// SPDX-License-Identifier: GPL-2.0 OR MIT

/*
 *  Xen para-virtual DRM device
 *
 * Copyright (C) 2016-2018 EPAM Systems Inc.
 *
 * Author: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
 */

#include <linux/delay.h>
#include <linux/dma-mapping.h>
#include <linux/module.h>

#include <drm/drm_atomic_helper.h>
#include <drm/drm_drv.h>
#include <drm/drm_ioctl.h>
#include <drm/drm_probe_helper.h>
#include <drm/drm_file.h>
#include <drm/drm_gem.h>
#include <drm/drm_print.h>

#include <xen/platform_pci.h>
#include <xen/xen.h>
#include <xen/xenbus.h>

#include <xen/xen-front-pgdir-shbuf.h>
#include <xen/interface/io/displif.h>

#include "xen_drm_front.h"
#include "xen_drm_front_cfg.h"
#include "xen_drm_front_evtchnl.h"
#include "xen_drm_front_gem.h"
#include "xen_drm_front_kms.h"

struct xen_drm_front_dbuf {
        struct list_head list;
        u64 dbuf_cookie;
        u64 fb_cookie;

        struct xen_front_pgdir_shbuf shbuf;
};

static void dbuf_add_to_list(struct xen_drm_front_info *front_info,
                             struct xen_drm_front_dbuf *dbuf, u64 dbuf_cookie)
{
        dbuf->dbuf_cookie = dbuf_cookie;
        list_add(&dbuf->list, &front_info->dbuf_list);
}

static struct xen_drm_front_dbuf *dbuf_get(struct list_head *dbuf_list,
                                           u64 dbuf_cookie)
{
        struct xen_drm_front_dbuf *buf, *q;

        list_for_each_entry_safe(buf, q, dbuf_list, list)
                if (buf->dbuf_cookie == dbuf_cookie)
                        return buf;

        return NULL;
}

static void dbuf_free(struct list_head *dbuf_list, u64 dbuf_cookie)
{
        struct xen_drm_front_dbuf *buf, *q;

        list_for_each_entry_safe(buf, q, dbuf_list, list)
                if (buf->dbuf_cookie == dbuf_cookie) {
                        list_del(&buf->list);
                        xen_front_pgdir_shbuf_unmap(&buf->shbuf);
                        xen_front_pgdir_shbuf_free(&buf->shbuf);
                        kfree(buf);
                        break;
                }
}

static void dbuf_free_all(struct list_head *dbuf_list)
{
        struct xen_drm_front_dbuf *buf, *q;

        list_for_each_entry_safe(buf, q, dbuf_list, list) {
                list_del(&buf->list);
                xen_front_pgdir_shbuf_unmap(&buf->shbuf);
                xen_front_pgdir_shbuf_free(&buf->shbuf);
                kfree(buf);
        }
}

static struct xendispl_req *
be_prepare_req(struct xen_drm_front_evtchnl *evtchnl, u8 operation)
{
        struct xendispl_req *req;

        req = RING_GET_REQUEST(&evtchnl->u.req.ring,
                               evtchnl->u.req.ring.req_prod_pvt);
        req->operation = operation;
        req->id = evtchnl->evt_next_id++;
        evtchnl->evt_id = req->id;
        return req;
}

static int be_stream_do_io(struct xen_drm_front_evtchnl *evtchnl,
                           struct xendispl_req *req)
{
        reinit_completion(&evtchnl->u.req.completion);
        if (unlikely(evtchnl->state != EVTCHNL_STATE_CONNECTED))
                return -EIO;

        xen_drm_front_evtchnl_flush(evtchnl);
        return 0;
}

static int be_stream_wait_io(struct xen_drm_front_evtchnl *evtchnl)
{
        if (wait_for_completion_timeout(&evtchnl->u.req.completion,
                        msecs_to_jiffies(XEN_DRM_FRONT_WAIT_BACK_MS)) <= 0)
                return -ETIMEDOUT;

        return evtchnl->u.req.resp_status;
}

int xen_drm_front_mode_set(struct xen_drm_front_drm_pipeline *pipeline,
                           u32 x, u32 y, u32 width, u32 height,
                           u32 bpp, u64 fb_cookie)
{
        struct xen_drm_front_evtchnl *evtchnl;
        struct xen_drm_front_info *front_info;
        struct xendispl_req *req;
        unsigned long flags;
        int ret;

