text: /*
   * Copyright © 2012-2014 Intel Corporation
   *
   * Permission is hereby granted, free of charge, to any person obtaining a
   * copy of this software and associated documentation files (the "Software"),
   * to deal in the Software without restriction, including without limitation
   * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   * and/or sell copies of the Software, and to permit persons to whom the
   * Software is furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice (including the next
   * paragraph) shall be included in all copies or substantial portions of the
   * Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
   * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
   * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
   * IN THE SOFTWARE.
   *
   */
  
  #include "drmP.h"
  #include "i915_drm.h"
  #include "i915_drv.h"
  #include "i915_trace.h"
  #include "intel_drv.h"
  #include <linux/mmu_context.h>
  #include <linux/mmu_notifier.h>
  #include <linux/mempolicy.h>
  #include <linux/swap.h>
  
  struct i915_mm_struct {
  	struct mm_struct *mm;
  	struct drm_device *dev;
  	struct i915_mmu_notifier *mn;
  	struct hlist_node node;
  	struct kref kref;
  	struct work_struct work;
  };
  
  #if defined(CONFIG_MMU_NOTIFIER)
  #include <linux/interval_tree.h>
  
  struct i915_mmu_notifier {
  	spinlock_t lock;
  	struct hlist_node node;
  	struct mmu_notifier mn;
  	struct rb_root objects;
  	struct list_head linear;
  	unsigned long serial;
  	bool has_linear;
  };
  
  struct i915_mmu_object {
  	struct i915_mmu_notifier *mn;
  	struct interval_tree_node it;
  	struct list_head link;
  	struct drm_i915_gem_object *obj;
  	bool is_linear;
  };
  
  static unsigned long cancel_userptr(struct drm_i915_gem_object *obj)
  {
  	struct drm_device *dev = obj->base.dev;
  	unsigned long end;
  
  	mutex_lock(&dev->struct_mutex);
  	/* Cancel any active worker and force us to re-evaluate gup */
  	obj->userptr.work = NULL;
  
  	if (obj->pages != NULL) {
  		struct drm_i915_private *dev_priv = to_i915(dev);
  		struct i915_vma *vma, *tmp;
  		bool was_interruptible;
  
  		was_interruptible = dev_priv->mm.interruptible;
  		dev_priv->mm.interruptible = false;
  
  		list_for_each_entry_safe(vma, tmp, &obj->vma_list, vma_link) {
  			int ret = i915_vma_unbind(vma);
  			WARN_ON(ret && ret != -EIO);
  		}
  		WARN_ON(i915_gem_object_put_pages(obj));
  
  		dev_priv->mm.interruptible = was_interruptible;
  	}
  
  	end = obj->userptr.ptr + obj->base.size;
  
  	drm_gem_object_unreference(&obj->base);
  	mutex_unlock(&dev->struct_mutex);
  
  	return end;
  }
  
  static void *invalidate_range__linear(struct i915_mmu_notifier *mn,
  				      struct mm_struct *mm,
  				      unsigned long start,
  				      unsigned long end)
  {
  	struct i915_mmu_object *mo;
  	unsigned long serial;
  
  restart:
  	serial = mn->serial;
  	list_for_each_entry(mo, &mn->linear, link) {
  		struct drm_i915_gem_object *obj;
  
  		if (mo->it.last < start || mo->it.start > end)
  			continue;
  
  		obj = mo->obj;
  		drm_gem_object_reference(&obj->base);
  		spin_unlock(&mn->lock);
  
  		cancel_userptr(obj);
  
  		spin_lock(&mn->lock);
  		if (serial != mn->serial)
  			goto restart;
  	}
  
  	return NULL;
  }
  
  static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
  						       struct mm_struct *mm,
  						       unsigned long start,
  						       unsigned long end)
  {
  	struct i915_mmu_notifier *mn = container_of(_mn, struct i915_mmu_notifier, mn);
  	struct interval_tree_node *it = NULL;
  	unsigned long next = start;
  	unsigned long serial = 0;
  
