rpmsg: avoid premature deallocation of endpoints

[karo-tx-linux.git] / drivers / rpmsg / virtio_rpmsg_bus.c

/*
 * Virtio-based remote processor messaging bus
 *
 * Copyright (C) 2011 Texas Instruments, Inc.
 * Copyright (C) 2011 Google, Inc.
 *
 * Ohad Ben-Cohen <ohad@wizery.com>
 * Brian Swetland <swetland@google.com>
 *
 * This software is licensed under the terms of the GNU General Public
 * License version 2, as published by the Free Software Foundation, and
 * may be copied, distributed, and modified under those terms.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 */

#define pr_fmt(fmt) "%s: " fmt, __func__

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/virtio.h>
#include <linux/virtio_ids.h>
#include <linux/virtio_config.h>
#include <linux/scatterlist.h>
#include <linux/dma-mapping.h>
#include <linux/slab.h>
#include <linux/idr.h>
#include <linux/jiffies.h>
#include <linux/sched.h>
#include <linux/wait.h>
#include <linux/rpmsg.h>
#include <linux/mutex.h>

/**
 * struct virtproc_info - virtual remote processor state
 * @vdev:       the virtio device
 * @rvq:        rx virtqueue
 * @svq:        tx virtqueue
 * @rbufs:      kernel address of rx buffers
 * @sbufs:      kernel address of tx buffers
 * @last_sbuf:  index of last tx buffer used
 * @bufs_dma:   dma base addr of the buffers
 * @tx_lock:    protects svq, sbufs and sleepers, to allow concurrent senders.
 *              sending a message might require waking up a dozing remote
 *              processor, which involves sleeping, hence the mutex.
 * @endpoints:  idr of local endpoints, allows fast retrieval
 * @endpoints_lock: lock of the endpoints set
 * @sendq:      wait queue of sending contexts waiting for a tx buffers
 * @sleepers:   number of senders that are waiting for a tx buffer
 * @ns_ept:     the bus's name service endpoint
 *
 * This structure stores the rpmsg state of a given virtio remote processor
 * device (there might be several virtio proc devices for each physical
 * remote processor).
 */
struct virtproc_info {
        struct virtio_device *vdev;
        struct virtqueue *rvq, *svq;
        void *rbufs, *sbufs;
        int last_sbuf;
        dma_addr_t bufs_dma;
        struct mutex tx_lock;
        struct idr endpoints;
        struct mutex endpoints_lock;
        wait_queue_head_t sendq;
        atomic_t sleepers;
        struct rpmsg_endpoint *ns_ept;
};

/**
 * struct rpmsg_channel_info - internal channel info representation
 * @name: name of service
 * @src: local address
 * @dst: destination address
 */
struct rpmsg_channel_info {
        char name[RPMSG_NAME_SIZE];
        u32 src;
        u32 dst;
};

#define to_rpmsg_channel(d) container_of(d, struct rpmsg_channel, dev)
#define to_rpmsg_driver(d) container_of(d, struct rpmsg_driver, drv)

/*
 * We're allocating 512 buffers of 512 bytes for communications, and then
 * using the first 256 buffers for RX, and the last 256 buffers for TX.
 *
 * Each buffer will have 16 bytes for the msg header and 496 bytes for
 * the payload.
 *
 * This will require a total space of 256KB for the buffers.
 *
 * We might also want to add support for user-provided buffers in time.
 * This will allow bigger buffer size flexibility, and can also be used
 * to achieve zero-copy messaging.
 *
 * Note that these numbers are purely a decision of this driver - we
 * can change this without changing anything in the firmware of the remote
 * processor.
 */
#define RPMSG_NUM_BUFS          (512)
#define RPMSG_BUF_SIZE          (512)
#define RPMSG_TOTAL_BUF_SPACE   (RPMSG_NUM_BUFS * RPMSG_BUF_SIZE)

/*
 * Local addresses are dynamically allocated on-demand.
 * We do not dynamically assign addresses from the low 1024 range,
 * in order to reserve that address range for predefined services.
 */
#define RPMSG_RESERVED_ADDRESSES        (1024)

/* Address 53 is reserved for advertising remote services */
#define RPMSG_NS_ADDR                   (53)

/* sysfs show configuration fields */
#define rpmsg_show_attr(field, path, format_string)                     \
static ssize_t                                                          \
field##_show(struct device *dev,                                        \
                        struct device_attribute *attr, char *buf)       \
{                                                                       \
        struct rpmsg_channel *rpdev = to_rpmsg_channel(dev);            \
                                                                        \
        return sprintf(buf, format_string, rpdev->path);                \
}

/* for more info, see Documentation/ABI/testing/sysfs-bus-rpmsg */
rpmsg_show_attr(name, id.name, "%s\n");
rpmsg_show_attr(src, src, "0x%x\n");
rpmsg_show_attr(dst, dst, "0x%x\n");
rpmsg_show_attr(announce, announce ? "true" : "false", "%s\n");