        front_info = pipeline->drm_info->front_info;
        evtchnl = &front_info->evt_pairs[pipeline->index].req;
        if (unlikely(!evtchnl))
                return -EIO;

        mutex_lock(&evtchnl->u.req.req_io_lock);

        spin_lock_irqsave(&front_info->io_lock, flags);
        req = be_prepare_req(evtchnl, XENDISPL_OP_SET_CONFIG);
        req->op.set_config.x = x;
        req->op.set_config.y = y;
        req->op.set_config.width = width;
        req->op.set_config.height = height;
        req->op.set_config.bpp = bpp;
        req->op.set_config.fb_cookie = fb_cookie;

        ret = be_stream_do_io(evtchnl, req);
        spin_unlock_irqrestore(&front_info->io_lock, flags);

        if (ret == 0)
                ret = be_stream_wait_io(evtchnl);

        mutex_unlock(&evtchnl->u.req.req_io_lock);
        return ret;
}

int xen_drm_front_dbuf_create(struct xen_drm_front_info *front_info,
                              u64 dbuf_cookie, u32 width, u32 height,
                              u32 bpp, u64 size, u32 offset,
                              struct page **pages)
{
        struct xen_drm_front_evtchnl *evtchnl;
        struct xen_drm_front_dbuf *dbuf;
        struct xendispl_req *req;
        struct xen_front_pgdir_shbuf_cfg buf_cfg;
        unsigned long flags;
        int ret;

        evtchnl = &front_info->evt_pairs[GENERIC_OP_EVT_CHNL].req;
        if (unlikely(!evtchnl))
                return -EIO;

        dbuf = kzalloc_obj(*dbuf);
        if (!dbuf)
                return -ENOMEM;

        dbuf_add_to_list(front_info, dbuf, dbuf_cookie);

        memset(&buf_cfg, 0, sizeof(buf_cfg));
        buf_cfg.xb_dev = front_info->xb_dev;
        buf_cfg.num_pages = DIV_ROUND_UP(size, PAGE_SIZE);
        buf_cfg.pages = pages;
        buf_cfg.pgdir = &dbuf->shbuf;
        buf_cfg.be_alloc = front_info->cfg.be_alloc;

        ret = xen_front_pgdir_shbuf_alloc(&buf_cfg);
        if (ret < 0)
                goto fail_shbuf_alloc;

        mutex_lock(&evtchnl->u.req.req_io_lock);

        spin_lock_irqsave(&front_info->io_lock, flags);
        req = be_prepare_req(evtchnl, XENDISPL_OP_DBUF_CREATE);
        req->op.dbuf_create.gref_directory =
                        xen_front_pgdir_shbuf_get_dir_start(&dbuf->shbuf);
        req->op.dbuf_create.buffer_sz = size;
        req->op.dbuf_create.data_ofs = offset;
        req->op.dbuf_create.dbuf_cookie = dbuf_cookie;
        req->op.dbuf_create.width = width;
        req->op.dbuf_create.height = height;
        req->op.dbuf_create.bpp = bpp;
        if (buf_cfg.be_alloc)
                req->op.dbuf_create.flags |= XENDISPL_DBUF_FLG_REQ_ALLOC;

        ret = be_stream_do_io(evtchnl, req);
        spin_unlock_irqrestore(&front_info->io_lock, flags);

        if (ret < 0)
                goto fail;

        ret = be_stream_wait_io(evtchnl);
        if (ret < 0)
                goto fail;

        ret = xen_front_pgdir_shbuf_map(&dbuf->shbuf);
        if (ret < 0)
                goto fail;

        mutex_unlock(&evtchnl->u.req.req_io_lock);
        return 0;

fail:
        mutex_unlock(&evtchnl->u.req.req_io_lock);
fail_shbuf_alloc:
        dbuf_free(&front_info->dbuf_list, dbuf_cookie);
        return ret;
}

static int xen_drm_front_dbuf_destroy(struct xen_drm_front_info *front_info,
                                      u64 dbuf_cookie)
{
        struct xen_drm_front_evtchnl *evtchnl;
        struct xendispl_req *req;
        unsigned long flags;
        bool be_alloc;
        int ret;

        evtchnl = &front_info->evt_pairs[GENERIC_OP_EVT_CHNL].req;
        if (unlikely(!evtchnl))
                return -EIO;

        be_alloc = front_info->cfg.be_alloc;