  	end--; /* interval ranges are inclusive, but invalidate range is exclusive */
  	while (next < end) {
  		struct drm_i915_gem_object *obj = NULL;
  
  		spin_lock(&mn->lock);
  		if (mn->has_linear)
  			it = invalidate_range__linear(mn, mm, start, end);
  		else if (serial == mn->serial)
  			it = interval_tree_iter_next(it, next, end);
  		else
  			it = interval_tree_iter_first(&mn->objects, start, end);
  		if (it != NULL) {
  			obj = container_of(it, struct i915_mmu_object, it)->obj;
  			drm_gem_object_reference(&obj->base);
  			serial = mn->serial;
  		}
  		spin_unlock(&mn->lock);
  		if (obj == NULL)
  			return;
  
  		next = cancel_userptr(obj);
  	}
  }
  
  static const struct mmu_notifier_ops i915_gem_userptr_notifier = {
  	.invalidate_range_start = i915_gem_userptr_mn_invalidate_range_start,
  };
  
  static struct i915_mmu_notifier *
  i915_mmu_notifier_create(struct mm_struct *mm)
  {
  	struct i915_mmu_notifier *mn;
  	int ret;
  
  	mn = kmalloc(sizeof(*mn), GFP_KERNEL);
  	if (mn == NULL)
  		return ERR_PTR(-ENOMEM);
  
  	spin_lock_init(&mn->lock);
  	mn->mn.ops = &i915_gem_userptr_notifier;
  	mn->objects = RB_ROOT;
  	mn->serial = 1;
  	INIT_LIST_HEAD(&mn->linear);
  	mn->has_linear = false;
  
  	 /* Protected by mmap_sem (write-lock) */
  	ret = __mmu_notifier_register(&mn->mn, mm);
  	if (ret) {
  		kfree(mn);
  		return ERR_PTR(ret);
  	}
  
  	return mn;
  }
  
  static void __i915_mmu_notifier_update_serial(struct i915_mmu_notifier *mn)
  {
  	if (++mn->serial == 0)
  		mn->serial = 1;
  }
  
  static int
  i915_mmu_notifier_add(struct drm_device *dev,
  		      struct i915_mmu_notifier *mn,
  		      struct i915_mmu_object *mo)
  {
  	struct interval_tree_node *it;
  	int ret;
  
  	ret = i915_mutex_lock_interruptible(dev);
  	if (ret)
  		return ret;
  
  	/* Make sure we drop the final active reference (and thereby
  	 * remove the objects from the interval tree) before we do
  	 * the check for overlapping objects.
  	 */
  	i915_gem_retire_requests(dev);
  
  	spin_lock(&mn->lock);
  	it = interval_tree_iter_first(&mn->objects,
  				      mo->it.start, mo->it.last);
  	if (it) {
  		struct drm_i915_gem_object *obj;
  
  		/* We only need to check the first object in the range as it
  		 * either has cancelled gup work queued and we need to
  		 * return back to the user to give time for the gup-workers
  		 * to flush their object references upon which the object will
  		 * be removed from the interval-tree, or the the range is
  		 * still in use by another client and the overlap is invalid.
  		 *
  		 * If we do have an overlap, we cannot use the interval tree
  		 * for fast range invalidation.
  		 */
  
  		obj = container_of(it, struct i915_mmu_object, it)->obj;
  		if (!obj->userptr.workers)
  			mn->has_linear = mo->is_linear = true;
  		else
  			ret = -EAGAIN;
  	} else
  		interval_tree_insert(&mo->it, &mn->objects);
  
  	if (ret == 0) {
  		list_add(&mo->link, &mn->linear);
  		__i915_mmu_notifier_update_serial(mn);
  	}
  	spin_unlock(&mn->lock);
  	mutex_unlock(&dev->struct_mutex);
  
  	return ret;
  }
  
  static bool i915_mmu_notifier_has_linear(struct i915_mmu_notifier *mn)
  {
  	struct i915_mmu_object *mo;
  
  	list_for_each_entry(mo, &mn->linear, link)
  		if (mo->is_linear)
  			return true;
  