/*
 * Unique (and free running) index for rpmsg devices.
 *
 * Yeah, we're not recycling those numbers (yet?). will be easy
 * to change if/when we want to.
 */
static unsigned int rpmsg_dev_index;

static ssize_t modalias_show(struct device *dev,
                             struct device_attribute *attr, char *buf)
{
        struct rpmsg_channel *rpdev = to_rpmsg_channel(dev);

        return sprintf(buf, RPMSG_DEVICE_MODALIAS_FMT "\n", rpdev->id.name);
}

static struct device_attribute rpmsg_dev_attrs[] = {
        __ATTR_RO(name),
        __ATTR_RO(modalias),
        __ATTR_RO(dst),
        __ATTR_RO(src),
        __ATTR_RO(announce),
        __ATTR_NULL
};

/* rpmsg devices and drivers are matched using the service name */
static inline int rpmsg_id_match(const struct rpmsg_channel *rpdev,
                                  const struct rpmsg_device_id *id)
{
        return strncmp(id->name, rpdev->id.name, RPMSG_NAME_SIZE) == 0;
}

/* match rpmsg channel and rpmsg driver */
static int rpmsg_dev_match(struct device *dev, struct device_driver *drv)
{
        struct rpmsg_channel *rpdev = to_rpmsg_channel(dev);
        struct rpmsg_driver *rpdrv = to_rpmsg_driver(drv);
        const struct rpmsg_device_id *ids = rpdrv->id_table;
        unsigned int i;

        for (i = 0; ids[i].name[0]; i++)
                if (rpmsg_id_match(rpdev, &ids[i]))
                        return 1;

        return 0;
}

static int rpmsg_uevent(struct device *dev, struct kobj_uevent_env *env)
{
        struct rpmsg_channel *rpdev = to_rpmsg_channel(dev);

        return add_uevent_var(env, "MODALIAS=" RPMSG_DEVICE_MODALIAS_FMT,
                                        rpdev->id.name);
}

/**
 * __ept_release() - deallocate an rpmsg endpoint
 * @kref: the ept's reference count
 *
 * This function deallocates an ept, and is invoked when its @kref refcount
 * drops to zero.
 *
 * Never invoke this function directly!
 */
static void __ept_release(struct kref *kref)
{
        struct rpmsg_endpoint *ept = container_of(kref, struct rpmsg_endpoint,
                                                  refcount);
        /*
         * At this point no one holds a reference to ept anymore,
         * so we can directly free it
         */
        kfree(ept);
}

/* for more info, see below documentation of rpmsg_create_ept() */
static struct rpmsg_endpoint *__rpmsg_create_ept(struct virtproc_info *vrp,
                struct rpmsg_channel *rpdev, rpmsg_rx_cb_t cb,
                void *priv, u32 addr)
{
        int err, tmpaddr, request;
        struct rpmsg_endpoint *ept;
        struct device *dev = rpdev ? &rpdev->dev : &vrp->vdev->dev;

        if (!idr_pre_get(&vrp->endpoints, GFP_KERNEL))
                return NULL;

        ept = kzalloc(sizeof(*ept), GFP_KERNEL);
        if (!ept) {
                dev_err(dev, "failed to kzalloc a new ept\n");
                return NULL;
        }

        kref_init(&ept->refcount);

        ept->rpdev = rpdev;
        ept->cb = cb;
        ept->priv = priv;

        /* do we need to allocate a local address ? */
        request = addr == RPMSG_ADDR_ANY ? RPMSG_RESERVED_ADDRESSES : addr;

        mutex_lock(&vrp->endpoints_lock);

        /* bind the endpoint to an rpmsg address (and allocate one if needed) */
        err = idr_get_new_above(&vrp->endpoints, ept, request, &tmpaddr);
        if (err) {
                dev_err(dev, "idr_get_new_above failed: %d\n", err);
                goto free_ept;
        }

        /* make sure the user's address request is fulfilled, if relevant */
        if (addr != RPMSG_ADDR_ANY && tmpaddr != addr) {
                dev_err(dev, "address 0x%x already in use\n", addr);
                goto rem_idr;
        }

        ept->addr = tmpaddr;

        mutex_unlock(&vrp->endpoints_lock);

        return ept;

rem_idr:
        idr_remove(&vrp->endpoints, request);
free_ept:
        mutex_unlock(&vrp->endpoints_lock);
        kref_put(&ept->refcount, __ept_release);
        return NULL;
}