        /*
         * For the backend allocated buffer release references now, so backend
         * can free the buffer.
         */
        if (be_alloc)
                dbuf_free(&front_info->dbuf_list, dbuf_cookie);

        mutex_lock(&evtchnl->u.req.req_io_lock);

        spin_lock_irqsave(&front_info->io_lock, flags);
        req = be_prepare_req(evtchnl, XENDISPL_OP_DBUF_DESTROY);
        req->op.dbuf_destroy.dbuf_cookie = dbuf_cookie;

        ret = be_stream_do_io(evtchnl, req);
        spin_unlock_irqrestore(&front_info->io_lock, flags);

        if (ret == 0)
                ret = be_stream_wait_io(evtchnl);

        /*
         * Do this regardless of communication status with the backend:
         * if we cannot remove remote resources remove what we can locally.
         */
        if (!be_alloc)
                dbuf_free(&front_info->dbuf_list, dbuf_cookie);

        mutex_unlock(&evtchnl->u.req.req_io_lock);
        return ret;
}

int xen_drm_front_fb_attach(struct xen_drm_front_info *front_info,
                            u64 dbuf_cookie, u64 fb_cookie, u32 width,
                            u32 height, u32 pixel_format)
{
        struct xen_drm_front_evtchnl *evtchnl;
        struct xen_drm_front_dbuf *buf;
        struct xendispl_req *req;
        unsigned long flags;
        int ret;

        evtchnl = &front_info->evt_pairs[GENERIC_OP_EVT_CHNL].req;
        if (unlikely(!evtchnl))
                return -EIO;

        buf = dbuf_get(&front_info->dbuf_list, dbuf_cookie);
        if (!buf)
                return -EINVAL;

        buf->fb_cookie = fb_cookie;

        mutex_lock(&evtchnl->u.req.req_io_lock);

        spin_lock_irqsave(&front_info->io_lock, flags);
        req = be_prepare_req(evtchnl, XENDISPL_OP_FB_ATTACH);
        req->op.fb_attach.dbuf_cookie = dbuf_cookie;
        req->op.fb_attach.fb_cookie = fb_cookie;
        req->op.fb_attach.width = width;
        req->op.fb_attach.height = height;
        req->op.fb_attach.pixel_format = pixel_format;

        ret = be_stream_do_io(evtchnl, req);
        spin_unlock_irqrestore(&front_info->io_lock, flags);

        if (ret == 0)
                ret = be_stream_wait_io(evtchnl);

        mutex_unlock(&evtchnl->u.req.req_io_lock);
        return ret;
}

int xen_drm_front_fb_detach(struct xen_drm_front_info *front_info,
                            u64 fb_cookie)
{
        struct xen_drm_front_evtchnl *evtchnl;
        struct xendispl_req *req;
        unsigned long flags;
        int ret;

        evtchnl = &front_info->evt_pairs[GENERIC_OP_EVT_CHNL].req;
        if (unlikely(!evtchnl))
                return -EIO;

        mutex_lock(&evtchnl->u.req.req_io_lock);

        spin_lock_irqsave(&front_info->io_lock, flags);
        req = be_prepare_req(evtchnl, XENDISPL_OP_FB_DETACH);
        req->op.fb_detach.fb_cookie = fb_cookie;

        ret = be_stream_do_io(evtchnl, req);
        spin_unlock_irqrestore(&front_info->io_lock, flags);

        if (ret == 0)
                ret = be_stream_wait_io(evtchnl);

        mutex_unlock(&evtchnl->u.req.req_io_lock);
        return ret;
}

int xen_drm_front_page_flip(struct xen_drm_front_info *front_info,
                            int conn_idx, u64 fb_cookie)
{
        struct xen_drm_front_evtchnl *evtchnl;
        struct xendispl_req *req;
        unsigned long flags;
        int ret;

        if (unlikely(conn_idx >= front_info->num_evt_pairs))
                return -EINVAL;

        evtchnl = &front_info->evt_pairs[conn_idx].req;

        mutex_lock(&evtchnl->u.req.req_io_lock);

        spin_lock_irqsave(&front_info->io_lock, flags);
        req = be_prepare_req(evtchnl, XENDISPL_OP_PG_FLIP);
        req->op.pg_flip.fb_cookie = fb_cookie;

        ret = be_stream_do_io(evtchnl, req);
        spin_unlock_irqrestore(&front_info->io_lock, flags);

        if (ret == 0)
                ret = be_stream_wait_io(evtchnl);

        mutex_unlock(&evtchnl->u.req.req_io_lock);
        return ret;
}

void xen_drm_front_on_frame_done(struct xen_drm_front_info *front_info,
                                 int conn_idx, u64 fb_cookie)
{
        struct xen_drm_front_drm_info *drm_info = front_info->drm_info;