  	return false;
  }
  
  static void
  i915_mmu_notifier_del(struct i915_mmu_notifier *mn,
  		      struct i915_mmu_object *mo)
  {
  	spin_lock(&mn->lock);
  	list_del(&mo->link);
  	if (mo->is_linear)
  		mn->has_linear = i915_mmu_notifier_has_linear(mn);
  	else
  		interval_tree_remove(&mo->it, &mn->objects);
  	__i915_mmu_notifier_update_serial(mn);
  	spin_unlock(&mn->lock);
  }
  
  static void
  i915_gem_userptr_release__mmu_notifier(struct drm_i915_gem_object *obj)
  {
  	struct i915_mmu_object *mo;
  
  	mo = obj->userptr.mmu_object;
  	if (mo == NULL)
  		return;
  
  	i915_mmu_notifier_del(mo->mn, mo);
  	kfree(mo);
  
  	obj->userptr.mmu_object = NULL;
  }
  
  static struct i915_mmu_notifier *
  i915_mmu_notifier_find(struct i915_mm_struct *mm)
  {
  	struct i915_mmu_notifier *mn = mm->mn;
  
  	mn = mm->mn;
  	if (mn)
  		return mn;
  
  	down_write(&mm->mm->mmap_sem);
  	mutex_lock(&to_i915(mm->dev)->mm_lock);
  	if ((mn = mm->mn) == NULL) {
  		mn = i915_mmu_notifier_create(mm->mm);
  		if (!IS_ERR(mn))
  			mm->mn = mn;
  	}
  	mutex_unlock(&to_i915(mm->dev)->mm_lock);
  	up_write(&mm->mm->mmap_sem);
  
  	return mn;
  }
  
  static int
  i915_gem_userptr_init__mmu_notifier(struct drm_i915_gem_object *obj,
  				    unsigned flags)
  {
  	struct i915_mmu_notifier *mn;
  	struct i915_mmu_object *mo;
  	int ret;
  
  	if (flags & I915_USERPTR_UNSYNCHRONIZED)
  		return capable(CAP_SYS_ADMIN) ? 0 : -EPERM;
  
  	if (WARN_ON(obj->userptr.mm == NULL))
  		return -EINVAL;
  
  	mn = i915_mmu_notifier_find(obj->userptr.mm);
  	if (IS_ERR(mn))
  		return PTR_ERR(mn);
  
  	mo = kzalloc(sizeof(*mo), GFP_KERNEL);
  	if (mo == NULL)
  		return -ENOMEM;
  
  	mo->mn = mn;
  	mo->it.start = obj->userptr.ptr;
  	mo->it.last = mo->it.start + obj->base.size - 1;
  	mo->obj = obj;
  
  	ret = i915_mmu_notifier_add(obj->base.dev, mn, mo);
  	if (ret) {
  		kfree(mo);
  		return ret;
  	}
  
  	obj->userptr.mmu_object = mo;
  	return 0;
  }
  
  static void
  i915_mmu_notifier_free(struct i915_mmu_notifier *mn,
  		       struct mm_struct *mm)
  {
  	if (mn == NULL)
  		return;
  
  	mmu_notifier_unregister(&mn->mn, mm);
  	kfree(mn);
  }
  
  #else
  
  static void
  i915_gem_userptr_release__mmu_notifier(struct drm_i915_gem_object *obj)
  {
  }
  
  static int
  i915_gem_userptr_init__mmu_notifier(struct drm_i915_gem_object *obj,
  				    unsigned flags)
  {
  	if ((flags & I915_USERPTR_UNSYNCHRONIZED) == 0)
  		return -ENODEV;
  
  	if (!capable(CAP_SYS_ADMIN))
  		return -EPERM;
  
  	return 0;
  }
  
  static void
  i915_mmu_notifier_free(struct i915_mmu_notifier *mn,
  		       struct mm_struct *mm)
  {
  }
  
  #endif
  
  static struct i915_mm_struct *
  __i915_mm_struct_find(struct drm_i915_private *dev_priv, struct mm_struct *real)
  {
  	struct i915_mm_struct *mm;
  