/**
 * rpmsg_create_ept() - create a new rpmsg_endpoint
 * @rpdev: rpmsg channel device
 * @cb: rx callback handler
 * @priv: private data for the driver's use
 * @addr: local rpmsg address to bind with @cb
 *
 * Every rpmsg address in the system is bound to an rx callback (so when
 * inbound messages arrive, they are dispatched by the rpmsg bus using the
 * appropriate callback handler) by means of an rpmsg_endpoint struct.
 *
 * This function allows drivers to create such an endpoint, and by that,
 * bind a callback, and possibly some private data too, to an rpmsg address
 * (either one that is known in advance, or one that will be dynamically
 * assigned for them).
 *
 * Simple rpmsg drivers need not call rpmsg_create_ept, because an endpoint
 * is already created for them when they are probed by the rpmsg bus
 * (using the rx callback provided when they registered to the rpmsg bus).
 *
 * So things should just work for simple drivers: they already have an
 * endpoint, their rx callback is bound to their rpmsg address, and when
 * relevant inbound messages arrive (i.e. messages which their dst address
 * equals to the src address of their rpmsg channel), the driver's handler
 * is invoked to process it.
 *
 * That said, more complicated drivers might do need to allocate
 * additional rpmsg addresses, and bind them to different rx callbacks.
 * To accomplish that, those drivers need to call this function.
 *
 * Drivers should provide their @rpdev channel (so the new endpoint would belong
 * to the same remote processor their channel belongs to), an rx callback
 * function, an optional private data (which is provided back when the
 * rx callback is invoked), and an address they want to bind with the
 * callback. If @addr is RPMSG_ADDR_ANY, then rpmsg_create_ept will
 * dynamically assign them an available rpmsg address (drivers should have
 * a very good reason why not to always use RPMSG_ADDR_ANY here).
 *
 * Returns a pointer to the endpoint on success, or NULL on error.
 */
struct rpmsg_endpoint *rpmsg_create_ept(struct rpmsg_channel *rpdev,
                                rpmsg_rx_cb_t cb, void *priv, u32 addr)
{
        return __rpmsg_create_ept(rpdev->vrp, rpdev, cb, priv, addr);
}
EXPORT_SYMBOL(rpmsg_create_ept);

/**
 * __rpmsg_destroy_ept() - destroy an existing rpmsg endpoint
 * @vrp: virtproc which owns this ept
 * @ept: endpoing to destroy
 *
 * An internal function which destroy an ept without assuming it is
 * bound to an rpmsg channel. This is needed for handling the internal
 * name service endpoint, which isn't bound to an rpmsg channel.
 * See also __rpmsg_create_ept().
 */
static void
__rpmsg_destroy_ept(struct virtproc_info *vrp, struct rpmsg_endpoint *ept)
{
        mutex_lock(&vrp->endpoints_lock);
        idr_remove(&vrp->endpoints, ept->addr);
        mutex_unlock(&vrp->endpoints_lock);

        kref_put(&ept->refcount, __ept_release);
}

/**
 * rpmsg_destroy_ept() - destroy an existing rpmsg endpoint
 * @ept: endpoing to destroy
 *
 * Should be used by drivers to destroy an rpmsg endpoint previously
 * created with rpmsg_create_ept().
 */
void rpmsg_destroy_ept(struct rpmsg_endpoint *ept)
{
        __rpmsg_destroy_ept(ept->rpdev->vrp, ept);
}
EXPORT_SYMBOL(rpmsg_destroy_ept);

/*
 * when an rpmsg driver is probed with a channel, we seamlessly create
 * it an endpoint, binding its rx callback to a unique local rpmsg
 * address.
 *
 * if we need to, we also announce about this channel to the remote
 * processor (needed in case the driver is exposing an rpmsg service).
 */
static int rpmsg_dev_probe(struct device *dev)
{
        struct rpmsg_channel *rpdev = to_rpmsg_channel(dev);
        struct rpmsg_driver *rpdrv = to_rpmsg_driver(rpdev->dev.driver);
        struct virtproc_info *vrp = rpdev->vrp;
        struct rpmsg_endpoint *ept;
        int err;

        ept = rpmsg_create_ept(rpdev, rpdrv->callback, NULL, rpdev->src);
        if (!ept) {
                dev_err(dev, "failed to create endpoint\n");
                err = -ENOMEM;
                goto out;
        }

        rpdev->ept = ept;
        rpdev->src = ept->addr;

        err = rpdrv->probe(rpdev);
        if (err) {
                dev_err(dev, "%s: failed: %d\n", __func__, err);
                rpmsg_destroy_ept(ept);
                goto out;
        }

        /* need to tell remote processor's name service about this channel ? */
        if (rpdev->announce &&
                        virtio_has_feature(vrp->vdev, VIRTIO_RPMSG_F_NS)) {
                struct rpmsg_ns_msg nsm;

                strncpy(nsm.name, rpdev->id.name, RPMSG_NAME_SIZE);
                nsm.addr = rpdev->src;
                nsm.flags = RPMSG_NS_CREATE;

                err = rpmsg_sendto(rpdev, &nsm, sizeof(nsm), RPMSG_NS_ADDR);
                if (err)
                        dev_err(dev, "failed to announce service %d\n", err);
        }

out:
        return err;
}

static int rpmsg_dev_remove(struct device *dev)
{
        struct rpmsg_channel *rpdev = to_rpmsg_channel(dev);
        struct rpmsg_driver *rpdrv = to_rpmsg_driver(rpdev->dev.driver);
        struct virtproc_info *vrp = rpdev->vrp;
        int err = 0;