        if (unlikely(conn_idx >= front_info->cfg.num_connectors))
                return;

        xen_drm_front_kms_on_frame_done(&drm_info->pipeline[conn_idx],
                                        fb_cookie);
}

void xen_drm_front_gem_object_free(struct drm_gem_object *obj)
{
        struct xen_drm_front_drm_info *drm_info = obj->dev->dev_private;
        int idx;

        if (drm_dev_enter(obj->dev, &idx)) {
                xen_drm_front_dbuf_destroy(drm_info->front_info,
                                           xen_drm_front_dbuf_to_cookie(obj));
                drm_dev_exit(idx);
        } else {
                dbuf_free(&drm_info->front_info->dbuf_list,
                          xen_drm_front_dbuf_to_cookie(obj));
        }

        xen_drm_front_gem_free_object_unlocked(obj);
}

static int xen_drm_drv_dumb_create(struct drm_file *filp,
                                   struct drm_device *dev,
                                   struct drm_mode_create_dumb *args)
{
        struct xen_drm_front_drm_info *drm_info = dev->dev_private;
        struct drm_gem_object *obj;
        int ret;

        /*
         * Dumb creation is a two stage process: first we create a fully
         * constructed GEM object which is communicated to the backend, and
         * only after that we can create GEM's handle. This is done so,
         * because of the possible races: once you create a handle it becomes
         * immediately visible to user-space, so the latter can try accessing
         * object without pages etc.
         * For details also see drm_gem_handle_create
         */
        args->pitch = DIV_ROUND_UP(args->width * args->bpp, 8);
        args->size = args->pitch * args->height;

        obj = xen_drm_front_gem_create(dev, args->size);
        if (IS_ERR(obj)) {
                ret = PTR_ERR(obj);
                goto fail;
        }

        ret = xen_drm_front_dbuf_create(drm_info->front_info,
                                        xen_drm_front_dbuf_to_cookie(obj),
                                        args->width, args->height, args->bpp,
                                        args->size, 0,
                                        xen_drm_front_gem_get_pages(obj));
        if (ret)
                goto fail_backend;

        /* This is the tail of GEM object creation */
        ret = drm_gem_handle_create(filp, obj, &args->handle);
        if (ret)
                goto fail_handle;

        /* Drop reference from allocate - handle holds it now */
        drm_gem_object_put(obj);
        return 0;

fail_handle:
        xen_drm_front_dbuf_destroy(drm_info->front_info,
                                   xen_drm_front_dbuf_to_cookie(obj));
fail_backend:
        /* drop reference from allocate */
        drm_gem_object_put(obj);
fail:
        DRM_ERROR("Failed to create dumb buffer: %d\n", ret);
        return ret;
}

static void xen_drm_drv_release(struct drm_device *dev)
{
        struct xen_drm_front_drm_info *drm_info = dev->dev_private;
        struct xen_drm_front_info *front_info = drm_info->front_info;

        xen_drm_front_kms_fini(drm_info);

        drm_atomic_helper_shutdown(dev);
        drm_mode_config_cleanup(dev);

        if (front_info->cfg.be_alloc)
                xenbus_switch_state(front_info->xb_dev,
                                    XenbusStateInitialising);

        kfree(drm_info);
}

DEFINE_DRM_GEM_FOPS(xen_drm_dev_fops);

static const struct drm_driver xen_drm_driver = {
        .driver_features           = DRIVER_GEM | DRIVER_MODESET | DRIVER_ATOMIC,
        .release                   = xen_drm_drv_release,
        .gem_prime_import_sg_table = xen_drm_front_gem_import_sg_table,
        .dumb_create               = xen_drm_drv_dumb_create,
        .fops                      = &xen_drm_dev_fops,
        .name                      = "xendrm-du",
        .desc                      = "Xen PV DRM Display Unit",
        .major                     = 1,
        .minor                     = 0,

};

static int xen_drm_drv_init(struct xen_drm_front_info *front_info)
{
        struct device *dev = &front_info->xb_dev->dev;
        struct xen_drm_front_drm_info *drm_info;
        struct drm_device *drm_dev;
        int ret;

        if (drm_firmware_drivers_only())
                return -ENODEV;