  	/* Protected by dev_priv->mm_lock */
  	hash_for_each_possible(dev_priv->mm_structs, mm, node, (unsigned long)real)
  		if (mm->mm == real)
  			return mm;
  
  	return NULL;
  }
  
  static int
  i915_gem_userptr_init__mm_struct(struct drm_i915_gem_object *obj)
  {
  	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
  	struct i915_mm_struct *mm;
  	int ret = 0;
  
  	/* During release of the GEM object we hold the struct_mutex. This
  	 * precludes us from calling mmput() at that time as that may be
  	 * the last reference and so call exit_mmap(). exit_mmap() will
  	 * attempt to reap the vma, and if we were holding a GTT mmap
  	 * would then call drm_gem_vm_close() and attempt to reacquire
  	 * the struct mutex. So in order to avoid that recursion, we have
  	 * to defer releasing the mm reference until after we drop the
  	 * struct_mutex, i.e. we need to schedule a worker to do the clean
  	 * up.
  	 */
  	mutex_lock(&dev_priv->mm_lock);
  	mm = __i915_mm_struct_find(dev_priv, current->mm);
  	if (mm == NULL) {
  		mm = kmalloc(sizeof(*mm), GFP_KERNEL);
  		if (mm == NULL) {
  			ret = -ENOMEM;
  			goto out;
  		}
  
  		kref_init(&mm->kref);
  		mm->dev = obj->base.dev;
  
  		mm->mm = current->mm;
  		atomic_inc(&current->mm->mm_count);
  
  		mm->mn = NULL;
  
  		/* Protected by dev_priv->mm_lock */
  		hash_add(dev_priv->mm_structs,
  			 &mm->node, (unsigned long)mm->mm);
  	} else
  		kref_get(&mm->kref);
  
  	obj->userptr.mm = mm;
  out:
  	mutex_unlock(&dev_priv->mm_lock);
  	return ret;
  }
  
  static void
  __i915_mm_struct_free__worker(struct work_struct *work)
  {
  	struct i915_mm_struct *mm = container_of(work, typeof(*mm), work);
  	i915_mmu_notifier_free(mm->mn, mm->mm);
  	mmdrop(mm->mm);
  	kfree(mm);
  }
  
  static void
  __i915_mm_struct_free(struct kref *kref)
  {
  	struct i915_mm_struct *mm = container_of(kref, typeof(*mm), kref);
  
  	/* Protected by dev_priv->mm_lock */
  	hash_del(&mm->node);
  	mutex_unlock(&to_i915(mm->dev)->mm_lock);
  
  	INIT_WORK(&mm->work, __i915_mm_struct_free__worker);
  	schedule_work(&mm->work);
  }
  
  static void
  i915_gem_userptr_release__mm_struct(struct drm_i915_gem_object *obj)
  {
  	if (obj->userptr.mm == NULL)
  		return;
  
  	kref_put_mutex(&obj->userptr.mm->kref,
  		       __i915_mm_struct_free,
  		       &to_i915(obj->base.dev)->mm_lock);
  	obj->userptr.mm = NULL;
  }
  
  struct get_pages_work {
  	struct work_struct work;
  	struct drm_i915_gem_object *obj;
  	struct task_struct *task;
  };
  
  #if IS_ENABLED(CONFIG_SWIOTLB)
  #define swiotlb_active() swiotlb_nr_tbl()
  #else
  #define swiotlb_active() 0
  #endif
  
  static int
  st_set_pages(struct sg_table **st, struct page **pvec, int num_pages)
  {
  	struct scatterlist *sg;
  	int ret, n;
  