        /* tell remote processor's name service we're removing this channel */
        if (rpdev->announce &&
                        virtio_has_feature(vrp->vdev, VIRTIO_RPMSG_F_NS)) {
                struct rpmsg_ns_msg nsm;

                strncpy(nsm.name, rpdev->id.name, RPMSG_NAME_SIZE);
                nsm.addr = rpdev->src;
                nsm.flags = RPMSG_NS_DESTROY;

                err = rpmsg_sendto(rpdev, &nsm, sizeof(nsm), RPMSG_NS_ADDR);
                if (err)
                        dev_err(dev, "failed to announce service %d\n", err);
        }

        rpdrv->remove(rpdev);

        rpmsg_destroy_ept(rpdev->ept);

        return err;
}

static struct bus_type rpmsg_bus = {
        .name           = "rpmsg",
        .match          = rpmsg_dev_match,
        .dev_attrs      = rpmsg_dev_attrs,
        .uevent         = rpmsg_uevent,
        .probe          = rpmsg_dev_probe,
        .remove         = rpmsg_dev_remove,
};

/**
 * register_rpmsg_driver() - register an rpmsg driver with the rpmsg bus
 * @rpdrv: pointer to a struct rpmsg_driver
 *
 * Returns 0 on success, and an appropriate error value on failure.
 */
int register_rpmsg_driver(struct rpmsg_driver *rpdrv)
{
        rpdrv->drv.bus = &rpmsg_bus;
        return driver_register(&rpdrv->drv);
}
EXPORT_SYMBOL(register_rpmsg_driver);

/**
 * unregister_rpmsg_driver() - unregister an rpmsg driver from the rpmsg bus
 * @rpdrv: pointer to a struct rpmsg_driver
 *
 * Returns 0 on success, and an appropriate error value on failure.
 */
void unregister_rpmsg_driver(struct rpmsg_driver *rpdrv)
{
        driver_unregister(&rpdrv->drv);
}
EXPORT_SYMBOL(unregister_rpmsg_driver);

static void rpmsg_release_device(struct device *dev)
{
        struct rpmsg_channel *rpdev = to_rpmsg_channel(dev);

        kfree(rpdev);
}

/*
 * match an rpmsg channel with a channel info struct.
 * this is used to make sure we're not creating rpmsg devices for channels
 * that already exist.
 */
static int rpmsg_channel_match(struct device *dev, void *data)
{
        struct rpmsg_channel_info *chinfo = data;
        struct rpmsg_channel *rpdev = to_rpmsg_channel(dev);

        if (chinfo->src != RPMSG_ADDR_ANY && chinfo->src != rpdev->src)
                return 0;

        if (chinfo->dst != RPMSG_ADDR_ANY && chinfo->dst != rpdev->dst)
                return 0;

        if (strncmp(chinfo->name, rpdev->id.name, RPMSG_NAME_SIZE))
                return 0;

        /* found a match ! */
        return 1;
}

/*
 * create an rpmsg channel using its name and address info.
 * this function will be used to create both static and dynamic
 * channels.
 */
static struct rpmsg_channel *rpmsg_create_channel(struct virtproc_info *vrp,
                                struct rpmsg_channel_info *chinfo)
{
        struct rpmsg_channel *rpdev;
        struct device *tmp, *dev = &vrp->vdev->dev;
        int ret;

        /* make sure a similar channel doesn't already exist */
        tmp = device_find_child(dev, chinfo, rpmsg_channel_match);
        if (tmp) {
                /* decrement the matched device's refcount back */
                put_device(tmp);
                dev_err(dev, "channel %s:%x:%x already exist\n",
                                chinfo->name, chinfo->src, chinfo->dst);
                return NULL;
        }

        rpdev = kzalloc(sizeof(struct rpmsg_channel), GFP_KERNEL);
        if (!rpdev) {
                pr_err("kzalloc failed\n");
                return NULL;
        }

        rpdev->vrp = vrp;
        rpdev->src = chinfo->src;
        rpdev->dst = chinfo->dst;

        /*
         * rpmsg server channels has predefined local address (for now),
         * and their existence needs to be announced remotely
         */
        rpdev->announce = rpdev->src != RPMSG_ADDR_ANY ? true : false;

        strncpy(rpdev->id.name, chinfo->name, RPMSG_NAME_SIZE);

