[PATCH] drm/i915: Allow userptr backchannel for passing around GTT mappings

[Chris Wilson <chris@chris-wilson.co.uk>]

Once userptr becomes part of client API, it is almost a certainty that
eventually someone will try to create a new object from a mapping of
another client object, e.g.

new = vaImport(vaMap(old, &size), size);

(using a hypothethical API, not meaning to pick on anyone!)

Since this is actually fairly safe to implement and to allow for a GTT
mapping (since it is within a single process space and the memory access
passes the standard permissions test) let us not limit the Client
possibilities.

v2: sfelling pixes

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
 drivers/gpu/drm/i915/i915_gem_userptr.c | 48 ++++++++++++++++++++++++++++++---
 1 file changed, 45 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 1b3b451b6658..afd4c2c4cc04 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -795,6 +795,37 @@ static const struct drm_i915_gem_object_ops i915_gem_userptr_ops = {
 	.release = i915_gem_userptr_release,
 };
 
+static struct drm_i915_gem_object *
+find_object_from_vma(struct drm_device *dev,
+		     struct drm_i915_gem_userptr *args)
+{
+	struct drm_i915_gem_object *obj = NULL;
+	struct vm_area_struct *vma;
+
+	down_read(&current->mm->mmap_sem);
+	vma = find_vma(current->mm, args->user_ptr);
+	if (vma == NULL) {
+		obj = ERR_PTR(-EFAULT);
+		goto out;
+	}
+
+	if (vma->vm_ops != dev->driver->gem_vm_ops)
+		goto out;
+
+	if (vma->vm_start != args->user_ptr ||
+	    vma->vm_end != args->user_ptr + args->user_size) {
+		obj = ERR_PTR(-EINVAL);
+		goto out;
+	}
+
+	obj = to_intel_bo(vma->vm_private_data);
+	drm_gem_object_reference(&obj->base);
+
+out:
+	up_read(&current->mm->mmap_sem);
+	return obj;
+}
+
 /**
  * Creates a new mm object that wraps some normal memory from the process
  * context - user memory.
@@ -802,8 +833,11 @@ static const struct drm_i915_gem_object_ops i915_gem_userptr_ops = {
  * 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).
+ * 2. It must either be:
+ *    a) normal system memory, not a pointer into another map of IO
+ *       space (e.g. it must not be part of a GTT mmapping of another object).
+ *    b) a pointer to the complete GTT mmap of another object in your
+ *       address space.
  * 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
@@ -853,6 +887,14 @@ i915_gem_userptr_ioctl(struct drm_device *dev, void *data, struct drm_file *file
 		return -ENODEV;
 	}
 
+	obj = find_object_from_vma(dev, args);
+	if (obj) {
+		if (!IS_ERR(obj))
+			goto out;
+
+		return PTR_ERR(obj);
+	}
+
 	obj = i915_gem_object_alloc(dev);
 	if (obj == NULL)
 		return -ENOMEM;
@@ -874,7 +916,7 @@ i915_gem_userptr_ioctl(struct drm_device *dev, void *data, struct drm_file *file
 	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);
+out:		 ret = drm_gem_handle_create(file, &obj->base, &handle);
 
 	/* drop reference from allocate - handle holds it now */
 	drm_gem_object_unreference_unlocked(&obj->base);
-- 
2.6.0

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