  	*st = kmalloc(sizeof(**st), GFP_KERNEL);
  	if (*st == NULL)
  		return -ENOMEM;
  
  	if (swiotlb_active()) {
  		ret = sg_alloc_table(*st, num_pages, GFP_KERNEL);
  		if (ret)
  			goto err;
  
  		for_each_sg((*st)->sgl, sg, num_pages, n)
  			sg_set_page(sg, pvec[n], PAGE_SIZE, 0);
  	} else {
  		ret = sg_alloc_table_from_pages(*st, pvec, num_pages,
  						0, num_pages << PAGE_SHIFT,
  						GFP_KERNEL);
  		if (ret)
  			goto err;
  	}
  
  	return 0;
  
  err:
  	kfree(*st);
  	*st = NULL;
  	return ret;
  }
  
  static void
  __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
  {
  	struct get_pages_work *work = container_of(_work, typeof(*work), work);
  	struct drm_i915_gem_object *obj = work->obj;
  	struct drm_device *dev = obj->base.dev;
  	const int num_pages = obj->base.size >> PAGE_SHIFT;
  	struct page **pvec;
  	int pinned, ret;
  
  	ret = -ENOMEM;
  	pinned = 0;
  
  	pvec = kmalloc(num_pages*sizeof(struct page *),
  		       GFP_TEMPORARY | __GFP_NOWARN | __GFP_NORETRY);
  	if (pvec == NULL)
  		pvec = drm_malloc_ab(num_pages, sizeof(struct page *));
  	if (pvec != NULL) {
  		struct mm_struct *mm = obj->userptr.mm->mm;
  
  		down_read(&mm->mmap_sem);
  		while (pinned < num_pages) {
  			ret = get_user_pages(work->task, mm,
  					     obj->userptr.ptr + pinned * PAGE_SIZE,
  					     num_pages - pinned,
  					     !obj->userptr.read_only, 0,
  					     pvec + pinned, NULL);
  			if (ret < 0)
  				break;
  
  			pinned += ret;
  		}
  		up_read(&mm->mmap_sem);
  	}
  
  	mutex_lock(&dev->struct_mutex);
  	if (obj->userptr.work != &work->work) {
  		ret = 0;
  	} else if (pinned == num_pages) {
  		ret = st_set_pages(&obj->pages, pvec, num_pages);
  		if (ret == 0) {
  			list_add_tail(&obj->global_list, &to_i915(dev)->mm.unbound_list);
  			pinned = 0;
  		}
  	}
  
  	obj->userptr.work = ERR_PTR(ret);
  	obj->userptr.workers--;
  	drm_gem_object_unreference(&obj->base);
  	mutex_unlock(&dev->struct_mutex);
  
  	release_pages(pvec, pinned, 0);
  	drm_free_large(pvec);
  
  	put_task_struct(work->task);
  	kfree(work);
  }
  
  static int
  i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
  {
  	const int num_pages = obj->base.size >> PAGE_SHIFT;
  	struct page **pvec;
  	int pinned, ret;
  
  	/* If userspace should engineer that these pages are replaced in
  	 * the vma between us binding this page into the GTT and completion
  	 * of rendering... Their loss. If they change the mapping of their
  	 * pages they need to create a new bo to point to the new vma.
  	 *
  	 * However, that still leaves open the possibility of the vma
  	 * being copied upon fork. Which falls under the same userspace
  	 * synchronisation issue as a regular bo, except that this time
  	 * the process may not be expecting that a particular piece of
  	 * memory is tied to the GPU.
  	 *
  	 * Fortunately, we can hook into the mmu_notifier in order to
  	 * discard the page references prior to anything nasty happening
  	 * to the vma (discard or cloning) which should prevent the more
  	 * egregious cases from causing harm.
  	 */
  
  	pvec = NULL;
  	pinned = 0;
  	if (obj->userptr.mm->mm == current->mm) {
  		pvec = kmalloc(num_pages*sizeof(struct page *),
  			       GFP_TEMPORARY | __GFP_NOWARN | __GFP_NORETRY);
  		if (pvec == NULL) {
  			pvec = drm_malloc_ab(num_pages, sizeof(struct page *));
  			if (pvec == NULL)
  				return -ENOMEM;
  		}
  