        /* very simple device indexing plumbing which is enough for now */
        dev_set_name(&rpdev->dev, "rpmsg%d", rpmsg_dev_index++);

        rpdev->dev.parent = &vrp->vdev->dev;
        rpdev->dev.bus = &rpmsg_bus;
        rpdev->dev.release = rpmsg_release_device;

        ret = device_register(&rpdev->dev);
        if (ret) {
                dev_err(dev, "device_register failed: %d\n", ret);
                put_device(&rpdev->dev);
                return NULL;
        }

        return rpdev;
}

/*
 * find an existing channel using its name + address properties,
 * and destroy it
 */
static int rpmsg_destroy_channel(struct virtproc_info *vrp,
                                        struct rpmsg_channel_info *chinfo)
{
        struct virtio_device *vdev = vrp->vdev;
        struct device *dev;

        dev = device_find_child(&vdev->dev, chinfo, rpmsg_channel_match);
        if (!dev)
                return -EINVAL;

        device_unregister(dev);

        put_device(dev);

        return 0;
}

/* super simple buffer "allocator" that is just enough for now */
static void *get_a_tx_buf(struct virtproc_info *vrp)
{
        unsigned int len;
        void *ret;

        /* support multiple concurrent senders */
        mutex_lock(&vrp->tx_lock);

        /*
         * either pick the next unused tx buffer
         * (half of our buffers are used for sending messages)
         */
        if (vrp->last_sbuf < RPMSG_NUM_BUFS / 2)
                ret = vrp->sbufs + RPMSG_BUF_SIZE * vrp->last_sbuf++;
        /* or recycle a used one */
        else
                ret = virtqueue_get_buf(vrp->svq, &len);

        mutex_unlock(&vrp->tx_lock);

        return ret;
}

/**
 * rpmsg_upref_sleepers() - enable "tx-complete" interrupts, if needed
 * @vrp: virtual remote processor state
 *
 * This function is called before a sender is blocked, waiting for
 * a tx buffer to become available.
 *
 * If we already have blocking senders, this function merely increases
 * the "sleepers" reference count, and exits.
 *
 * Otherwise, if this is the first sender to block, we also enable
 * virtio's tx callbacks, so we'd be immediately notified when a tx
 * buffer is consumed (we rely on virtio's tx callback in order
 * to wake up sleeping senders as soon as a tx buffer is used by the
 * remote processor).
 */
static void rpmsg_upref_sleepers(struct virtproc_info *vrp)
{
        /* support multiple concurrent senders */
        mutex_lock(&vrp->tx_lock);

        /* are we the first sleeping context waiting for tx buffers ? */
        if (atomic_inc_return(&vrp->sleepers) == 1)
                /* enable "tx-complete" interrupts before dozing off */
                virtqueue_enable_cb(vrp->svq);

        mutex_unlock(&vrp->tx_lock);
}

/**
 * rpmsg_downref_sleepers() - disable "tx-complete" interrupts, if needed
 * @vrp: virtual remote processor state
 *
 * This function is called after a sender, that waited for a tx buffer
 * to become available, is unblocked.
 *
 * If we still have blocking senders, this function merely decreases
 * the "sleepers" reference count, and exits.
 *
 * Otherwise, if there are no more blocking senders, we also disable
 * virtio's tx callbacks, to avoid the overhead incurred with handling
 * those (now redundant) interrupts.
 */
static void rpmsg_downref_sleepers(struct virtproc_info *vrp)
{
        /* support multiple concurrent senders */
        mutex_lock(&vrp->tx_lock);

        /* are we the last sleeping context waiting for tx buffers ? */
        if (atomic_dec_and_test(&vrp->sleepers))
                /* disable "tx-complete" interrupts */
                virtqueue_disable_cb(vrp->svq);

        mutex_unlock(&vrp->tx_lock);
}

/**
 * rpmsg_send_offchannel_raw() - send a message across to the remote processor
 * @rpdev: the rpmsg channel
 * @src: source address
 * @dst: destination address
 * @data: payload of message
 * @len: length of payload
 * @wait: indicates whether caller should block in case no TX buffers available
 *
 * This function is the base implementation for all of the rpmsg sending API.
 *
 * It will send @data of length @len to @dst, and say it's from @src. The
 * message will be sent to the remote processor which the @rpdev channel
 * belongs to.
 *
 * The message is sent using one of the TX buffers that are available for
 * communication with this remote processor.
 *
 * If @wait is true, the caller will be blocked until either a TX buffer is
 * available, or 15 seconds elapses (we don't want callers to
 * sleep indefinitely due to misbehaving remote processors), and in that
 * case -ERESTARTSYS is returned. The number '15' itself was picked
 * arbitrarily; there's little point in asking drivers to provide a timeout
 * value themselves.
 *
 * Otherwise, if @wait is false, and there are no TX buffers available,
 * the function will immediately fail, and -ENOMEM will be returned.
 *
 * Normally drivers shouldn't use this function directly; instead, drivers
 * should use the appropriate rpmsg_{try}send{to, _offchannel} API
 * (see include/linux/rpmsg.h).
 *
 * Returns 0 on success and an appropriate error value on failure.
 */
int rpmsg_send_offchannel_raw(struct rpmsg_channel *rpdev, u32 src, u32 dst,
                                        void *data, int len, bool wait)
{
        struct virtproc_info *vrp = rpdev->vrp;
        struct device *dev = &rpdev->dev;
        struct scatterlist sg;
        struct rpmsg_hdr *msg;
        int err;