        DRM_INFO("Creating %s\n", xen_drm_driver.desc);

        drm_info = kzalloc_obj(*drm_info);
        if (!drm_info) {
                ret = -ENOMEM;
                goto fail;
        }

        drm_info->front_info = front_info;
        front_info->drm_info = drm_info;

        drm_dev = drm_dev_alloc(&xen_drm_driver, dev);
        if (IS_ERR(drm_dev)) {
                ret = PTR_ERR(drm_dev);
                goto fail_dev;
        }

        drm_info->drm_dev = drm_dev;

        drm_dev->dev_private = drm_info;

        ret = xen_drm_front_kms_init(drm_info);
        if (ret) {
                DRM_ERROR("Failed to initialize DRM/KMS, ret %d\n", ret);
                goto fail_modeset;
        }

        ret = drm_dev_register(drm_dev, 0);
        if (ret)
                goto fail_register;

        return 0;

fail_register:
        drm_dev_unregister(drm_dev);
fail_modeset:
        drm_kms_helper_poll_fini(drm_dev);
        drm_mode_config_cleanup(drm_dev);
        drm_dev_put(drm_dev);
fail_dev:
        kfree(drm_info);
        front_info->drm_info = NULL;
fail:
        return ret;
}

static void xen_drm_drv_fini(struct xen_drm_front_info *front_info)
{
        struct xen_drm_front_drm_info *drm_info = front_info->drm_info;
        struct drm_device *dev;

        if (!drm_info)
                return;

        dev = drm_info->drm_dev;
        if (!dev)
                return;

        /* Nothing to do if device is already unplugged */
        if (drm_dev_is_unplugged(dev))
                return;

        drm_kms_helper_poll_fini(dev);
        drm_dev_unplug(dev);
        drm_dev_put(dev);

        front_info->drm_info = NULL;

        xen_drm_front_evtchnl_free_all(front_info);
        dbuf_free_all(&front_info->dbuf_list);

        /*
         * If we are not using backend allocated buffers, then tell the
         * backend we are ready to (re)initialize. Otherwise, wait for
         * drm_driver.release.
         */
        if (!front_info->cfg.be_alloc)
                xenbus_switch_state(front_info->xb_dev,
                                    XenbusStateInitialising);
}

static int displback_initwait(struct xen_drm_front_info *front_info)
{
        struct xen_drm_front_cfg *cfg = &front_info->cfg;
        int ret;

        cfg->front_info = front_info;
        ret = xen_drm_front_cfg_card(front_info, cfg);
        if (ret < 0)
                return ret;

        DRM_INFO("Have %d connector(s)\n", cfg->num_connectors);
        /* Create event channels for all connectors and publish */
        ret = xen_drm_front_evtchnl_create_all(front_info);
        if (ret < 0)
                return ret;

        return xen_drm_front_evtchnl_publish_all(front_info);
}

static int displback_connect(struct xen_drm_front_info *front_info)
{
        xen_drm_front_evtchnl_set_state(front_info, EVTCHNL_STATE_CONNECTED);
        return xen_drm_drv_init(front_info);
}

static void displback_disconnect(struct xen_drm_front_info *front_info)
{
        if (!front_info->drm_info)
                return;

        /* Tell the backend to wait until we release the DRM driver. */
        xenbus_switch_state(front_info->xb_dev, XenbusStateReconfiguring);

        xen_drm_drv_fini(front_info);
}

static void displback_changed(struct xenbus_device *xb_dev,
                              enum xenbus_state backend_state)
{
        struct xen_drm_front_info *front_info = dev_get_drvdata(&xb_dev->dev);
        int ret;

        DRM_DEBUG("Backend state is %s, front is %s\n",
                  xenbus_strstate(backend_state),
                  xenbus_strstate(xb_dev->state));

        switch (backend_state) {
        case XenbusStateReconfiguring:
        case XenbusStateReconfigured:
        case XenbusStateInitialised:
                break;

        case XenbusStateInitialising:
                if (xb_dev->state == XenbusStateReconfiguring)
                        break;

                /* recovering after backend unexpected closure */
                displback_disconnect(front_info);
                break;

        case XenbusStateInitWait:
                if (xb_dev->state == XenbusStateReconfiguring)
                        break;