  		pinned = __get_user_pages_fast(obj->userptr.ptr, num_pages,
  					       !obj->userptr.read_only, pvec);
  	}
  	if (pinned < num_pages) {
  		if (pinned < 0) {
  			ret = pinned;
  			pinned = 0;
  		} else {
  			/* Spawn a worker so that we can acquire the
  			 * user pages without holding our mutex. Access
  			 * to the user pages requires mmap_sem, and we have
  			 * a strict lock ordering of mmap_sem, struct_mutex -
  			 * we already hold struct_mutex here and so cannot
  			 * call gup without encountering a lock inversion.
  			 *
  			 * Userspace will keep on repeating the operation
  			 * (thanks to EAGAIN) until either we hit the fast
  			 * path or the worker completes. If the worker is
  			 * cancelled or superseded, the task is still run
  			 * but the results ignored. (This leads to
  			 * complications that we may have a stray object
  			 * refcount that we need to be wary of when
  			 * checking for existing objects during creation.)
  			 * If the worker encounters an error, it reports
  			 * that error back to this function through
  			 * obj->userptr.work = ERR_PTR.
  			 */
  			ret = -EAGAIN;
  			if (obj->userptr.work == NULL &&
  			    obj->userptr.workers < I915_GEM_USERPTR_MAX_WORKERS) {
  				struct get_pages_work *work;
  
  				work = kmalloc(sizeof(*work), GFP_KERNEL);
  				if (work != NULL) {
  					obj->userptr.work = &work->work;
  					obj->userptr.workers++;
  
  					work->obj = obj;
  					drm_gem_object_reference(&obj->base);
  
  					work->task = current;
  					get_task_struct(work->task);
  
  					INIT_WORK(&work->work, __i915_gem_userptr_get_pages_worker);
  					schedule_work(&work->work);
  				} else
  					ret = -ENOMEM;
  			} else {
  				if (IS_ERR(obj->userptr.work)) {
  					ret = PTR_ERR(obj->userptr.work);
  					obj->userptr.work = NULL;
  				}
  			}
  		}
  	} else {
  		ret = st_set_pages(&obj->pages, pvec, num_pages);
  		if (ret == 0) {
  			obj->userptr.work = NULL;
  			pinned = 0;
  		}
  	}
  
  	release_pages(pvec, pinned, 0);
  	drm_free_large(pvec);
  	return ret;
  }
  
  static void
  i915_gem_userptr_put_pages(struct drm_i915_gem_object *obj)
  {
  	struct sg_page_iter sg_iter;
  
  	BUG_ON(obj->userptr.work != NULL);
  
  	if (obj->madv != I915_MADV_WILLNEED)
  		obj->dirty = 0;
  
  	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, 0) {
  		struct page *page = sg_page_iter_page(&sg_iter);
  
  		if (obj->dirty)
  			set_page_dirty(page);
  
  		mark_page_accessed(page);
  		page_cache_release(page);
  	}
  	obj->dirty = 0;
  
  	sg_free_table(obj->pages);
  	kfree(obj->pages);
  }
  
  static void
  i915_gem_userptr_release(struct drm_i915_gem_object *obj)
  {
  	i915_gem_userptr_release__mmu_notifier(obj);
  	i915_gem_userptr_release__mm_struct(obj);
  }
  
  static int
  i915_gem_userptr_dmabuf_export(struct drm_i915_gem_object *obj)
  {
  	if (obj->userptr.mmu_object)
  		return 0;
  
  	return i915_gem_userptr_init__mmu_notifier(obj, 0);
  }
  
  static const struct drm_i915_gem_object_ops i915_gem_userptr_ops = {
  	.dmabuf_export = i915_gem_userptr_dmabuf_export,
  	.get_pages = i915_gem_userptr_get_pages,
  	.put_pages = i915_gem_userptr_put_pages,
  	.release = i915_gem_userptr_release,
  };
  