        /* bcasting isn't allowed */
        if (src == RPMSG_ADDR_ANY || dst == RPMSG_ADDR_ANY) {
                dev_err(dev, "invalid addr (src 0x%x, dst 0x%x)\n", src, dst);
                return -EINVAL;
        }

        /*
         * We currently use fixed-sized buffers, and therefore the payload
         * length is limited.
         *
         * One of the possible improvements here is either to support
         * user-provided buffers (and then we can also support zero-copy
         * messaging), or to improve the buffer allocator, to support
         * variable-length buffer sizes.
         */
        if (len > RPMSG_BUF_SIZE - sizeof(struct rpmsg_hdr)) {
                dev_err(dev, "message is too big (%d)\n", len);
                return -EMSGSIZE;
        }

        /* grab a buffer */
        msg = get_a_tx_buf(vrp);
        if (!msg && !wait)
                return -ENOMEM;

        /* no free buffer ? wait for one (but bail after 15 seconds) */
        while (!msg) {
                /* enable "tx-complete" interrupts, if not already enabled */
                rpmsg_upref_sleepers(vrp);

                /*
                 * sleep until a free buffer is available or 15 secs elapse.
                 * the timeout period is not configurable because there's
                 * little point in asking drivers to specify that.
                 * if later this happens to be required, it'd be easy to add.
                 */
                err = wait_event_interruptible_timeout(vrp->sendq,
                                        (msg = get_a_tx_buf(vrp)),
                                        msecs_to_jiffies(15000));

                /* disable "tx-complete" interrupts if we're the last sleeper */
                rpmsg_downref_sleepers(vrp);

                /* timeout ? */
                if (!err) {
                        dev_err(dev, "timeout waiting for a tx buffer\n");
                        return -ERESTARTSYS;
                }
        }

        msg->len = len;
        msg->flags = 0;
        msg->src = src;
        msg->dst = dst;
        msg->reserved = 0;
        memcpy(msg->data, data, len);

        dev_dbg(dev, "TX From 0x%x, To 0x%x, Len %d, Flags %d, Reserved %d\n",
                                        msg->src, msg->dst, msg->len,
                                        msg->flags, msg->reserved);
        print_hex_dump(KERN_DEBUG, "rpmsg_virtio TX: ", DUMP_PREFIX_NONE, 16, 1,
                                        msg, sizeof(*msg) + msg->len, true);

        sg_init_one(&sg, msg, sizeof(*msg) + len);

        mutex_lock(&vrp->tx_lock);

        /* add message to the remote processor's virtqueue */
        err = virtqueue_add_buf(vrp->svq, &sg, 1, 0, msg, GFP_KERNEL);
        if (err < 0) {
                /*
                 * need to reclaim the buffer here, otherwise it's lost
                 * (memory won't leak, but rpmsg won't use it again for TX).
                 * this will wait for a buffer management overhaul.
                 */
                dev_err(dev, "virtqueue_add_buf failed: %d\n", err);
                goto out;
        }

        /* tell the remote processor it has a pending message to read */
        virtqueue_kick(vrp->svq);

        err = 0;
out:
        mutex_unlock(&vrp->tx_lock);
        return err;
}
EXPORT_SYMBOL(rpmsg_send_offchannel_raw);

/* called when an rx buffer is used, and it's time to digest a message */
static void rpmsg_recv_done(struct virtqueue *rvq)
{
        struct rpmsg_hdr *msg;
        unsigned int len;
        struct rpmsg_endpoint *ept;
        struct scatterlist sg;
        struct virtproc_info *vrp = rvq->vdev->priv;
        struct device *dev = &rvq->vdev->dev;
        int err;

        msg = virtqueue_get_buf(rvq, &len);
        if (!msg) {
                dev_err(dev, "uhm, incoming signal, but no used buffer ?\n");
                return;
        }

        dev_dbg(dev, "From: 0x%x, To: 0x%x, Len: %d, Flags: %d, Reserved: %d\n",
                                        msg->src, msg->dst, msg->len,
                                        msg->flags, msg->reserved);
        print_hex_dump(KERN_DEBUG, "rpmsg_virtio RX: ", DUMP_PREFIX_NONE, 16, 1,
                                        msg, sizeof(*msg) + msg->len, true);

