc_instance(void)
> +{
> +	struct f_usb9pfs_opts *usb9pfs_opts;
> +
> +	usb9pfs_opts = kzalloc(sizeof(*usb9pfs_opts), GFP_KERNEL);
> +	if (!usb9pfs_opts)
> +		return ERR_PTR(-ENOMEM);
> +
> +	mutex_init(&usb9pfs_opts->lock);
> +
> +	usb9pfs_opts->func_inst.free_func_inst = usb9pfs_free_instance;
> +
> +	usb9pfs_opts->buflen = DEFAULT_BUFLEN;
> +
> +	config_group_init_type_name(&usb9pfs_opts->func_inst.group, "",
> +				    &usb9pfs_func_type);
> +
> +	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_function_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");
> 

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