  /**
   * Creates a new mm object that wraps some normal memory from the process
   * context - user memory.
   *
   * We impose several restrictions upon the memory being mapped
   * into the GPU.
   * 1. It must be page aligned (both start/end addresses, i.e ptr and size).
   * 2. It must be normal system memory, not a pointer into another map of IO
   *    space (e.g. it must not be a GTT mmapping of another object).
   * 3. We only allow a bo as large as we could in theory map into the GTT,
   *    that is we limit the size to the total size of the GTT.
   * 4. The bo is marked as being snoopable. The backing pages are left
   *    accessible directly by the CPU, but reads and writes by the GPU may
   *    incur the cost of a snoop (unless you have an LLC architecture).
   *
   * Synchronisation between multiple users and the GPU is left to userspace
   * through the normal set-domain-ioctl. The kernel will enforce that the
   * GPU relinquishes the VMA before it is returned back to the system
   * i.e. upon free(), munmap() or process termination. However, the userspace
   * malloc() library may not immediately relinquish the VMA after free() and
   * instead reuse it whilst the GPU is still reading and writing to the VMA.
   * Caveat emptor.
   *
   * Also note, that the object created here is not currently a "first class"
   * object, in that several ioctls are banned. These are the CPU access
   * ioctls: mmap(), pwrite and pread. In practice, you are expected to use
   * direct access via your pointer rather than use those ioctls.
   *
   * If you think this is a good interface to use to pass GPU memory between
   * drivers, please use dma-buf instead. In fact, wherever possible use
   * dma-buf instead.
   */
  int
  i915_gem_userptr_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
  {
  	struct drm_i915_private *dev_priv = dev->dev_private;
  	struct drm_i915_gem_userptr *args = data;
  	struct drm_i915_gem_object *obj;
  	int ret;
  	u32 handle;
  
  	if (args->flags & ~(I915_USERPTR_READ_ONLY |
  			    I915_USERPTR_UNSYNCHRONIZED))
  		return -EINVAL;
  
  	if (offset_in_page(args->user_ptr | args->user_size))
  		return -EINVAL;
  
  	if (args->user_size > dev_priv->gtt.base.total)
  		return -E2BIG;
  
  	if (!access_ok(args->flags & I915_USERPTR_READ_ONLY ? VERIFY_READ : VERIFY_WRITE,
  		       (char __user *)(unsigned long)args->user_ptr, args->user_size))
  		return -EFAULT;
  
  	if (args->flags & I915_USERPTR_READ_ONLY) {
  		/* On almost all of the current hw, we cannot tell the GPU that a
  		 * page is readonly, so this is just a placeholder in the uAPI.
  		 */
  		return -ENODEV;
  	}
  
  	obj = i915_gem_object_alloc(dev);
  	if (obj == NULL)
  		return -ENOMEM;
  
  	drm_gem_private_object_init(dev, &obj->base, args->user_size);
  	i915_gem_object_init(obj, &i915_gem_userptr_ops);
  	obj->cache_level = I915_CACHE_LLC;
  	obj->base.write_domain = I915_GEM_DOMAIN_CPU;
  	obj->base.read_domains = I915_GEM_DOMAIN_CPU;
  
  	obj->userptr.ptr = args->user_ptr;
  	obj->userptr.read_only = !!(args->flags & I915_USERPTR_READ_ONLY);
  
  	/* And keep a pointer to the current->mm for resolving the user pages
  	 * at binding. This means that we need to hook into the mmu_notifier
  	 * in order to detect if the mmu is destroyed.
  	 */
  	ret = i915_gem_userptr_init__mm_struct(obj);
  	if (ret == 0)
  		ret = i915_gem_userptr_init__mmu_notifier(obj, args->flags);
  	if (ret == 0)
  		ret = drm_gem_handle_create(file, &obj->base, &handle);
  
  	/* drop reference from allocate - handle holds it now */
  	drm_gem_object_unreference_unlocked(&obj->base);
  	if (ret)
  		return ret;
  
  	args->handle = handle;
  	return 0;
  }
  
  int
  i915_gem_init_userptr(struct drm_device *dev)
  {
  	struct drm_i915_private *dev_priv = to_i915(dev);
  	mutex_init(&dev_priv->mm_lock);
  	hash_init(dev_priv->mm_structs);
  	return 0;
  }
  