        /*
         * We currently use fixed-sized buffers, so trivially sanitize
         * the reported payload length.
         */
        if (len > RPMSG_BUF_SIZE ||
                msg->len > (len - sizeof(struct rpmsg_hdr))) {
                dev_warn(dev, "inbound msg too big: (%d, %d)\n", len, msg->len);
                return;
        }

        /* use the dst addr to fetch the callback of the appropriate user */
        mutex_lock(&vrp->endpoints_lock);

        ept = idr_find(&vrp->endpoints, msg->dst);

        /* let's make sure no one deallocates ept while we use it */
        if (ept)
                kref_get(&ept->refcount);

        mutex_unlock(&vrp->endpoints_lock);

        if (ept && ept->cb)
                ept->cb(ept->rpdev, msg->data, msg->len, ept->priv, msg->src);
        else
                dev_warn(dev, "msg received with no recepient\n");

        /* farewell, ept, we don't need you anymore */
        if (ept)
                kref_put(&ept->refcount, __ept_release);

        /* publish the real size of the buffer */
        sg_init_one(&sg, msg, RPMSG_BUF_SIZE);

        /* add the buffer back to the remote processor's virtqueue */
        err = virtqueue_add_buf(vrp->rvq, &sg, 0, 1, msg, GFP_KERNEL);
        if (err < 0) {
                dev_err(dev, "failed to add a virtqueue buffer: %d\n", err);
                return;
        }

        /* tell the remote processor we added another available rx buffer */
        virtqueue_kick(vrp->rvq);
}

/*
 * This is invoked whenever the remote processor completed processing
 * a TX msg we just sent it, and the buffer is put back to the used ring.
 *
 * Normally, though, we suppress this "tx complete" interrupt in order to
 * avoid the incurred overhead.
 */
static void rpmsg_xmit_done(struct virtqueue *svq)
{
        struct virtproc_info *vrp = svq->vdev->priv;

        dev_dbg(&svq->vdev->dev, "%s\n", __func__);

        /* wake up potential senders that are waiting for a tx buffer */
        wake_up_interruptible(&vrp->sendq);
}

/* invoked when a name service announcement arrives */
static void rpmsg_ns_cb(struct rpmsg_channel *rpdev, void *data, int len,
                                                        void *priv, u32 src)
{
        struct rpmsg_ns_msg *msg = data;
        struct rpmsg_channel *newch;
        struct rpmsg_channel_info chinfo;
        struct virtproc_info *vrp = priv;
        struct device *dev = &vrp->vdev->dev;
        int ret;

        print_hex_dump(KERN_DEBUG, "NS announcement: ",
                        DUMP_PREFIX_NONE, 16, 1,
                        data, len, true);

        if (len != sizeof(*msg)) {
                dev_err(dev, "malformed ns msg (%d)\n", len);
                return;
        }

        /*
         * the name service ept does _not_ belong to a real rpmsg channel,
         * and is handled by the rpmsg bus itself.
         * for sanity reasons, make sure a valid rpdev has _not_ sneaked
         * in somehow.
         */
        if (rpdev) {
                dev_err(dev, "anomaly: ns ept has an rpdev handle\n");
                return;
        }

        /* don't trust the remote processor for null terminating the name */
        msg->name[RPMSG_NAME_SIZE - 1] = '\0';

        dev_info(dev, "%sing channel %s addr 0x%x\n",
                        msg->flags & RPMSG_NS_DESTROY ? "destroy" : "creat",
                        msg->name, msg->addr);

        strncpy(chinfo.name, msg->name, sizeof(chinfo.name));
        chinfo.src = RPMSG_ADDR_ANY;
        chinfo.dst = msg->addr;

        if (msg->flags & RPMSG_NS_DESTROY) {
                ret = rpmsg_destroy_channel(vrp, &chinfo);
                if (ret)
                        dev_err(dev, "rpmsg_destroy_channel failed: %d\n", ret);
        } else {
                newch = rpmsg_create_channel(vrp, &chinfo);
                if (!newch)
                        dev_err(dev, "rpmsg_create_channel failed\n");
        }
}

static int rpmsg_probe(struct virtio_device *vdev)
{
        vq_callback_t *vq_cbs[] = { rpmsg_recv_done, rpmsg_xmit_done };
        const char *names[] = { "input", "output" };
        struct virtqueue *vqs[2];
        struct virtproc_info *vrp;
        void *bufs_va;
        int err = 0, i;

        vrp = kzalloc(sizeof(*vrp), GFP_KERNEL);
        if (!vrp)
                return -ENOMEM;

        vrp->vdev = vdev;

        idr_init(&vrp->endpoints);
        mutex_init(&vrp->endpoints_lock);
        mutex_init(&vrp->tx_lock);
        init_waitqueue_head(&vrp->sendq);

        /* We expect two virtqueues, rx and tx (and in this order) */
        err = vdev->config->find_vqs(vdev, 2, vqs, vq_cbs, names);
        if (err)
                goto free_vrp;

        vrp->rvq = vqs[0];
        vrp->svq = vqs[1];