                /* recovering after backend unexpected closure */
                displback_disconnect(front_info);
                if (xb_dev->state != XenbusStateInitialising)
                        break;

                ret = displback_initwait(front_info);
                if (ret < 0)
                        xenbus_dev_fatal(xb_dev, ret, "initializing frontend");
                else
                        xenbus_switch_state(xb_dev, XenbusStateInitialised);
                break;

        case XenbusStateConnected:
                if (xb_dev->state != XenbusStateInitialised)
                        break;

                ret = displback_connect(front_info);
                if (ret < 0) {
                        displback_disconnect(front_info);
                        xenbus_dev_fatal(xb_dev, ret, "connecting backend");
                } else {
                        xenbus_switch_state(xb_dev, XenbusStateConnected);
                }
                break;

        case XenbusStateClosing:
                /*
                 * in this state backend starts freeing resources,
                 * so let it go into closed state, so we can also
                 * remove ours
                 */
                break;

        case XenbusStateUnknown:
        case XenbusStateClosed:
                if (xb_dev->state == XenbusStateClosed)
                        break;

                displback_disconnect(front_info);
                break;
        }
}

static int xen_drv_probe(struct xenbus_device *xb_dev,
                         const struct xenbus_device_id *id)
{
        struct xen_drm_front_info *front_info;
        struct device *dev = &xb_dev->dev;
        int ret;

        ret = dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(64));
        if (ret < 0) {
                DRM_ERROR("Cannot setup DMA mask, ret %d", ret);
                return ret;
        }

        front_info = devm_kzalloc(&xb_dev->dev,
                                  sizeof(*front_info), GFP_KERNEL);
        if (!front_info)
                return -ENOMEM;

        front_info->xb_dev = xb_dev;
        spin_lock_init(&front_info->io_lock);
        INIT_LIST_HEAD(&front_info->dbuf_list);
        dev_set_drvdata(&xb_dev->dev, front_info);

        return xenbus_switch_state(xb_dev, XenbusStateInitialising);
}

static void xen_drv_remove(struct xenbus_device *dev)
{
        struct xen_drm_front_info *front_info = dev_get_drvdata(&dev->dev);
        int to = 100;

        xenbus_switch_state(dev, XenbusStateClosing);

        /*
         * On driver removal it is disconnected from XenBus,
         * so no backend state change events come via .otherend_changed
         * callback. This prevents us from exiting gracefully, e.g.
         * signaling the backend to free event channels, waiting for its
         * state to change to XenbusStateClosed and cleaning at our end.
         * Normally when front driver removed backend will finally go into
         * XenbusStateInitWait state.
         *
         * Workaround: read backend's state manually and wait with time-out.
         */
        while ((xenbus_read_unsigned(front_info->xb_dev->otherend, "state",
                                     XenbusStateUnknown) != XenbusStateInitWait) &&
                                     --to)
                msleep(10);

        if (!to) {
                unsigned int state;

                state = xenbus_read_unsigned(front_info->xb_dev->otherend,
                                             "state", XenbusStateUnknown);
                DRM_ERROR("Backend state is %s while removing driver\n",
                          xenbus_strstate(state));
        }

        xen_drm_drv_fini(front_info);
        xenbus_frontend_closed(dev);
}

static const struct xenbus_device_id xen_driver_ids[] = {
        { XENDISPL_DRIVER_NAME },
        { "" }
};

static struct xenbus_driver xen_driver = {
        .ids = xen_driver_ids,
        .probe = xen_drv_probe,
        .remove = xen_drv_remove,
        .otherend_changed = displback_changed,
        .not_essential = true,
};

static int __init xen_drv_init(void)
{
        /* At the moment we only support case with XEN_PAGE_SIZE == PAGE_SIZE */
        if (XEN_PAGE_SIZE != PAGE_SIZE) {
                DRM_ERROR(XENDISPL_DRIVER_NAME ": different kernel and Xen page sizes are not supported: XEN_PAGE_SIZE (%lu) != PAGE_SIZE (%lu)\n",
                          XEN_PAGE_SIZE, PAGE_SIZE);
                return -ENODEV;
        }

        if (!xen_domain())
                return -ENODEV;

        if (!xen_has_pv_devices())
                return -ENODEV;

        DRM_INFO("Registering XEN PV " XENDISPL_DRIVER_NAME "\n");
        return xenbus_register_frontend(&xen_driver);
}

static void __exit xen_drv_fini(void)
{
        DRM_INFO("Unregistering XEN PV " XENDISPL_DRIVER_NAME "\n");
        xenbus_unregister_driver(&xen_driver);
}

module_init(xen_drv_init);
module_exit(xen_drv_fini);

MODULE_DESCRIPTION("Xen para-virtualized display device frontend");
MODULE_LICENSE("GPL");
MODULE_ALIAS("xen:" XENDISPL_DRIVER_NAME);