        /* allocate coherent memory for the buffers */
        bufs_va = dma_alloc_coherent(vdev->dev.parent, RPMSG_TOTAL_BUF_SPACE,
                                &vrp->bufs_dma, GFP_KERNEL);
        if (!bufs_va)
                goto vqs_del;

        dev_dbg(&vdev->dev, "buffers: va %p, dma 0x%llx\n", bufs_va,
                                        (unsigned long long)vrp->bufs_dma);

        /* half of the buffers is dedicated for RX */
        vrp->rbufs = bufs_va;

        /* and half is dedicated for TX */
        vrp->sbufs = bufs_va + RPMSG_TOTAL_BUF_SPACE / 2;

        /* set up the receive buffers */
        for (i = 0; i < RPMSG_NUM_BUFS / 2; i++) {
                struct scatterlist sg;
                void *cpu_addr = vrp->rbufs + i * RPMSG_BUF_SIZE;

                sg_init_one(&sg, cpu_addr, RPMSG_BUF_SIZE);

                err = virtqueue_add_buf(vrp->rvq, &sg, 0, 1, cpu_addr,
                                                                GFP_KERNEL);
                WARN_ON(err < 0); /* sanity check; this can't really happen */
        }

        /* suppress "tx-complete" interrupts */
        virtqueue_disable_cb(vrp->svq);

        vdev->priv = vrp;

        /* if supported by the remote processor, enable the name service */
        if (virtio_has_feature(vdev, VIRTIO_RPMSG_F_NS)) {
                /* a dedicated endpoint handles the name service msgs */
                vrp->ns_ept = __rpmsg_create_ept(vrp, NULL, rpmsg_ns_cb,
                                                vrp, RPMSG_NS_ADDR);
                if (!vrp->ns_ept) {
                        dev_err(&vdev->dev, "failed to create the ns ept\n");
                        err = -ENOMEM;
                        goto free_coherent;
                }
        }

        /* tell the remote processor it can start sending messages */
        virtqueue_kick(vrp->rvq);

        dev_info(&vdev->dev, "rpmsg host is online\n");

        return 0;

free_coherent:
        dma_free_coherent(vdev->dev.parent, RPMSG_TOTAL_BUF_SPACE, bufs_va,
                                        vrp->bufs_dma);
vqs_del:
        vdev->config->del_vqs(vrp->vdev);
free_vrp:
        kfree(vrp);
        return err;
}

static int rpmsg_remove_device(struct device *dev, void *data)
{
        device_unregister(dev);

        return 0;
}

static void __devexit rpmsg_remove(struct virtio_device *vdev)
{
        struct virtproc_info *vrp = vdev->priv;
        int ret;

        vdev->config->reset(vdev);

        ret = device_for_each_child(&vdev->dev, NULL, rpmsg_remove_device);
        if (ret)
                dev_warn(&vdev->dev, "can't remove rpmsg device: %d\n", ret);

        if (vrp->ns_ept)
                __rpmsg_destroy_ept(vrp, vrp->ns_ept);

        idr_remove_all(&vrp->endpoints);
        idr_destroy(&vrp->endpoints);

        vdev->config->del_vqs(vrp->vdev);

        dma_free_coherent(vdev->dev.parent, RPMSG_TOTAL_BUF_SPACE,
                                        vrp->rbufs, vrp->bufs_dma);

        kfree(vrp);
}

static struct virtio_device_id id_table[] = {
        { VIRTIO_ID_RPMSG, VIRTIO_DEV_ANY_ID },
        { 0 },
};

static unsigned int features[] = {
        VIRTIO_RPMSG_F_NS,
};

static struct virtio_driver virtio_ipc_driver = {
        .feature_table  = features,
        .feature_table_size = ARRAY_SIZE(features),
        .driver.name    = KBUILD_MODNAME,
        .driver.owner   = THIS_MODULE,
        .id_table       = id_table,
        .probe          = rpmsg_probe,
        .remove         = __devexit_p(rpmsg_remove),
};

static int __init rpmsg_init(void)
{
        int ret;

        ret = bus_register(&rpmsg_bus);
        if (ret) {
                pr_err("failed to register rpmsg bus: %d\n", ret);
                return ret;
        }

        ret = register_virtio_driver(&virtio_ipc_driver);
        if (ret) {
                pr_err("failed to register virtio driver: %d\n", ret);
                bus_unregister(&rpmsg_bus);
        }

        return ret;
}
module_init(rpmsg_init);

static void __exit rpmsg_fini(void)
{
        unregister_virtio_driver(&virtio_ipc_driver);
        bus_unregister(&rpmsg_bus);
}
module_exit(rpmsg_fini);

MODULE_DEVICE_TABLE(virtio, id_table);
MODULE_DESCRIPTION("Virtio-based remote processor messaging bus");
MODULE_LICENSE("GPL v2");